This book is the primary reference for the Rust programming language. It provides three kinds of material:

• Chapters that informally describe each language construct and their use.
• Chapters that informally describe the memory model, concurrency model, runtime services, linkage model and debugging facilities.
• Appendix chapters providing rationale and references to languages that influenced the design.

This book does not serve as an introduction to the language. Background familiarity with the language is assumed. A separate book is available to help acquire such background familiarity.

This book also does not serve as a reference to the standard library included in the language distribution. Those libraries are documented separately by extracting documentation attributes from their source code. Many of the features that one might expect to be language features are library features in Rust, so what you're looking for may be there, not here.

Similarly, this book does not usually document the specifics of rustc as a tool or of Cargo. rustc has its own book. Cargo has a book that contains a reference. There are a few pages such as linkage that still describe how rustc works.

This book also only serves as a reference to what is available in stable Rust. For unstable features being worked on, see the Unstable Book.

Finally, this book is not normative. It may include details that are specific to rustc itself, and should not be taken as a specification for the Rust language. We intend to produce such a book someday, and until then, the reference is the closest thing we have to one.

This book does not assume you are reading this book sequentially. Each chapter generally can be read standalone, but will cross-link to other chapters for facets of the language they refer to, but do not discuss.

There are two main ways to read this document.

The first is to answer a specific question. If you know which chapter answers that question, you can jump to that chapter in the table of contents. Otherwise, you can press s or the click the magnifying glass on the top bar to search for keywords related to your question. For example, say you wanted to know when a temporary value created in a let statement is dropped. If you didn't already know that the lifetime of temporaries is defined in the expressions chapter, you could search "temporary let" and the first search result will take you to that section.

The second is to generally improve your knowledge of a facet of the language. In that case, just browse the table of contents until you see something you want to know more about, and just start reading. If a link looks interesting, click it, and read about that section.

That said, there is no wrong way to read this book. Read it however you feel helps you best.

Like all technical books, this book has certain conventions in how it displays information. These conventions are documented here.

• Statements that define a term contain that term in italics. Whenever that term is used outside of that chapter, it is usually a link to the section that has this definition.

  An example term is an example of a term being defined.

• Differences in the language by which edition the crate is compiled under are in a blockquote that start with the words "Edition Differences:" in bold.

  Edition Differences: In the 2015 edition, this syntax is valid that is disallowed as of the 2018 edition.

• Notes that contain useful information about the state of the book or point out useful, but mostly out of scope, information are in blockquotes that start with the word "Note:" in bold.

  Note: This is an example note.

• Warnings that show unsound behavior in the language or possibly confusing interactions of language features are in a special warning box.

  Warning: This is an example warning.

• Code snippets inline in the text are inside <code> tags.

  Longer code examples are in a syntax highlighted box that has controls for copying, executing, and showing hidden lines in the top right corner.

  # // This is a hidden line.
  fn main() {
      println!("This is a code example");
  }
  

• The grammar and lexical structure is in blockquotes with either "Lexer" or "Syntax" in ^{bold superscript} as the first line.

  ^Syntax
  ExampleGrammar:
        ~ Expression
     | box Expression

  See Notation for more detail.

We welcome contributions of all kinds.

You can contribute to this book by opening an issue or sending a pull request to the Rust Reference repository. If this book does not answer your question, and you think its answer is in scope of it, please do not hesitate to file an issue or ask about it in the #docs channels on Discord. Knowing what people use this book for the most helps direct our attention to making those sections the best that they can be.

The following notations are used by the Lexer and Syntax grammar snippets:

Some rules in the grammar — notably unary operators, binary operators, and keywords — are given in a simplified form: as a listing of printable strings. These cases form a subset of the rules regarding the token rule, and are assumed to be the result of a lexical-analysis phase feeding the parser, driven by a DFA, operating over the disjunction of all such string table entries.

When such a string in monospace font occurs inside the grammar, it is an implicit reference to a single member of such a string table production. See tokens for more information.

Rust input is interpreted as a sequence of Unicode code points encoded in UTF-8.

Rust divides keywords into three categories:

• strict
• reserved
• weak

These keywords can only be used in their correct contexts. They cannot be used as the names of:

• Items
• Variables and function parameters
• Fields and variants
• Type parameters
• Lifetime parameters or loop labels
• Macros or attributes
• Macro placeholders
• Crates

^Lexer:
KW_AS : as
KW_BREAK : break
KW_CONST : const
KW_CONTINUE : continue
KW_CRATE : crate
KW_ELSE : else
KW_ENUM : enum
KW_EXTERN : extern
KW_FALSE : false
KW_FN : fn
KW_FOR : for
KW_IF : if
KW_IMPL : impl
KW_IN : in
KW_LET : let
KW_LOOP : loop
KW_MATCH : match
KW_MOD : mod
KW_MOVE : move
KW_MUT : mut
KW_PUB : pub
KW_REF : ref
KW_RETURN : return
KW_SELFVALUE : self
KW_SELFTYPE : Self
KW_STATIC : static
KW_STRUCT : struct
KW_SUPER : super
KW_TRAIT : trait
KW_TRUE : true
KW_TYPE : type
KW_UNSAFE : unsafe
KW_USE : use
KW_WHERE : where
KW_WHILE : while

The following keywords were added beginning in the 2018 edition.

^{Lexer 2018+}
KW_DYN : dyn

These keywords aren't used yet, but they are reserved for future use. They have the same restrictions as strict keywords. The reasoning behind this is to make current programs forward compatible with future versions of Rust by forbidding them to use these keywords.

^Lexer
KW_ABSTRACT : abstract
KW_BECOME : become
KW_BOX : box
KW_DO : do
KW_FINAL : final
KW_MACRO : macro
KW_OVERRIDE : override
KW_PRIV : priv
KW_TYPEOF : typeof
KW_UNSIZED : unsized
KW_VIRTUAL : virtual
KW_YIELD : yield

The following keywords are reserved beginning in the 2018 edition.

^{Lexer 2018+}
KW_ASYNC : async
KW_AWAIT : await
KW_TRY : try

These keywords have special meaning only in certain contexts. For example, it is possible to declare a variable or method with the name union.

• union is used to declare a union and is only a keyword when used in a union declaration.

• 'static is used for the static lifetime and cannot be used as a generic lifetime parameter

  // error[E0262]: invalid lifetime parameter name: `'static`
  fn invalid_lifetime_parameter<'static>(s: &'static str) -> &'static str { s }
  

• In the 2015 edition, dyn is a keyword when used in a type position followed by a path that does not start with ::.

  Beginning in the 2018 edition, dyn has been promoted to a strict keyword.

^Lexer
KW_UNION : union
KW_STATICLIFETIME : 'static

^{Lexer 2015}
KW_DYN : dyn

^Lexer:
IDENTIFIER_OR_KEYWORD :
      [a-z A-Z] [a-z A-Z 0-9 _]^*
   | _ [a-z A-Z 0-9 _]⁺

RAW_IDENTIFIER : r# IDENTIFIER_OR_KEYWORD _{Except crate, extern, self, super, Self}

NON_KEYWORD_IDENTIFIER : IDENTIFIER_OR_KEYWORD _{Except a strict or reserved keyword}

IDENTIFIER :
NON_KEYWORD_IDENTIFIER | RAW_IDENTIFIER

An identifier is any nonempty ASCII string of the following form:

Either

• The first character is a letter.
• The remaining characters are alphanumeric or _.

Or

• The first character is _.
• The identifier is more than one character. _ alone is not an identifier.
• The remaining characters are alphanumeric or _.

A raw identifier is like a normal identifier, but prefixed by r#. (Note that the r# prefix is not included as part of the actual identifier.) Unlike a normal identifier, a raw identifier may be any strict or reserved keyword except the ones listed above for RAW_IDENTIFIER.

^Lexer
LINE_COMMENT :
      // (~[/ !] | //) ~\n^*
   | //

BLOCK_COMMENT :
      /* (~[* !] | ** | BlockCommentOrDoc) (BlockCommentOrDoc | ~*/)^* */
   | /**/
   | /***/

INNER_LINE_DOC :
   //! ~[\n IsolatedCR]^*

INNER_BLOCK_DOC :
   /*! ( BlockCommentOrDoc | ~[*/ IsolatedCR] )^* */

OUTER_LINE_DOC :
   /// (~/ ~[\n IsolatedCR]^*)^?

OUTER_BLOCK_DOC :
   /** (~* | BlockCommentOrDoc ) (BlockCommentOrDoc | ~[*/ IsolatedCR])^* */

BlockCommentOrDoc :
      BLOCK_COMMENT
   | OUTER_BLOCK_DOC
   | INNER_BLOCK_DOC

IsolatedCR :
   A \r not followed by a \n

Comments in Rust code follow the general C++ style of line (//) and block (/* ... */) comment forms. Nested block comments are supported.

Non-doc comments are interpreted as a form of whitespace.

Line doc comments beginning with exactly three slashes (///), and block doc comments (/** ... */), both inner doc comments, are interpreted as a special syntax for doc attributes. That is, they are equivalent to writing #[doc="..."] around the body of the comment, i.e., /// Foo turns into #[doc="Foo"] and /** Bar */ turns into #[doc="Bar"].

Line comments beginning with //! and block comments /*! ... */ are doc comments that apply to the parent of the comment, rather than the item that follows. That is, they are equivalent to writing #![doc="..."] around the body of the comment. //! comments are usually used to document modules that occupy a source file.

Isolated CRs (\r), i.e. not followed by LF (\n), are not allowed in doc comments.

# #![allow(unused_variables)]
#fn main() {
//! A doc comment that applies to the implicit anonymous module of this crate

pub mod outer_module {

    //!  - Inner line doc
    //!! - Still an inner line doc (but with a bang at the beginning)

    /*!  - Inner block doc */
    /*!! - Still an inner block doc (but with a bang at the beginning) */

    //   - Only a comment
    ///  - Outer line doc (exactly 3 slashes)
    //// - Only a comment

    /*   - Only a comment */
    /**  - Outer block doc (exactly) 2 asterisks */
    /*** - Only a comment */

    pub mod inner_module {}

    pub mod nested_comments {
        /* In Rust /* we can /* nest comments */ */ */

        // All three types of block comments can contain or be nested inside
        // any other type:

        /*   /* */  /** */  /*! */  */
        /*!  /* */  /** */  /*! */  */
        /**  /* */  /** */  /*! */  */
        pub mod dummy_item {}
    }

    pub mod degenerate_cases {
        // empty inner line doc
        //!

        // empty inner block doc
        /*!*/

        // empty line comment
        //

        // empty outer line doc
        ///

        // empty block comment
        /**/

        pub mod dummy_item {}

        // empty 2-asterisk block isn't a doc block, it is a block comment
        /***/

    }

    /* The next one isn't allowed because outer doc comments
       require an item that will receive the doc */

    /// Where is my item?
#   mod boo {}
}
#}

Whitespace is any non-empty string containing only characters that have the Pattern_White_Space Unicode property, namely:

• U+0009 (horizontal tab, '\t')
• U+000A (line feed, '\n')
• U+000B (vertical tab)
• U+000C (form feed)
• U+000D (carriage return, '\r')
• U+0020 (space, ' ')
• U+0085 (next line)
• U+200E (left-to-right mark)
• U+200F (right-to-left mark)
• U+2028 (line separator)
• U+2029 (paragraph separator)

Rust is a "free-form" language, meaning that all forms of whitespace serve only to separate tokens in the grammar, and have no semantic significance.

A Rust program has identical meaning if each whitespace element is replaced with any other legal whitespace element, such as a single space character.

Tokens are primitive productions in the grammar defined by regular (non-recursive) languages. Rust source input can be broken down into the following kinds of tokens:

• Keywords
• Identifiers
• Literals
• Lifetimes
• Punctuation
• Delimiters

Within this documentation's grammar, "simple" tokens are given in string table production form, and appear in monospace font.

A literal is an expression consisting of a single token, rather than a sequence of tokens, that immediately and directly denotes the value it evaluates to, rather than referring to it by name or some other evaluation rule. A literal is a form of constant expression, so is evaluated (primarily) at compile time.

* The number of #s on each side of the same literal must be equivalent

* All number literals allow _ as a visual separator: 1_234.0E+18f64

^Lexer
CHAR_LITERAL :
   ' ( ~[' \ \n \r \t] | QUOTE_ESCAPE | ASCII_ESCAPE | UNICODE_ESCAPE ) '

QUOTE_ESCAPE :
   \' | \"

ASCII_ESCAPE :
      \x OCT_DIGIT HEX_DIGIT
   | \n | \r | \t | \\ | \0

UNICODE_ESCAPE :
   \u{ ( HEX_DIGIT _^* )^1..6 }

A character literal is a single Unicode character enclosed within two U+0027 (single-quote) characters, with the exception of U+0027 itself, which must be escaped by a preceding U+005C character (\).

^Lexer
STRING_LITERAL :
   " (
      ~[" \ IsolatedCR]
      | QUOTE_ESCAPE
      | ASCII_ESCAPE
      | UNICODE_ESCAPE
      | STRING_CONTINUE
   )^* "

STRING_CONTINUE :
   \ followed by \n

A string literal is a sequence of any Unicode characters enclosed within two U+0022 (double-quote) characters, with the exception of U+0022 itself, which must be escaped by a preceding U+005C character (\).

Line-break characters are allowed in string literals. Normally they represent themselves (i.e. no translation), but as a special exception, when an unescaped U+005C character (\) occurs immediately before the newline (U+000A), the U+005C character, the newline, and all whitespace at the beginning of the next line are ignored. Thus a and b are equal:

# #![allow(unused_variables)]
#fn main() {
let a = "foobar";
let b = "foo\
         bar";

assert_eq!(a,b);
#}

Some additional escapes are available in either character or non-raw string literals. An escape starts with a U+005C (\) and continues with one of the following forms:

• A 7-bit code point escape starts with U+0078 (x) and is followed by exactly two hex digits with value up to 0x7F. It denotes the ASCII character with value equal to the provided hex value. Higher values are not permitted because it is ambiguous whether they mean Unicode code points or byte values.
• A 24-bit code point escape starts with U+0075 (u) and is followed by up to six hex digits surrounded by braces U+007B ({) and U+007D (}). It denotes the Unicode code point equal to the provided hex value.
• A whitespace escape is one of the characters U+006E (n), U+0072 (r), or U+0074 (t), denoting the Unicode values U+000A (LF), U+000D (CR) or U+0009 (HT) respectively.
• The null escape is the character U+0030 (0) and denotes the Unicode value U+0000 (NUL).
• The backslash escape is the character U+005C (\) which must be escaped in order to denote itself.

^Lexer
RAW_STRING_LITERAL :
   r RAW_STRING_CONTENT

RAW_STRING_CONTENT :
      " ( ~ IsolatedCR )^{* (non-greedy)} "
   | # RAW_STRING_CONTENT #

Raw string literals do not process any escapes. They start with the character U+0072 (r), followed by zero or more of the character U+0023 (#) and a U+0022 (double-quote) character. The raw string body can contain any sequence of Unicode characters and is terminated only by another U+0022 (double-quote) character, followed by the same number of U+0023 (#) characters that preceded the opening U+0022 (double-quote) character.

All Unicode characters contained in the raw string body represent themselves, the characters U+0022 (double-quote) (except when followed by at least as many U+0023 (#) characters as were used to start the raw string literal) or U+005C (\) do not have any special meaning.

Examples for string literals:

# #![allow(unused_variables)]
#fn main() {
"foo"; r"foo";                     // foo
"\"foo\""; r#""foo""#;             // "foo"

"foo #\"# bar";
r##"foo #"# bar"##;                // foo #"# bar

"\x52"; "R"; r"R";                 // R
"\\x52"; r"\x52";                  // \x52
#}

^Lexer
BYTE_LITERAL :
   b' ( ASCII_FOR_CHAR | BYTE_ESCAPE ) '

ASCII_FOR_CHAR :
   any ASCII (i.e. 0x00 to 0x7F), except ', \, \n, \r or \t

BYTE_ESCAPE :
      \x HEX_DIGIT HEX_DIGIT
   | \n | \r | \t | \\ | \0

A byte literal is a single ASCII character (in the U+0000 to U+007F range) or a single escape preceded by the characters U+0062 (b) and U+0027 (single-quote), and followed by the character U+0027. If the character U+0027 is present within the literal, it must be escaped by a preceding U+005C (\) character. It is equivalent to a u8 unsigned 8-bit integer number literal.

^Lexer
BYTE_STRING_LITERAL :
   b" ( ASCII_FOR_STRING | BYTE_ESCAPE | STRING_CONTINUE )^* "

ASCII_FOR_STRING :
   any ASCII (i.e 0x00 to 0x7F), except ", \ and IsolatedCR

A non-raw byte string literal is a sequence of ASCII characters and escapes, preceded by the characters U+0062 (b) and U+0022 (double-quote), and followed by the character U+0022. If the character U+0022 is present within the literal, it must be escaped by a preceding U+005C (\) character. Alternatively, a byte string literal can be a raw byte string literal, defined below. The type of a byte string literal of length n is &'static [u8; n].

Some additional escapes are available in either byte or non-raw byte string literals. An escape starts with a U+005C (\) and continues with one of the following forms:

• A byte escape escape starts with U+0078 (x) and is followed by exactly two hex digits. It denotes the byte equal to the provided hex value.
• A whitespace escape is one of the characters U+006E (n), U+0072 (r), or U+0074 (t), denoting the bytes values 0x0A (ASCII LF), 0x0D (ASCII CR) or 0x09 (ASCII HT) respectively.
• The null escape is the character U+0030 (0) and denotes the byte value 0x00 (ASCII NUL).
• The backslash escape is the character U+005C (\) which must be escaped in order to denote its ASCII encoding 0x5C.

^Lexer
RAW_BYTE_STRING_LITERAL :
   br RAW_BYTE_STRING_CONTENT

RAW_BYTE_STRING_CONTENT :
      " ASCII^{* (non-greedy)} "
   | # RAW_STRING_CONTENT #

ASCII :
   any ASCII (i.e. 0x00 to 0x7F)

Raw byte string literals do not process any escapes. They start with the character U+0062 (b), followed by U+0072 (r), followed by zero or more of the character U+0023 (#), and a U+0022 (double-quote) character. The raw string body can contain any sequence of ASCII characters and is terminated only by another U+0022 (double-quote) character, followed by the same number of U+0023 (#) characters that preceded the opening U+0022 (double-quote) character. A raw byte string literal can not contain any non-ASCII byte.

All characters contained in the raw string body represent their ASCII encoding, the characters U+0022 (double-quote) (except when followed by at least as many U+0023 (#) characters as were used to start the raw string literal) or U+005C (\) do not have any special meaning.

Examples for byte string literals:

# #![allow(unused_variables)]
#fn main() {
b"foo"; br"foo";                     // foo
b"\"foo\""; br#""foo""#;             // "foo"

b"foo #\"# bar";
br##"foo #"# bar"##;                 // foo #"# bar

b"\x52"; b"R"; br"R";                // R
b"\\x52"; br"\x52";                  // \x52
#}

A number literal is either an integer literal or a floating-point literal. The grammar for recognizing the two kinds of literals is mixed.

^Lexer
INTEGER_LITERAL :
   ( DEC_LITERAL | BIN_LITERAL | OCT_LITERAL | HEX_LITERAL ) INTEGER_SUFFIX^?

DEC_LITERAL :
   DEC_DIGIT (DEC_DIGIT|_)^*

TUPLE_INDEX :
      0    | NON_ZERO_DEC_DIGIT DEC_DIGIT^*

BIN_LITERAL :
   0b (BIN_DIGIT|_)^* BIN_DIGIT (BIN_DIGIT|_)^*

OCT_LITERAL :
   0o (OCT_DIGIT|_)^* OCT_DIGIT (OCT_DIGIT|_)^*

HEX_LITERAL :
   0x (HEX_DIGIT|_)^* HEX_DIGIT (HEX_DIGIT|_)^*

BIN_DIGIT : [0-1]

OCT_DIGIT : [0-7]

DEC_DIGIT : [0-9]

NON_ZERO_DEC_DIGIT : [1-9]

HEX_DIGIT : [0-9 a-f A-F]

INTEGER_SUFFIX :
      u8 | u16 | u32 | u64 | u128 | usize
   | i8 | i16 | i32 | i64 | i128 | isize

An integer literal has one of four forms:

• A decimal literal starts with a decimal digit and continues with any mixture of decimal digits and underscores.
• A tuple index is either 0, or starts with a non-zero decimal digit and continues with zero or more decimal digits. Tuple indexes are used to refer to the fields of tuples, tuple structs and tuple variants.
• A hex literal starts with the character sequence U+0030 U+0078 (0x) and continues as any mixture (with at least one digit) of hex digits and underscores.
• An octal literal starts with the character sequence U+0030 U+006F (0o) and continues as any mixture (with at least one digit) of octal digits and underscores.
• A binary literal starts with the character sequence U+0030 U+0062 (0b) and continues as any mixture (with at least one digit) of binary digits and underscores.

Like any literal, an integer literal may be followed (immediately, without any spaces) by an integer suffix, which forcibly sets the type of the literal. The integer suffix must be the name of one of the integral types: u8, i8, u16, i16, u32, i32, u64, i64, u128, i128, usize, or isize.

The type of an unsuffixed integer literal is determined by type inference:

• If an integer type can be uniquely determined from the surrounding program context, the unsuffixed integer literal has that type.

• If the program context under-constrains the type, it defaults to the signed 32-bit integer i32.

• If the program context over-constrains the type, it is considered a static type error.

Examples of integer literals of various forms:

# #![allow(unused_variables)]
#fn main() {
123;                               // type i32
123i32;                            // type i32
123u32;                            // type u32
123_u32;                           // type u32
let a: u64 = 123;                  // type u64

0xff;                              // type i32
0xff_u8;                           // type u8

0o70;                              // type i32
0o70_i16;                          // type i16

0b1111_1111_1001_0000;             // type i32
0b1111_1111_1001_0000i64;          // type i64
0b________1;                       // type i32

0usize;                            // type usize
#}

Examples of invalid integer literals:

// invalid suffixes

0invalidSuffix;

// uses numbers of the wrong base

123AFB43;
0b0102;
0o0581;

// integers too big for their type (they overflow)

128_i8;
256_u8;

// bin, hex and octal literals must have at least one digit

0b_;
0b____;

Note that the Rust syntax considers -1i8 as an application of the unary minus operator to an integer literal 1i8, rather than a single integer literal.

^Lexer
FLOAT_LITERAL :
      DEC_LITERAL . (not immediately followed by ., _ or an identifier)
   | DEC_LITERAL FLOAT_EXPONENT
   | DEC_LITERAL . DEC_LITERAL FLOAT_EXPONENT^?
   | DEC_LITERAL (. DEC_LITERAL)^? FLOAT_EXPONENT^? FLOAT_SUFFIX

FLOAT_EXPONENT :
   (e|E) (+|-)? (DEC_DIGIT|_)^* DEC_DIGIT (DEC_DIGIT|_)^*

FLOAT_SUFFIX :
   f32 | f64

A floating-point literal has one of two forms:

• A decimal literal followed by a period character U+002E (.). This is optionally followed by another decimal literal, with an optional exponent.
• A single decimal literal followed by an exponent.

Like integer literals, a floating-point literal may be followed by a suffix, so long as the pre-suffix part does not end with U+002E (.). The suffix forcibly sets the type of the literal. There are two valid floating-point suffixes, f32 and f64 (the 32-bit and 64-bit floating point types), which explicitly determine the type of the literal.

The type of an unsuffixed floating-point literal is determined by type inference:

• If a floating-point type can be uniquely determined from the surrounding program context, the unsuffixed floating-point literal has that type.

• If the program context under-constrains the type, it defaults to f64.

• If the program context over-constrains the type, it is considered a static type error.

Examples of floating-point literals of various forms:

# #![allow(unused_variables)]
#fn main() {
123.0f64;        // type f64
0.1f64;          // type f64
0.1f32;          // type f32
12E+99_f64;      // type f64
let x: f64 = 2.; // type f64
#}

This last example is different because it is not possible to use the suffix syntax with a floating point literal ending in a period. 2.f64 would attempt to call a method named f64 on 2.

The representation semantics of floating-point numbers are described in "Machine Types".

^Lexer
BOOLEAN_LITERAL :
      true
   | false

The two values of the boolean type are written true and false.

^Lexer
LIFETIME_TOKEN :
      ' IDENTIFIER_OR_KEYWORD
   | '_

LIFETIME_OR_LABEL :
      ' NON_KEYWORD_IDENTIFIER

Lifetime parameters and loop labels use LIFETIME_OR_LABEL tokens. Any LIFETIME_TOKEN will be accepted by the lexer, and for example, can be used in macros.

Punctuation symbol tokens are listed here for completeness. Their individual usages and meanings are defined in the linked pages.

Bracket punctuation is used in various parts of the grammar. An open bracket must always be paired with a close bracket. Brackets and the tokens within them are referred to as "token trees" in macros. The three types of brackets are:

A path is a sequence of one or more path segments logically separated by a namespace qualifier (::). If a path consists of only one segment, it refers to either an item or a variable in a local control scope. If a path has multiple segments, it always refers to an item.

Two examples of simple paths consisting of only identifier segments:

x;
x::y::z;

^Syntax
SimplePath :
   ::^? SimplePathSegment (:: SimplePathSegment)^*

SimplePathSegment :
   IDENTIFIER | super | self | crate | $crate

Simple paths are used in visibility markers, attributes, macros, and use items. Examples:

# #![allow(unused_variables)]
#fn main() {
use std::io::{self, Write};
mod m {
    #[clippy::cyclomatic_complexity = "0"]
    pub (in super) fn f1() {}
}
#}

^Syntax
PathInExpression :
   ::^? PathExprSegment (:: PathExprSegment)^*

PathExprSegment :
   PathIdentSegment (:: GenericArgs)^?

PathIdentSegment :
   IDENTIFIER | super | self | Self | crate | $crate

GenericArgs :
      < >
   | < GenericArgsLifetimes ,^? >
   | < GenericArgsTypes ,^? >
   | < GenericArgsBindings ,^? >
   | < GenericArgsTypes , GenericArgsBindings ,^? >
   | < GenericArgsLifetimes , GenericArgsTypes ,^? >
   | < GenericArgsLifetimes , GenericArgsBindings ,^? >
   | < GenericArgsLifetimes , GenericArgsTypes , GenericArgsBindings ,^? >

GenericArgsLifetimes :
   Lifetime (, Lifetime)^*

GenericArgsTypes :
   Type (, Type)^*

GenericArgsBindings :
   GenericArgsBinding (, GenericArgsBinding)^*

GenericArgsBinding :
   IDENTIFIER = Type

Paths in expressions allow for paths with generic arguments to be specified. They are used in various places in expressions and patterns.

The :: token is required before the opening < for generic arguments to avoid ambiguity with the less-than operator. This is colloquially known as "turbofish" syntax.

# #![allow(unused_variables)]
#fn main() {
(0..10).collect::<Vec<_>>();
Vec::<u8>::with_capacity(1024);
#}

^Syntax
QualifiedPathInExpression :
   QualifiedPathType (:: PathExprSegment)⁺

QualifiedPathType :
   < Type (as TypePath)? >

QualifiedPathInType :
   QualifiedPathType (:: TypePathSegment)⁺

Fully qualified paths allow for disambiguating the path for trait implementations and for specifying canonical paths. When used in a type specification, it supports using the type syntax specified below.

# #![allow(unused_variables)]
#fn main() {
struct S;
impl S {
    fn f() { println!("S"); }
}
trait T1 {
    fn f() { println!("T1 f"); }
}
impl T1 for S {}
trait T2 {
    fn f() { println!("T2 f"); }
}
impl T2 for S {}
S::f();  // Calls the inherent impl.
<S as T1>::f();  // Calls the T1 trait function.
<S as T2>::f();  // Calls the T2 trait function.
#}

^Syntax
TypePath :
   ::^? TypePathSegment (:: TypePathSegment)^*

TypePathSegment :
   PathIdentSegment ::^? (GenericArgs | TypePathFn)^?

TypePathFn :
( TypePathFnInputs^? ) (-> Type)^?

TypePathFnInputs :
Type (, Type)^* ,^?

Type paths are used within type definitions, trait bounds, type parameter bounds, and qualified paths.

Although the :: token is allowed before the generics arguments, it is not required because there is no ambiguity like there is in PathInExpression.

impl ops::Index<ops::Range<usize>> for S { /*...*/ }
fn i() -> impl Iterator<Item = op::Example<'a>> { /*...*/ }
type G = std::boxed::Box<std::ops::FnOnce(isize) -> isize>;

Paths can be denoted with various leading qualifiers to change the meaning of how it is resolved.

Paths starting with :: are considered to be global paths where the segments of the path start being resolved from the crate root. Each identifier in the path must resolve to an item.

Edition Differences: In the 2015 Edition, the crate root contains a variety of different items, including external crates, default crates such as std and core, and items in the top level of the crate (including use imports).

Beginning with the 2018 Edition, paths starting with :: can only reference crates.

mod a {
    pub fn foo() {}
}
mod b {
    pub fn foo() {
        ::a::foo(); // call a's foo function
    }
}
# fn main() {}

self resolves the path relative to the current module. self can only be used as the first segment, without a preceding ::.

fn foo() {}
fn bar() {
    self::foo();
}
# fn main() {}

Self, with a capital "S", is used to refer to the implementing type within traits and implementations.

Self can only be used as the first segment, without a preceding ::.

# #![allow(unused_variables)]
#fn main() {
trait T {
    type Item;
    const C: i32;
    // `Self` will be whatever type that implements `T`.
    fn new() -> Self;
    // `Self::Item` will be the type alias in the implementation.
    fn f(&self) -> Self::Item;
}
struct S;
impl T for S {
    type Item = i32;
    const C: i32 = 9;
    fn new() -> Self {           // `Self` is the type `S`.
        S
    }
    fn f(&self) -> Self::Item {  // `Self::Item` is the type `i32`.
        Self::C                  // `Self::C` is the constant value `9`.
    }
}
#}

super in a path resolves to the parent module. It may only be used in leading segments of the path, possibly after an initial self segment.

mod a {
    pub fn foo() {}
}
mod b {
    pub fn foo() {
        super::a::foo(); // call a's foo function
    }
}
# fn main() {}

super may be repeated several times after the first super or self to refer to ancestor modules.

mod a {
    fn foo() {}

    mod b {
        mod c {
            fn foo() {
                super::super::foo(); // call a's foo function
                self::super::super::foo(); // call a's foo function
            }
        }
    }
}
# fn main() {}

crate resolves the path relative to the current crate. crate can only be used as the first segment, without a preceding ::.

fn foo() {}
mod a {
    fn bar() {
        crate::foo();
    }
}
# fn main() {}

$crate is only used within macro transcribers, and can only be used as the first segment, without a preceding ::. $crate will expand to a path to access items from the top level of the crate where the macro is defined, regardless of which crate the macro is invoked.

pub fn increment(x: u32) -> u32 {
    x + 1
}

#[macro_export]
macro_rules! inc {
    ($x:expr) => ( $crate::increment($x) )
}
# fn main() { }

Items defined in a module or implementation have a canonical path that corresponds to where within its crate it is defined. All other paths to these items are aliases. The canonical path is defined as a path prefix appended by the path segment the item itself defines.

Implementations and use declarations do not have canonical paths, although the items that implementations define do have them. Items defined in block expressions do not have canonical paths. Items defined in a module that does not have a canonical path do not have a canonical path. Associated items defined in an implementation that refers to an item without a canonical path, e.g. as the implementing type, the trait being implemented, a type parameter or bound on a type parameter, do not have canonical paths.

The path prefix for modules is the canonical path to that module. For bare implementations, it is the canonical path of the item being implemented surrounded by angle (<>) brackets. For trait implementations, it is the canonical path of the item being implemented followed by as followed by the canonical path to the trait all surrounded in angle (<>) brackets.

The canonical path is only meaningful within a given crate. There is no global namespace across crates; an item's canonical path merely identifies it within the crate.

// Comments show the canonical path of the item.

mod a { // ::a
    pub struct Struct; // ::a::Struct

    pub trait Trait { // ::a::Trait
        fn f(&self); // a::Trait::f
    }

    impl Trait for Struct {
        fn f(&self) {} // <::a::Struct as ::a::Trait>::f
    }

    impl Struct {
        fn g(&self) {} // <::a::Struct>::g
    }
}

mod without { // ::without
    fn canonicals() { // ::without::canonicals
        struct OtherStruct; // None

        trait OtherTrait { // None
            fn g(&self); // None
        }

        impl OtherTrait for OtherStruct {
            fn g(&self) {} // None
        }

        impl OtherTrait for ::a::Struct {
            fn g(&self) {} // None
        }

        impl ::a::Trait for OtherStruct {
            fn f(&self) {} // None
        }
    }
}

# fn main() {}

The functionality and syntax of Rust can be extended with custom definitions called macros. They are given names, and invoked through a consistent syntax:some_extension!(...).

There are two ways to define new macros:

• Macros by Example define new syntax in a higher-level, declarative way.
• Procedural Macros can be used to implement custom derive.

^Syntax
MacroInvocation :
   SimplePath ! DelimTokenTree

DelimTokenTree :
      ( TokenTree^* )
   | [ TokenTree^* ]
   | { TokenTree^* }

TokenTree :
   Token_{except delimiters} | DelimTokenTree

MacroInvocationSemi :
      SimplePath ! ( TokenTree^* ) ;
   | SimplePath ! [ TokenTree^* ] ;
   | SimplePath ! { TokenTree^* }

A macro invocation executes a macro at compile time and replaces the invocation with the result of the macro. Macros may be invoked in the following situations:

• Expressions and statements
• Patterns
• Types
• Items including associated items
• macro_rules transcribers

When used as an item or a statement, the MacroInvocationSemi form is used where a semicolon is required at the end when not using curly braces. Visibility qualifiers are never allowed before a macro invocation or macro_rules definition.

# #![allow(unused_variables)]
#fn main() {
// Used as an expression.
let x = vec![1,2,3];

// Used as a statement.
println!("Hello!");

// Used in a pattern.
macro_rules! pat {
    ($i:ident) => (Some($i))
}

if let pat!(x) = Some(1) {
    assert_eq!(x, 1);
}

// Used in a type.
macro_rules! Tuple {
    { $A:ty, $B:ty } => { ($A, $B) };
}

type N2 = Tuple!(i32, i32);

// Used as an item.
# use std::cell::RefCell;
thread_local!(static FOO: RefCell<u32> = RefCell::new(1));

// Used as an associated item.
macro_rules! const_maker {
    ($t:ty, $v:tt) => { const CONST: $t = $v; };
}
trait T {
    const_maker!{i32, 7}
}

// Macro calls within macros.
macro_rules! example {
    () => { println!("Macro call in a macro!") };
}
// Outer macro `example` is expanded, then inner macro `println` is expanded.
example!();
#}

^Syntax
MacroRulesDefinition :
   macro_rules ! IDENTIFIER MacroRulesDef

MacroRulesDef :
      ( MacroRules ) ;
   | [ MacroRules ] ;
   | { MacroRules }

MacroRules :
   MacroRule ( ; MacroRule )^* ;^?

MacroRule :
   MacroMatcher => MacroTranscriber

MacroMatcher :
      ( MacroMatch^* )
   | [ MacroMatch^* ]
   | { MacroMatch^* }

MacroMatch :
      Token_{except $ and delimiters}
   | MacroMatcher
   | $ IDENTIFIER : MacroFragSpec
   | $ ( MacroMatch⁺ ) MacroRepSep^? MacroRepOp

MacroFragSpec :
      block | expr | ident | item | lifetime | literal
   | meta | pat | path | stmt | tt | ty | vis

MacroRepSep :
   Token_{except delimiters and repetition operators}

MacroRepOp₂₀₁₈₊ :
   * | + | ?₂₀₁₈₊

MacroTranscriber :
   DelimTokenTree

macro_rules allows users to define syntax extension in a declarative way. We call such extensions "macros by example" or simply "macros".

Each macro by example has a name, and one or more rules. Each rule has two parts: a matcher, describing the syntax that it matches, and a transcriber, describing the syntax that will replace a successfully matched invocation. Both the matcher and the transcriber must be surrounded by delimiters. Macros can expand to expressions, statements, items (including traits, impls, and foreign items), types, or patterns.

When a macro is invoked, the macro expander looks up macro invocations by name, and tries each macro rule in turn. It transcribes the first successful match; if this results in an error, then future matches are not tried. When matching, no lookahead is performed; if the compiler cannot unambiguously determine how to parse the macro invocation one token at a time, then it is an error. In the following example, the compiler does not look ahead past the identifier to see if the following token is a ), even though that would allow it to parse the invocation unambiguously:

# #![allow(unused_variables)]
#fn main() {
macro_rules! ambiguity {
    ($($i:ident)* $j:ident) => { };
}

ambiguity!(error); // Error: local ambiguity
#}

In both the matcher and the transcriber, the $ token is used to invoke special behaviours from the macro engine (described below in Metavariables and Repetitions). Tokens that aren't part of such an invocation are matched and transcribed literally, with one exception. The exception is that the outer delimiters for the matcher will match any pair of delimiters. Thus, for instance, the matcher (()) will match {()} but not {{}}. The character $ cannot be matched or transcribed literally.

When forwarding a matched fragment to another macro-by-example, matchers in the second macro will see an opaque AST of the fragment type. The second macro can't use literal tokens to match the fragments in the matcher, only a fragment specifier of the same type. The ident, lifetime, and tt fragment types are an exception, and can be matched by literal tokens. The following illustrates this restriction:

# #![allow(unused_variables)]
#fn main() {
macro_rules! foo {
    ($l:expr) => { bar!($l); }
// ERROR:               ^^ no rules expected this token in macro call
}

macro_rules! bar {
    (3) => {}
}

foo!(3);
#}

The following illustrates how tokens can be directly matched after matching a tt fragment:

# #![allow(unused_variables)]
#fn main() {
// compiles OK
macro_rules! foo {
    ($l:tt) => { bar!($l); }
}

macro_rules! bar {
    (3) => {}
}

foo!(3);
#}

In the matcher, $ name : fragment-specifier matches a Rust syntax fragment of the kind specified and binds it to the metavariable $name. Valid fragment specifiers are:

• item: an Item
• block: a BlockExpression
• stmt: a Statement without the trailing semicolon (except for item statements that require semicolons)
• pat: a Pattern
• expr: an Expression
• ty: a Type
• ident: an IDENTIFIER_OR_KEYWORD
• path: a TypePath style path
• tt: a TokenTree (a single token or tokens in matching delimiters (), [], or {})
• meta: a MetaItem, the contents of an attribute
• lifetime: a LIFETIME_TOKEN
• vis: a possibly empty Visibility qualifier
• literal: matches -^?LiteralExpression

In the transcriber, metavariables are referred to simply by $name, since the fragment kind is specified in the matcher. Metavariables are replaced with the syntax element that matched them. The keyword metavariable $crate can be used to refer to the current crate; see Hygiene below. Metavariables can be transcribed more than once or not at all.

In both the matcher and transcriber, repetitions are indicated by placing the tokens to be repeated inside $(…), followed by a repetition operator, optionally with a separator token between. The separator token can be any token other than a delimiter or one of the repetition operators, but ; and , are the most common. For instance, $( $i:ident ),* represents any number of identifiers separated by commas. Nested repetitions are permitted.

The repetition operators are:

• * — indicates any number of repetitions.
• + — indicates any number but at least one.
• ? — indicates an optional fragment with zero or one occurrences.

Since ? represents at most one occurrence, it cannot be used with a separator.

The repeated fragment both matches and transcribes to the specified number of the fragment, separated by the separator token. Metavariables are matched to every repetition of their corresponding fragment. For instance, the $( $i:ident ),* example above matches $i to all of the identifiers in the list.

During transcription, additional restrictions apply to repetitions so that the compiler knows how to expand them properly:

1. A metavariable must appear in exactly the same number, kind, and nesting order of repetitions in the transcriber as it did in the matcher. So for the matcher $( $i:ident ),*, the transcribers => { $i }, => { $( $( $i)* )* }, and => { $( $i )+ } are all illegal, but => { $( $i );* } is correct and replaces a comma-separated list of identifiers with a semicolon-separated list.
2. Second, each repetition in the transcriber must contain at least one metavariable to decide now many times to expand it. If multiple metavariables appear in the same repetition, they must be bound to the same number of fragments. For instance, ( $( $i:ident ),* ; $( $j:ident ),* ) => ( $( ($i,$j) ),* must bind the same number of $i fragments as $j fragments. This means that invoking the macro with (a, b, c; d, e, f) is legal and expands to ((a,d), (b,e), c,f)), but (a, b, c; d, e) is illegal because it does not have the same number. This requirement applies to every layer of nested repetitions.

Edition Differences: The ? repetition operator did not exist before the 2018 edition. Prior to the 2018 Edition, ? was an allowed separator token, rather than a repetition operator.

For historical reasons, the scoping of macros by example does not work entirely like items. Macros have two forms of scope: textual scope, and path-based scope. Textual scope is based on the order that things appear in source files, or even across multiple files, and is the default scoping. It is explained further below. Path-based scope works exactly the same way that item scoping does. The scoping, exporting, and importing of macros is controlled largely by attributes.

When a macro is invoked by an unqualified identifier (not part of a multi-part path), it is first looked up in textual scoping. If this does not yield any results, then it is looked up in path-based scoping. If the macro's name is qualified with a path, then it is only looked up in path-based scoping.

use lazy_static::lazy_static; // Path-based import.

macro_rules! lazy_static { // Textual definition.
    (lazy) => {};
}

lazy_static!{lazy} // Textual lookup finds our macro first.
self::lazy_static!{} // Path-based lookup ignores our macro, finds imported one.

Textual scope is based largely on the order that things appear in source files, and works similarly to the scope of local variables declared with let except it also applies at the module level. When macro_rules! is used to define a macro, the macro enters the scope after the definition (note that it can still be used recursively, since names are looked up from the invocation site), up until its surrounding scope, typically a module, is closed. This can enter child modules and even span across multiple files:

//// src/lib.rs
mod has_macro {
    // m!{} // Error: m is not in scope.

    macro_rules! m {
        () => {};
    }
    m!{} // OK: appears after declaration of m.

    mod uses_macro;
}

// m!{} // Error: m is not in scope.

//// src/has_macro/uses_macro.rs

m!{} // OK: appears after declaration of m in src/lib.rs

It is not an error to define a macro multiple times; the most recent declaration will shadow the previous one unless it has gone out of scope.

# #![allow(unused_variables)]
#fn main() {
macro_rules! m {
    (1) => {};
}

m!(1);

mod inner {
    m!(1);

    macro_rules! m {
        (2) => {};
    }
    // m!(1); // Error: no rule matches '1'
    m!(2);

    macro_rules! m {
        (3) => {};
    }
    m!(3);
}

m!(1);
#}

Macros can be declared and used locally inside functions as well, and work similarly:

# #![allow(unused_variables)]
#fn main() {
fn foo() {
    // m!(); // Error: m is not in scope.
    macro_rules! m {
        () => {};
    }
    m!();
}


// m!(); // Error: m is not in scope.
#}

The macro_use attribute has two purposes. First, it can be used to make a module's macro scope not end when the module is closed, by applying it to a module:

# #![allow(unused_variables)]
#fn main() {
#[macro_use]
mod inner {
    macro_rules! m {
        () => {};
    }
}

m!();
#}

Second, it can be used to import macros from another crate, by attaching it to an extern crate declaration appearing in the crate's root module. Macros imported this way are imported into the prelude of the crate, not textually, which means that they can be shadowed by any other name. While macros imported by #[macro_use] can be used before the import statement, in case of a conflict, the last macro imported wins. Optionally, a list of macros to import can be specified using the MetaListIdents syntax; this is not supported when #[macro_use] is applied to a module.

#[macro_use(lazy_static)] // Or #[macro_use] to import all macros.
extern crate lazy_static;

lazy_static!{};
// self::lazy_static!{} // Error: lazy_static is not defined in `self`

Macros to be imported with #[macro_use] must be exported with #[macro_export], which is described below.

By default, a macro has no path-based scope. However, if it has the #[macro_export] attribute, then it is declared in the crate root scope and can be referred to normally as such:

# #![allow(unused_variables)]
#fn main() {
self::m!();
m!(); // OK: Path-based lookup finds m in the current module.

mod inner {
    super::m!();
    crate::m!();
}

mod mac {
    #[macro_export]
    macro_rules! m {
        () => {};
    }
}
#}

Macros labeled with #[macro_export] are always pub and can be referred to by other crates, either by path or by #[macro_use] as described above.

By default, all identifiers referred to in a macro are expanded as-is, and are looked up at the macro's invocation site. This can lead to issues if a macro refers to an item or macro which isn't in scope at the invocation site. To alleviate this, the $crate metavariable can be used at the start of a path to force lookup to occur inside the crate defining the macro.

//// Definitions in the `helper_macro` crate.
#[macro_export]
macro_rules! helped {
    // () => { helper!() } // This might lead to an error due to 'helper' not being in scope.
    () => { $crate::helper!() }
}

#[macro_export]
macro_rules! helper {
    () => { () }
}

//// Usage in another crate.
// Note that `helper_macro::helper` is not imported!
use helper_macro::helped;

fn unit() {
    helped!();
}

Note that, because $crate refers to the current crate, it must be used with a fully qualified module path when referring to non-macro items:

# #![allow(unused_variables)]
#fn main() {
pub mod inner {
    #[macro_export]
    macro_rules! call_foo {
        () => { $crate::inner::foo() };
    }

    pub fn foo() {}
}
#}

Additionally, even though $crate allows a macro to refer to items within its own crate when expanding, its use has no effect on visibility. An item or macro referred to must still be visible from the invocation site. In the following example, any attempt to invoke call_foo!() from outside its crate will fail because foo() is not public.

# #![allow(unused_variables)]
#fn main() {
#[macro_export]
macro_rules! call_foo {
    () => { $crate::foo() };
}

fn foo() {}
#}

Version & Edition Differences: Prior to Rust 1.30, $crate and local_inner_macros (below) were unsupported. They were added alongside path-based imports of macros (described above), to ensure that helper macros did not need to be manually imported by users of a macro-exporting crate. Crates written for earlier versions of Rust that use helper macros need to be modified to use $crate or local_inner_macros to work well with path-based imports.

When a macro is exported, the #[macro_export] attribute can have the local_inner_macros keyword added to automatically prefix all contained macro invocations with $crate::. This is intended primarily as a tool to migrate code written before $crate was added to the language to work with Rust 2018's path-based imports of macros. Its use is discouraged in new code.

# #![allow(unused_variables)]
#fn main() {
#[macro_export(local_inner_macros)]
macro_rules! helped {
    () => { helper!() } // Automatically converted to $crate::helper!().
}

#[macro_export]
macro_rules! helper {
    () => { () }
}
#}

The parser used by the macro system is reasonably powerful, but it is limited in order to prevent ambiguity in current or future versions of the language. In particular, in addition to the rule about ambiguous expansions, a nonterminal matched by a metavariable must be followed by a token which has been decided can be safely used after that kind of match.

As an example, a macro matcher like $i:expr [ , ] could in theory be accepted in Rust today, since [,] cannot be part of a legal expression and therefore the parse would always be unambiguous. However, because [ can start trailing expressions, [ is not a character which can safely be ruled out as coming after an expression. If [,] were accepted in a later version of Rust, this matcher would become ambiguous or would misparse, breaking working code. Matchers like $i:expr, or $i:expr; would be legal, however, because , and ; are legal expression separators. The specific rules are:

• expr and stmt may only be followed by one of: =>, ,, or ;.
• pat may only be followed by one of: =>, ,, =, |, if, or in.
• path and ty may only be followed by one of: =>, ,, =, |, ;, :, >, >>, [, {, as, where, or a macro variable of block fragment specifier.
• vis may only be followed by one of: ,, an identifier other than a non-raw priv, any token that can begin a type, or a metavariable with a ident, ty, or path fragment specifier.
• All other fragment specifiers have no restrictions.

When repetitions are involved, then the rules apply to every possible number of expansions, taking separators into account. This means:

• If the repetition includes a separator, that separator must be able to follow the contents of the repetition.
• If the repetition can repeat multiple times (* or +), then the contents must be able to follow themselves.
• The contents of the repetition must be able to follow whatever comes before, and whatever comes after must be able to follow the contents of the repetition.
• If the repetition can match zero times (* or ?), then whatever comes after must be able to follow whatever comes before.

For more detail, see the formal specification.

Procedural macros allow creating syntax extensions as execution of a function. Procedural macros come in one of three flavors:

• Function-like macros - custom!(...)
• Derive macros - #[derive(CustomDerive)]
• Attribute macros - #[CustomAttribute]

Procedural macros allow you to run code at compile time that operates over Rust syntax, both consuming and producing Rust syntax. You can sort of think of procedural macros as functions from an AST to another AST.

Procedural macros must be defined in a crate with the crate type of proc-macro.

Note: When using Cargo, Procedural macro crates are defined with the proc-macro key in your manifest:

[lib]
proc-macro = true

As functions, they must either return syntax, panic, or loop endlessly. Returned syntax either replaces or adds the syntax depending on the kind of procedural macro. Panics are caught by the compiler and are turned into a compiler error. Endless loops are not caught by the compiler which hangs the compiler.

Procedural macros run during compilation, and thus have the same resources that the compiler has. For example, standard input, error, and output are the same that the compiler has access to. Similarly, file access is the same. Because of this, procedural macros have the same security concerns that Cargo's build scripts have.

Procedural macros have two ways of reporting errors. The first is to panic. The second is to emit a compile_error macro invocation.

Procedural macro crates almost always will link to the compiler-provided proc_macro crate. The proc_macro crate provides types required for writing procedural macros and facilities to make it easier.

This crate primarily contains a TokenStream type. Procedural macros operate over token streams instead of AST nodes, which is a far more stable interface over time for both the compiler and for procedural macros to target. A token stream is roughly equivalent to Vec<TokenTree> where a TokenTree can roughly be thought of as lexical token. For example foo is an Ident token, . is a Punct token, and 1.2 is a Literal token. The TokenStream type, unlike Vec<TokenTree>, is cheap to clone.

All tokens have an associated Span. A Span is an opaque value that cannot be modified but can be manufactured. Spans represent an extent of source code within a program and are primarily used for error reporting. You can modify the Span of any token.

Procedural macros are unhygienic. This means they behave as if the output token stream was simply written inline to the code it's next to. This means that it's affected by external items and also affects external imports.

Macro authors need to be careful to ensure their macros work in as many contexts as possible given this limitation. This often includes using absolute paths to items in libraries (for example, ::std::option::Option instead of Option) or by ensuring that generated functions have names that are unlikely to clash with other functions (like __internal_foo instead of foo).

Function-like procedural macros are procedural macros that are invoked using the macro invocation operator (!).

These macros are defined by a public function with the proc_macro attribute and a signature of (TokenStream) -> TokenStream. The input TokenStream is what is inside the delimiters of the macro invocation and the output TokenStream replaces the entire macro invocation. It may contain an arbitrary number of items. These macros cannot expand to syntax that defines new macro_rules style macros.

For example, the following macro definition ignores its input and outputs a function answer into its scope.

extern crate proc_macro;
use proc_macro::TokenStream;

#[proc_macro]
pub fn make_answer(_item: TokenStream) -> TokenStream {
    "fn answer() -> u32 { 42 }".parse().unwrap()
}

And then we use it a binary crate to print "42" to standard output.

extern crate proc_macro_examples;
use proc_macro_examples::make_answer;

make_answer!();

fn main() {
    println!("{}", answer());
}

These macros are only invokable in modules. They cannot even be invoked to create item declaration statements. Furthermore, they must either be invoked with curly braces and no semicolon or a different delimiter followed by a semicolon. For example, make_answer from the previous example can be invoked as make_answer!{}, make_answer!(); or make_answer![];.

Derive macros define new inputs for the derive attribute. These macros can create new items given the token stream of a struct, enum, or union. They can also define derive macro helper attributes.

Custom derive macros are defined by a public function with the proc_macro_derive attribute and a signature of (TokenStream) -> TokenStream.

The input TokenStream is the token stream of the item that has the derive attribute on it. The output TokenStream must be a set of items that are then appended to the module or block that the item from the input TokenStream is in.

The following is an example of a derive macro. Instead of doing anything useful with its input, it just appends a function answer.

extern crate proc_macro;
use proc_macro::TokenStream;

#[proc_macro_derive(AnswerFn)]
pub fn derive_answer_fn(_item: TokenStream) -> TokenStream {
    "fn answer() -> u32 { 42 }".parse().unwrap()
}

And then using said derive macro:

extern crate proc_macro_examples;
use proc_macro_examples::AnswerFn;

#[derive(AnswerFn)]
struct Struct;

fn main() {
    assert_eq!(42, answer());
}

Derive macros can add additional attributes into the scope of the item they are on. Said attributes are called derive macro helper attributes. These attributes are inert, and their only purpose is to be fed into the derive macro that defined them. That said, they can be seen by all macros.

The way to define helper attributes is to put an attributes key in the proc_macro_derive macro with a comma separated list of identifiers that are the names of the helper attributes.

For example, the following derive macro defines a helper attribute helper, but ultimately doesn't do anything with it.

# #[crate_type="proc-macro"]
# extern crate proc_macro;
# use proc_macro::TokenStream;

#[proc_macro_derive(HelperAttr, attributes(helper))]
pub fn derive_helper_attr(_item: TokenStream) -> TokenStream {
    TokenStream::new();
}

And then usage on the derive macro on a struct:

# #![crate_type="proc-macro"]
# extern crate proc_macro_examples;
# use proc_macro_examples::HelperAttr;

#[derive(HelperAttr)]
struct Struct {
    #[helper] field: ()
}

Attribute macros define new attributes which can be attached to items.

Attribute macros are defined by a public function with the proc_macro_attribute attribute that has a signature of (TokenStream, TokenStream) -> TokenStream. The first TokenStream is the delimited token tree following the attribute's name, not including the outer delimiters. If the attribute is written as a bare attribute name, the attribute TokenStream is empty. The second TokenStream is the rest of the item including other attributes on the item. The returned TokenStream replaces the item with an arbitrary number of items. These macros cannot expand to syntax that defines new macro_rules style macros.

For example, this attribute macro takes the input stream and returns it as is, effectively being the no-op of attributes.

# #![crate_type = "proc-macro"]
# extern crate proc_macro;
# use proc_macro::TokenStream;

#[proc_macro_attribute]
pub fn return_as_is(_attr: TokenStream, item: TokenStream) -> TokenStream {
    item
}

This following example shows the stringified TokenStreams that the attribute macros see. The output will show in the output of the compiler. The output is shown in the comments after the function prefixed with "out:".

// my-macro/src/lib.rs
# extern crate proc_macro;
# use proc_macro::TokenStream;

#[proc_macro_attribute]
pub fn show_streams(attr: TokenStream, item: TokenStream) -> TokenStream {
    println!("attr: \"{}\"", attr.to_string());
    println!("item: \"{}\"", item.to_string());
    item
}

// src/lib.rs
extern crate my_macro;

use my_macro::show_streams;

// Example: Basic function
#[show_streams]
fn invoke1() {}
// out: attr: ""
// out: item: "fn invoke1() { }"

// Example: Attribute with input
#[show_streams(bar)]
fn invoke2() {}
// out: attr: "bar"
// out: item: "fn invoke2() {}"

// Example: Multiple tokens in the input
#[show_streams(multiple => tokens)]
fn invoke3() {}
// out: attr: "multiple => tokens"
// out: item: "fn invoke3() {}"

// Example:
#[show_streams { delimiters }]
fn invoke4() {}
// out: attr: "delimiters"
// out: item: "fn invoke4() {}"

^Syntax
Crate :
   UTF8BOM^?
   SHEBANG^?
   InnerAttribute^*
   Item^*

^Lexer
UTF8BOM : \uFEFF
SHEBANG : #! ~[[ \n] ~\n^*

Note: Although Rust, like any other language, can be implemented by an interpreter as well as a compiler, the only existing implementation is a compiler, and the language has always been designed to be compiled. For these reasons, this section assumes a compiler.

Rust's semantics obey a phase distinction between compile-time and run-time.¹ Semantic rules that have a static interpretation govern the success or failure of compilation, while semantic rules that have a dynamic interpretation govern the behavior of the program at run-time.

The compilation model centers on artifacts called crates. Each compilation processes a single crate in source form, and if successful, produces a single crate in binary form: either an executable or some sort of library.²

A crate is a unit of compilation and linking, as well as versioning, distribution and runtime loading. A crate contains a tree of nested module scopes. The top level of this tree is a module that is anonymous (from the point of view of paths within the module) and any item within a crate has a canonical module path denoting its location within the crate's module tree.

The Rust compiler is always invoked with a single source file as input, and always produces a single output crate. The processing of that source file may result in other source files being loaded as modules. Source files have the extension .rs.

A Rust source file describes a module, the name and location of which — in the module tree of the current crate — are defined from outside the source file: either by an explicit Module item in a referencing source file, or by the name of the crate itself. Every source file is a module, but not every module needs its own source file: module definitions can be nested within one file.

Each source file contains a sequence of zero or more Item definitions, and may optionally begin with any number of attributes that apply to the containing module, most of which influence the behavior of the compiler. The anonymous crate module can have additional attributes that apply to the crate as a whole.

# #![allow(unused_variables)]
#fn main() {
// Specify the crate name.
#![crate_name = "projx"]

// Specify the type of output artifact.
#![crate_type = "lib"]

// Turn on a warning.
// This can be done in any module, not just the anonymous crate module.
#![warn(non_camel_case_types)]
#}

The optional UTF8 byte order mark (UTF8BOM production) indicates that the file is encoded in UTF8. It can only occur at the beginning of the file and is ignored by the compiler.

A source file can have a shebang (SHEBANG production), which indicates to the operating system what program to use to execute this file. It serves essentially to treat the source file as an executable script. The shebang can only occur at the beginning of the file (but after the optional UTF8BOM). It is ignored by the compiler. For example:

#!/usr/bin/env rustx

fn main() {
    println!("Hello!");
}

All crates have a prelude that automatically inserts names from a specific module, the prelude module, into scope of each module and an extern crate into the crate root module. By default, the standard prelude is used. The linked crate is std and the prelude module is std::prelude::v1.

The prelude can be changed to the core prelude by using the no_std attribute on the root crate module. The linked crate is core and the prelude module is core::prelude::v1. Using the core prelude over the standard prelude is useful when either the crate is targeting a platform that does not support the standard library or is purposefully not using the capabilities of the standard library. Those capabilities are mainly dynamic memory allocation (e.g. Box and Vec) and file and network capabilities (e.g. std::fs and std::io).

A crate that contains a main function can be compiled to an executable. If a main function is present, it must take no arguments, must not declare any trait or lifetime bounds, must not have any where clauses, and its return type must be one of the following:

• ()
• Result<(), E> where E: Error

Note: The implementation of which return types are allowed is determined by the unstable Termination trait.

The no_main attribute may be applied at the crate level to disable emitting the main symbol for an executable binary. This is useful when some other object being linked to defines main.

The crate_name attribute may be applied at the crate level to specify the name of the crate with the MetaNameValueStr syntax.

#![crate_name = "mycrate"]

The crate name must not be empty, and must only contain Unicode alphanumeric or - (U+002D) characters.

^Syntax
ConfigurationPredicate :
      ConfigurationOption
   | ConfigurationAll
   | ConfigurationAny
   | ConfigurationNot

ConfigurationOption :
   IDENTIFIER (= (STRING_LITERAL | RAW_STRING_LITERAL))^?

ConfigurationAll
   all ( ConfigurationPredicateList^? )

ConfigurationAny
   any ( ConfigurationPredicateList^? )

ConfigurationNot
   not ( ConfigurationPredicate )

ConfigurationPredicateList
   ConfigurationPredicate (, ConfigurationPredicate)^* ,^?

Conditionally compiled source code is source code that may or may not be considered a part of the source code depending on certain conditions. Source code can be conditionally compiled using attributes, cfg and cfg_attr, and the built-in cfg macro. These conditions are based on the target architecture of the compiled crate, arbitrary values passed to the compiler, and a few other miscellaneous things further described below in detail.

Each form of conditional compilation takes a configuration predicate that evaluates to true or false. The predicate is one of the following:

• A configuration option. It is true if the option is set and false if it is unset.
• all() with a comma separated list of configuration predicates. It is false if at least one predicate is false. If there are no predicates, it is true.
• any() with a comma separated list of configuration predicates. It is true if at least one predicate is true. If there are no predicates, it is false.
• not() with a configuration predicate. It is true if its predicate is false and false if its predicate is true.

Configuration options are names and key-value pairs that are either set or unset. Names are written as a single identifier such as, for example, unix. Key-value pairs are written as an identifier, =, and then a string. For example, target_arch = "x86_64" is a configuration option.

Note: Whitespace around the = is ignored. foo="bar" and foo = "bar" are equivalent configuration options.

Keys are not unique in the set of key-value configuration options. For example, both feature = "std" and feature = "serde" can be set at the same time.

Which configuration options are set is determined statically during the compilation of the crate. Certain options are compiler-set based on data about the compilation. Other options are arbitrarily-set, set based on input passed to the compiler outside of the code. It is not possible to set a configuration option from within the source code of the crate being compiled.

Note: For rustc, arbitrary-set configuration options are set using the --cfg flag.

Key-value option set once with the target's CPU architecture. The value is similar to the first element of the platform's target triple, but not identical.

Example values:

• "x86"
• "x86_64"
• "mips"
• "powerpc"
• "powerpc64"
• "arm"
• "aarch64"

Key-value option set for each platform feature available for the current compilation target.

Example values:

• "avx"
• "avx2"
• "crt-static"
• "rdrand"
• "sse"
• "sse2"
• "sse4.1"

See the target_feature attribute for more details on the available features. An additional feature of crt-static is available to the target_feature option to indicate that a static C runtime is available.

Key-value option set once with the target's operating system. This value is similar to the second and third element of the platform's target triple.

Example values:

• "windows"
• "macos"
• "ios"
• "linux"
• "android"
• "freebsd"
• "dragonfly"
• "bitrig"
• "openbsd"
• "netbsd"

Key-value option set at most once with the target's operating system value.

Example values:

• "unix"
• "windows"

unix is set if target_family = "unix" is set and windows is set if target_family = "windows" is set.

Key-value option set with further disambiguating information about the target platform with information about the ABI or libc used. For historical reasons, this value is only defined as not the empty-string when actually needed for disambiguation. Thus, for example, on many GNU platforms, this value will be empty. This value is similar to the fourth element of the platform's target triple. One difference is that embedded ABIs such as gnueabihf will simply define target_env as "gnu".

Example values:

• ""
• "gnu"
• "msvc"
• "musl"
• "sgx"

Key-value option set once with either a value of "little" or "big" depending on the endianness of the target's CPU.

Key-value option set once with the target's pointer width in bits. For example, for targets with 32-bit pointers, this is set to "32". Likewise, it is set to "64" for targets with 64-bit pointers.

Key-value option set once with the vendor of the target.

Example values:

• "apple"
• "fortanix"
• "pc"
• "unknown"

Enabled when compiling the test harness. Done with rustc by using the --test flag. See Testing for more on testing support.

Enabled by default when compiling without optimizations. This can be used to enable extra debugging code in development but not in production. For example, it controls the behavior of the standard library's debug_assert! macro.

Set when the crate being compiled is being compiled with the proc_macro crate type.

^Syntax
CfgAttrAttribute :
   cfg ( ConfigurationPredicate )

The cfg attribute conditionally includes the thing it is attached to based on a configuration predicate.

It is written as cfg, (, a configuration predicate, and finally ).

If the predicate is true, the thing is rewritten to not have the cfg attribute on it. If the predicate is false, the thing is removed from the source code.

Some examples on functions:

# #![allow(unused_variables)]
#fn main() {
// The function is only included in the build when compiling for macOS
#[cfg(target_os = "macos")]
fn macos_only() {
  // ...
}

// This function is only included when either foo or bar is defined
#[cfg(any(foo, bar))]
fn needs_foo_or_bar() {
  // ...
}

// This function is only included when compiling for a unixish OS with a 32-bit
// architecture
#[cfg(all(unix, target_pointer_width = "32"))]
fn on_32bit_unix() {
  // ...
}

// This function is only included when foo is not defined
#[cfg(not(foo))]
fn needs_not_foo() {
  // ...
}
#}

The cfg attribute is allowed anywhere attributes are allowed except on generic parameters.

^Syntax
CfgAttrAttribute :
   cfg_attr ( ConfigurationPredicate , CfgAttrs^? )

CfgAttrs :
   Attr (, Attr)^* ,^?

The cfg_attr attribute conditionally includes attributes based on a configuration predicate.

When the configuration predicate is true, this attribute expands out to the attributes listed after the predicate. For example, the following module will either be found at linux.rs or windows.rs based on the target.

#[cfg_attr(linux, path = "linux.rs")]
#[cfg_attr(windows, path = "windows.rs")]
mod os;

Zero, one, or more attributes may be listed. Multiple attributes will each be expanded into separate attributes. For example:

#[cfg_attr(feature = "magic", sparkles, crackles)]
fn bewitched() {}

// When the `magic` feature flag is enabled, the above will expand to:
#[sparkles]
#[crackles]
fn bewitched() {}

Note: The cfg_attr can expand to another cfg_attr. For example, #[cfg_attr(linux, cfg_attr(feature = "multithreaded", some_other_attribute)) is valid. This example would be equivalent to #[cfg_attr(all(linux, feature ="multithreaded"), some_other_attribute)].

The cfg_attr attribute is allowed anywhere attributes are allowed except on generic parameters.

The built-in cfg macro takes in a single configuration predicate and evaluates to the true literal when the predicate is true and the false literal when it is false.

For example:

# #![allow(unused_variables)]
#fn main() {
let machine_kind = if cfg!(unix) {
  "unix"
} else if cfg!(windows) {
  "windows"
} else {
  "unknown"
};

println!("I'm running on a {} machine!", machine_kind);
#}

^Syntax:
Item:
   OuterAttribute^*
      VisItem
   | MacroItem

VisItem:
   Visibility^?
   (
         Module
      | ExternCrate
      | UseDeclaration
      | Function
      | TypeAlias
      | Struct
      | Enumeration
      | Union
      | ConstantItem
      | StaticItem
      | Trait
      | Implementation
      | ExternBlock
   )

MacroItem:
      MacroInvocationSemi
   | MacroRulesDefinition

An item is a component of a crate. Items are organized within a crate by a nested set of modules. Every crate has a single "outermost" anonymous module; all further items within the crate have paths within the module tree of the crate.

Items are entirely determined at compile-time, generally remain fixed during execution, and may reside in read-only memory.

There are several kinds of items:

• modules
• extern crate declarations
• use declarations
• function definitions
• type definitions
• struct definitions
• enumeration definitions
• union definitions
• constant items
• static items
• trait definitions
• implementations
• extern blocks

Some items form an implicit scope for the declaration of sub-items. In other words, within a function or module, declarations of items can (in many cases) be mixed with the statements, control blocks, and similar artifacts that otherwise compose the item body. The meaning of these scoped items is the same as if the item was declared outside the scope — it is still a static item — except that the item's path name within the module namespace is qualified by the name of the enclosing item, or is private to the enclosing item (in the case of functions). The grammar specifies the exact locations in which sub-item declarations may appear.

^Syntax:
Module :
      mod IDENTIFIER ;
   | mod IDENTIFIER {
        InnerAttribute^*
        Item^*
      }

A module is a container for zero or more items.

A module item is a module, surrounded in braces, named, and prefixed with the keyword mod. A module item introduces a new, named module into the tree of modules making up a crate. Modules can nest arbitrarily.

An example of a module:

# #![allow(unused_variables)]
#fn main() {
mod math {
    type Complex = (f64, f64);
    fn sin(f: f64) -> f64 {
        /* ... */
#       unimplemented!();
    }
    fn cos(f: f64) -> f64 {
        /* ... */
#       unimplemented!();
    }
    fn tan(f: f64) -> f64 {
        /* ... */
#       unimplemented!();
    }
}
#}

Modules and types share the same namespace. Declaring a named type with the same name as a module in scope is forbidden: that is, a type definition, trait, struct, enumeration, union, type parameter or crate can't shadow the name of a module in scope, or vice versa. Items brought into scope with use also have this restriction.

A module without a body is loaded from an external file. When the module does not have a path attribute, the path to the file mirrors the logical module path. Ancestor module path components are directories, and the module's contents are in a file with the name of the module plus the .rs extension. For example, the following module structure can have this corresponding filesystem structure:

Module filenames may also be the name of the module as a directory with the contents in a file named mod.rs within that directory. The above example can alternately be expressed with crate::util's contents in a file named util/mod.rs. It is not allowed to have both util.rs and util/mod.rs.

Note: Previous to rustc 1.30, using mod.rs files was the way to load a module with nested children. It is encouraged to use the new naming convention as it is more consistent, and avoids having many files named mod.rs within a project.

The directories and files used for loading external file modules can be influenced with the path attribute.

For path attributes on modules not inside inline module blocks, the file path is relative to the directory the source file is located. For example, the following code snippet would use the paths shown based on where it is located:

#[path = "foo.rs"]
mod c;

For path attributes inside inline module blocks, the relative location of the file path depends on the kind of source file the path attribute is located in. "mod-rs" source files are root modules (such as lib.rs or main.rs) and modules with files named mod.rs. "non-mod-rs" source files are all other module files. Paths for path attributes inside inline module blocks in a mod-rs file are relative to the directory of the mod-rs file including the inline module components as directories. For non-mod-rs files, it is the same except the path starts with a directory with the name of the non-mod-rs module. For example, the following code snippet would use the paths shown based on where it is located:

mod inline {
    #[path = "other.rs"]
    mod inner;
}

An example of combining the above rules of path attributes on inline modules and nested modules within (applies to both mod-rs and non-mod-rs files):

#[path = "thread_files"]
mod thread {
    // Load the `local_data` module from `thread_files/tls.rs` relative to
    // this source file's directory.
    #[path = "tls.rs"]
    mod local_data;
}

Modules implicitly have some names in scope. These name are to built-in types, macros imported with #[macro_use] on an extern crate, and by the crate's prelude. These names are all made of a single identifier. These names are not part of the module, so for example, any name name, self::name is not a valid path. The names added by the prelude can be removed by placing the no_implicit_prelude attribute onto the module.

Modules, like all items, accept outer attributes. They also accept inner attributes: either after { for a module with a body, or at the beginning of the source file, after the optional BOM and shebang.

The built-in attributes that have meaning on a function are cfg, deprecated, doc, the lint check attributes, path, and no_implicit_prelude. Modules also accept macro attributes.

^Syntax:
ExternCrate :
   extern crate CrateRef AsClause^? ;

CrateRef :
   IDENTIFIER | self

AsClause :
   as ( IDENTIFIER | _ )

An extern crate declaration specifies a dependency on an external crate. The external crate is then bound into the declaring scope as the identifier provided in the extern crate declaration. The as clause can be used to bind the imported crate to a different name.

The external crate is resolved to a specific soname at compile time, and a runtime linkage requirement to that soname is passed to the linker for loading at runtime. The soname is resolved at compile time by scanning the compiler's library path and matching the optional crateid provided against the crateid attributes that were declared on the external crate when it was compiled. If no crateid is provided, a default name attribute is assumed, equal to the identifier given in the extern crate declaration.

The self crate may be imported which creates a binding to the current crate. In this case the as clause must be used to specify the name to bind it to.

Three examples of extern crate declarations:

extern crate pcre;

extern crate std; // equivalent to: extern crate std as std;

extern crate std as ruststd; // linking to 'std' under another name

When naming Rust crates, hyphens are disallowed. However, Cargo packages may make use of them. In such case, when Cargo.toml doesn't specify a crate name, Cargo will transparently replace - with _ (Refer to RFC 940 for more details).

Here is an example:

// Importing the Cargo package hello-world
extern crate hello_world; // hyphen replaced with an underscore

External crates imported with extern crate in the root module or provided to the compiler (as with the --extern flag with rustc) are added to the "extern prelude". Crates in the extern prelude are in scope in the entire crate, including inner modules. If imported with extern crate orig_name as new_name, then the symbol new_name is instead added to the prelude.

The core crate is always added to the extern prelude. The std crate is added as long as the no_std attribute is not specified in the crate root.

The no_implicit_prelude attribute can be used on a module to disable prelude lookups within that module.

Edition Differences: In the 2015 edition, crates in the extern prelude cannot be referenced via use declarations, so it is generally standard practice to include extern crate declarations to bring them into scope.

Beginning in the 2018 edition, use declarations can reference crates in the extern prelude, so it is considered unidiomatic to use extern crate.

Note: Additional crates that ship with rustc, such as proc_macro, alloc, and test, are not automatically included with the --extern flag when using Cargo. They must be brought into scope with an extern crate declaration, even in the 2018 edition.

# #![allow(unused_variables)]
#fn main() {
extern crate proc_macro;
use proc_macro::TokenStream;
#}

An external crate dependency can be declared without binding its name in scope by using an underscore with the form extern crate foo as _. This may be useful for crates that only need to be linked, but are never referenced, and will avoid being reported as unused.

The macro_use attribute works as usual and import the macro names into the macro-use prelude.

The no_link attribute may be specified on an extern crate item to prevent linking the crate into the output. This is commonly used to load a crate to access only its macros.

^Syntax:
UseDeclaration :
   use UseTree ;

UseTree :
      (SimplePath^? ::)^? *
   | (SimplePath^? ::)^? { (UseTree ( , UseTree )^* ,^?)^? }
   | SimplePath ( as ( IDENTIFIER | _ ) )^?

A use declaration creates one or more local name bindings synonymous with some other path. Usually a use declaration is used to shorten the path required to refer to a module item. These declarations may appear in modules and blocks, usually at the top.

Use declarations support a number of convenient shortcuts:

• Simultaneously binding a list of paths with a common prefix, using the glob-like brace syntax use a::b::{c, d, e::f, g::h::i};
• Simultaneously binding a list of paths with a common prefix and their common parent module, using the self keyword, such as use a::b::{self, c, d::e};
• Rebinding the target name as a new local name, using the syntax use p::q::r as x;. This can also be used with the last two features: use a::b::{self as ab, c as abc}.
• Binding all paths matching a given prefix, using the asterisk wildcard syntax use a::b::*;.
• Nesting groups of the previous features multiple times, such as use a::b::{self as ab, c, d::{*, e::f}};

An example of use declarations:

use std::option::Option::{Some, None};
use std::collections::hash_map::{self, HashMap};

fn foo<T>(_: T){}
fn bar(map1: HashMap<String, usize>, map2: hash_map::HashMap<String, usize>){}

fn main() {
    // Equivalent to 'foo(vec![std::option::Option::Some(1.0f64),
    // std::option::Option::None]);'
    foo(vec![Some(1.0f64), None]);

    // Both `hash_map` and `HashMap` are in scope.
    let map1 = HashMap::new();
    let map2 = hash_map::HashMap::new();
    bar(map1, map2);
}

Like items, use declarations are private to the containing module, by default. Also like items, a use declaration can be public, if qualified by the pub keyword. Such a use declaration serves to re-export a name. A public use declaration can therefore redirect some public name to a different target definition: even a definition with a private canonical path, inside a different module. If a sequence of such redirections form a cycle or cannot be resolved unambiguously, they represent a compile-time error.

An example of re-exporting:

# fn main() { }
mod quux {
    pub use quux::foo::{bar, baz};

    pub mod foo {
        pub fn bar() { }
        pub fn baz() { }
    }
}

In this example, the module quux re-exports two public names defined in foo.

Paths in use items must start with a crate name or one of the path qualifiers crate, self, super, or ::. crate refers to the current crate. self refers to the current module. super refers to the parent module. :: can be used to explicitly refer to a crate, requiring an extern crate name to follow.

An example of what will and will not work for use items:

# #![allow(unused_imports)]
use std::path::{self, Path, PathBuf};  // good: std is a crate name
use crate::foo::baz::foobaz;    // good: foo is at the root of the crate

mod foo {

    mod example {
        pub mod iter {}
    }

    use crate::foo::example::iter; // good: foo is at crate root
//  use example::iter;      // bad: relative paths are not allowed without `self`
    use self::baz::foobaz;  // good: self refers to module 'foo'
    use crate::foo::bar::foobar;   // good: foo is at crate root

    pub mod bar {
        pub fn foobar() { }
    }

    pub mod baz {
        use super::bar::foobar; // good: super refers to module 'foo'
        pub fn foobaz() { }
    }
}

fn main() {}

Edition Differences: In the 2015 edition, use paths also allow accessing items in the crate root. Using the example above, the following use paths work in 2015 but not 2018:

use foo::example::iter;
use ::foo::baz::foobaz;

The 2015 edition does not allow use declarations to reference the extern prelude. Thus extern crate declarations are still required in 2015 to reference an external crate in a use declaration. Beginning with the 2018 edition, use declarations can specify an external crate dependency the same way extern crate can.

In the 2018 edition, if an in-scope item has the same name as an external crate, then use of that crate name requires a leading :: to unambiguously select the crate name. This is to retain compatibility with potential future changes.

// use std::fs; // Error, this is ambiguous.
use ::std::fs;  // Imports from the `std` crate, not the module below.
use self::std::fs as self_fs;  // Imports the module below.

mod std {
    pub mod fs {}
}
# fn main() {}

Items can be imported without binding to a name by using an underscore with the form use path as _. This is particularly useful to import a trait so that its methods may be used without importing the trait's symbol, for example if the trait's symbol may conflict with another symbol. Another example is to link an external crate without importing its name.

Asterisk glob imports will import items imported with _ in their unnameable form.

mod foo {
    pub trait Zoo {
        fn zoo(&self) {}
    }

    impl<T> Zoo for T {}
}

use self::foo::Zoo as _;
struct Zoo;  // Underscore import avoids name conflict with this item.

fn main() {
    let z = Zoo;
    z.zoo();
}

The unique, unnameable symbols are created after macro expansion so that macros may safely emit multiple references to _ imports. For example, the following should not produce an error:

# #![allow(unused_variables)]
#fn main() {
macro_rules! m {
    ($item: item) => { $item $item }
}

m!(use std as _;);
// This expands to:
// use std as _;
// use std as _;
#}

^Syntax
Function :
   FunctionQualifiers fn IDENTIFIER Generics^?
      ( FunctionParameters^? )
      FunctionReturnType^? WhereClause^?
      BlockExpression

FunctionQualifiers :
   const^? unsafe^? (extern Abi^?)^?

Abi :
   STRING_LITERAL | RAW_STRING_LITERAL

FunctionParameters :
   FunctionParam (, FunctionParam)^* ,^?

FunctionParam :
   Pattern : Type

FunctionReturnType :
   -> Type

A function consists of a block, along with a name and a set of parameters. Other than a name, all these are optional. Functions are declared with the keyword fn. Functions may declare a set of input variables as parameters, through which the caller passes arguments into the function, and the output type of the value the function will return to its caller on completion.

When referred to, a function yields a first-class value of the corresponding zero-sized function item type, which when called evaluates to a direct call to the function.

For example, this is a simple function:

# #![allow(unused_variables)]
#fn main() {
fn answer_to_life_the_universe_and_everything() -> i32 {
    return 42;
}
#}

As with let bindings, function arguments are irrefutable patterns, so any pattern that is valid in a let binding is also valid as an argument:

# #![allow(unused_variables)]
#fn main() {
fn first((value, _): (i32, i32)) -> i32 { value }
#}

The block of a function is conceptually wrapped in a block that binds the argument patterns and then returns the value of the function's block. This means that the tail expression of the block, if evaluated, ends up being returned to the caller. As usual, an explicit return expression within the body of the function will short-cut that implicit return, if reached.

For example, the function above behaves as if it was written as:

// argument_0 is the actual first argument passed from the caller
let (value, _) = argument_0;
return {
    value
};

A generic function allows one or more parameterized types to appear in its signature. Each type parameter must be explicitly declared in an angle-bracket-enclosed and comma-separated list, following the function name.

# #![allow(unused_variables)]
#fn main() {
// foo is generic over A and B

fn foo<A, B>(x: A, y: B) {
# }
#}

Inside the function signature and body, the name of the type parameter can be used as a type name. Trait bounds can be specified for type parameters to allow methods with that trait to be called on values of that type. This is specified using the where syntax:

# #![allow(unused_variables)]
#fn main() {
# use std::fmt::Debug;
fn foo<T>(x: T) where T: Debug {
# }
#}

When a generic function is referenced, its type is instantiated based on the context of the reference. For example, calling the foo function here:

# #![allow(unused_variables)]
#fn main() {
use std::fmt::Debug;

fn foo<T>(x: &[T]) where T: Debug {
    // details elided
}

foo(&[1, 2]);
#}

will instantiate type parameter T with i32.

The type parameters can also be explicitly supplied in a trailing path component after the function name. This might be necessary if there is not sufficient context to determine the type parameters. For example, mem::size_of::<u32>() == 4.

Extern functions are part of Rust's foreign function interface, providing the opposite functionality to external blocks. Whereas external blocks allow Rust code to call foreign code, extern functions with bodies defined in Rust code can be called by foreign code. They are defined in the same way as any other Rust function, except that they have the extern qualifier.

# #![allow(unused_variables)]
#fn main() {
// Declares an extern fn, the ABI defaults to "C"
extern fn new_i32() -> i32 { 0 }

// Declares an extern fn with "stdcall" ABI
# #[cfg(target_arch = "x86_64")]
extern "stdcall" fn new_i32_stdcall() -> i32 { 0 }
#}

Unlike normal functions, extern fns have type extern "ABI" fn(). This is the same type as the functions declared in an extern block.

# #![allow(unused_variables)]
#fn main() {
# extern fn new_i32() -> i32 { 0 }
let fptr: extern "C" fn() -> i32 = new_i32;
#}

As non-Rust calling conventions do not support unwinding, unwinding past the end of an extern function will cause the process to abort. In LLVM, this is implemented by executing an illegal instruction.

Functions qualified with the const keyword are const functions. Const functions can be called from within const contexts. When called from a const context, the function is interpreted by the compiler at compile time. The interpretation happens in the environment of the compilation target and not the host. So usize is 32 bits if you are compiling against a 32 bit system, irrelevant of whether you are building on a 64 bit or a 32 bit system.

If a const function is called outside a const context, it is indistinguishable from any other function. You can freely do anything with a const function that you can do with a regular function.

Const functions have various restrictions to make sure that they can be evaluated at compile-time. It is, for example, not possible to write a random number generator as a const function. Calling a const function at compile-time will always yield the same result as calling it at runtime, even when called multiple times. There's one exception to this rule: if you are doing complex floating point operations in extreme situations, then you might get (very slightly) different results. It is advisable to not make array lengths and enum discriminants depend on floating point computations.

Exhaustive list of permitted structures in const functions:

Note: this list is more restrictive than what you can write in regular constants

• Type parameters where the parameters only have any trait bounds of the following kind:

  • lifetimes
  • Sized or ?Sized

  This means that <T: 'a + ?Sized>, <T: 'b + Sized> and <T> are all permitted.

  This rule also applies to type parameters of impl blocks that contain const methods

• Arithmetic and comparison operators on integers

• All boolean operators except for && and || which are banned since they are short-circuiting.

• Any kind of aggregate constructor (array, struct, enum, tuple, ...)

• Calls to other safe const functions (whether by function call or method call)

• Index expressions on arrays and slices

• Field accesses on structs and tuples

• Reading from constants (but not statics, not even taking a reference to a static)

• & and * (only dereferencing of references, not raw pointers)

• Casts except for raw pointer to integer casts

• unsafe blocks and const unsafe fn are allowed, but the body/block may only do the following unsafe operations:

  • calls to const unsafe functions

Outer attributes are allowed on functions. Inner attributes are allowed directly after the { inside its block.

This example shows an inner attribute on a function. The function will only be available while running tests.

fn test_only() {
    #![test]
}

Note: Except for lints, it is idiomatic to only use outer attributes on function items.

The attributes that have meaning on a function are cfg, deprecated, doc, export_name, link_section, no_mangle, the lint check attributes, must_use, the procedural macro attributes, the testing attributes, and the optimization hint attributes. Functions also accept attributes macros.

^Syntax
TypeAlias :
   type IDENTIFIER Generics^? WhereClause^? = Type ;

A type alias defines a new name for an existing type. Type aliases are declared with the keyword type. Every value has a single, specific type, but may implement several different traits, or be compatible with several different type constraints.

For example, the following defines the type Point as a synonym for the type (u8, u8), the type of pairs of unsigned 8 bit integers:

# #![allow(unused_variables)]
#fn main() {
type Point = (u8, u8);
let p: Point = (41, 68);
#}

A type alias to an enum type cannot be used to qualify the constructors:

# #![allow(unused_variables)]
#fn main() {
enum E { A }
type F = E;
let _: F = E::A;  // OK
// let _: F = F::A;  // Doesn't work
#}

^Syntax
Struct :
      StructStruct
   | TupleStruct

StructStruct :
   struct IDENTIFIER  Generics^? WhereClause^? ( { StructFields^? } | ; )

TupleStruct :
   struct IDENTIFIER  Generics^? ( TupleFields^? ) WhereClause^? ;

StructFields :
   StructField (, StructField)^* ,^?

StructField :
   OuterAttribute^*
   Visibility^?
   IDENTIFIER : Type

TupleFields :
   TupleField (, TupleField)^* ,^?

TupleField :
   OuterAttribute^*
   Visibility^?
   Type

A struct is a nominal struct type defined with the keyword struct.

An example of a struct item and its use:

# #![allow(unused_variables)]
#fn main() {
struct Point {x: i32, y: i32}
let p = Point {x: 10, y: 11};
let px: i32 = p.x;
#}

A tuple struct is a nominal tuple type, also defined with the keyword struct. For example:

# #![allow(unused_variables)]
#fn main() {
struct Point(i32, i32);
let p = Point(10, 11);
let px: i32 = match p { Point(x, _) => x };
#}

A unit-like struct is a struct without any fields, defined by leaving off the list of fields entirely. Such a struct implicitly defines a constant of its type with the same name. For example:

# #![allow(unused_variables)]
#fn main() {
struct Cookie;
let c = [Cookie, Cookie {}, Cookie, Cookie {}];
#}

is equivalent to

# #![allow(unused_variables)]
#fn main() {
struct Cookie {}
const Cookie: Cookie = Cookie {};
let c = [Cookie, Cookie {}, Cookie, Cookie {}];
#}

The precise memory layout of a struct is not specified. One can specify a particular layout using the repr attribute.

^Syntax
Enumeration :
   enum IDENTIFIER  Generics^? WhereClause^? { EnumItems^? }

EnumItems :
   EnumItem ( , EnumItem )^* ,^?

EnumItem :
   OuterAttribute^*
   IDENTIFIER ( EnumItemTuple | EnumItemStruct | EnumItemDiscriminant )^?

EnumItemTuple :
   ( TupleFields^? )

EnumItemStruct :
   { StructFields^? }

EnumItemDiscriminant :
   = Expression

An enumeration, also referred to as enum is a simultaneous definition of a nominal enumerated type as well as a set of constructors, that can be used to create or pattern-match values of the corresponding enumerated type.

Enumerations are declared with the keyword enum.

An example of an enum item and its use:

# #![allow(unused_variables)]
#fn main() {
enum Animal {
    Dog,
    Cat,
}

let mut a: Animal = Animal::Dog;
a = Animal::Cat;
#}

Enum constructors can have either named or unnamed fields:

# #![allow(unused_variables)]
#fn main() {
enum Animal {
    Dog(String, f64),
    Cat { name: String, weight: f64 },
}

let mut a: Animal = Animal::Dog("Cocoa".to_string(), 37.2);
a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 };
#}

In this example, Cat is a struct-like enum variant, whereas Dog is simply called an enum variant. Each enum instance has a discriminant which is an integer associated to it that is used to determine which variant it holds. An opaque reference to this discriminant can be obtained with the mem::discriminant function.

If there is no data attached to any of the variants of an enumeration, then the discriminant can be directly chosen and accessed.

These enumerations can be cast to integer types with the as operator by a numeric cast. The enumeration can optionally specify which integer each discriminant gets by following the variant name with = followed by a constant expression. If the first variant in the declaration is unspecified, then it is set to zero. For every other unspecified discriminant, it is set to one higher than the previous variant in the declaration.

# #![allow(unused_variables)]
#fn main() {
enum Foo {
    Bar,            // 0
    Baz = 123,      // 123
    Quux,           // 124
}

let baz_discriminant = Foo::Baz as u32;
assert_eq!(baz_discriminant, 123);
#}

Under the default representation, the specified discriminant is interpreted as an isize value although the compiler is allowed to use a smaller type in the actual memory layout. The size and thus acceptable values can be changed by using a primitive representation or the C representation.

It is an error when two variants share the same discriminant.

enum SharedDiscriminantError {
    SharedA = 1,
    SharedB = 1
}

enum SharedDiscriminantError2 {
    Zero,       // 0
    One,        // 1
    OneToo = 1  // 1 (collision with previous!)
}

It is also an error to have an unspecified discriminant where the previous discriminant is the maximum value for the size of the discriminant.

#[repr(u8)]
enum OverflowingDiscriminantError {
    Max = 255,
    MaxPlusOne // Would be 256, but that overflows the enum.
}

#[repr(u8)]
enum OverflowingDiscriminantError2 {
    MaxMinusOne = 254, // 254
    Max,               // 255
    MaxPlusOne         // Would be 256, but that overflows the enum.
}

Enums with zero variants are known as zero-variant enums. As they have no valid values, they cannot be instantiated.

# #![allow(unused_variables)]
#fn main() {
enum ZeroVariants {}
#}

^Syntax
Union :
   union IDENTIFIER Generics^? WhereClause^? {StructFields }

A union declaration uses the same syntax as a struct declaration, except with union in place of struct.

# #![allow(unused_variables)]
#fn main() {
#[repr(C)]
union MyUnion {
    f1: u32,
    f2: f32,
}
#}

The key property of unions is that all fields of a union share common storage. As a result writes to one field of a union can overwrite its other fields, and size of a union is determined by the size of its largest field.

A value of a union type can be created using the same syntax that is used for struct types, except that it must specify exactly one field:

# #![allow(unused_variables)]
#fn main() {
# union MyUnion { f1: u32, f2: f32 }
#
let u = MyUnion { f1: 1 };
#}

The expression above creates a value of type MyUnion and initializes the storage using field f1. The union can be accessed using the same syntax as struct fields:

let f = u.f1;

Unions have no notion of an "active field". Instead, every union access just interprets the storage at the type of the field used for the access. Reading a union field reads the bits of the union at the field's type. It is the programmer's responsibility to make sure that the data is valid at that type. Failing to do so results in undefined behavior. For example, reading the value 3 at type bool is undefined behavior. Effectively, writing to and then reading from a union is analogous to a transmute from the type used for writing to the type used for reading.

Consequently, all reads of union fields have to be placed in unsafe blocks:

# #![allow(unused_variables)]
#fn main() {
# union MyUnion { f1: u32, f2: f32 }
# let u = MyUnion { f1: 1 };
#
unsafe {
    let f = u.f1;
}
#}

Writes to Copy union fields do not require reads for running destructors, so these writes don't have to be placed in unsafe blocks

# #![allow(unused_variables)]
#fn main() {
# union MyUnion { f1: u32, f2: f32 }
# let mut u = MyUnion { f1: 1 };
#
u.f1 = 2;
#}

Commonly, code using unions will provide safe wrappers around unsafe union field accesses.

Another way to access union fields is to use pattern matching. Pattern matching on union fields uses the same syntax as struct patterns, except that the pattern must specify exactly one field. Since pattern matching is like reading the union with a particular field, it has to be placed in unsafe blocks as well.

# #![allow(unused_variables)]
#fn main() {
# union MyUnion { f1: u32, f2: f32 }
#
fn f(u: MyUnion) {
    unsafe {
        match u {
            MyUnion { f1: 10 } => { println!("ten"); }
            MyUnion { f2 } => { println!("{}", f2); }
        }
    }
}
#}

Pattern matching may match a union as a field of a larger structure. In particular, when using a Rust union to implement a C tagged union via FFI, this allows matching on the tag and the corresponding field simultaneously:

# #![allow(unused_variables)]
#fn main() {
#[repr(u32)]
enum Tag { I, F }

#[repr(C)]
union U {
    i: i32,
    f: f32,
}

#[repr(C)]
struct Value {
    tag: Tag,
    u: U,
}

fn is_zero(v: Value) -> bool {
    unsafe {
        match v {
            Value { tag: I, u: U { i: 0 } } => true,
            Value { tag: F, u: U { f: 0.0 } } => true,
            _ => false,
        }
    }
}
#}

Since union fields share common storage, gaining write access to one field of a union can give write access to all its remaining fields. Borrow checking rules have to be adjusted to account for this fact. As a result, if one field of a union is borrowed, all its remaining fields are borrowed as well for the same lifetime.

// ERROR: cannot borrow `u` (via `u.f2`) as mutable more than once at a time
fn test() {
    let mut u = MyUnion { f1: 1 };
    unsafe {
        let b1 = &mut u.f1;
                      ---- first mutable borrow occurs here (via `u.f1`)
        let b2 = &mut u.f2;
                      ^^^^ second mutable borrow occurs here (via `u.f2`)
        *b1 = 5;
    }
    - first borrow ends here
    assert_eq!(unsafe { u.f1 }, 5);
}

As you could see, in many aspects (except for layouts, safety and ownership) unions behave exactly like structs, largely as a consequence of inheriting their syntactic shape from structs. This is also true for many unmentioned aspects of Rust language (such as privacy, name resolution, type inference, generics, trait implementations, inherent implementations, coherence, pattern checking, etc etc etc).

^Syntax
ConstantItem :
   const IDENTIFIER : Type = Expression ;

A constant item is a named constant value which is not associated with a specific memory location in the program. Constants are essentially inlined wherever they are used, meaning that they are copied directly into the relevant context when used. References to the same constant are not necessarily guaranteed to refer to the same memory address.

Constants must be explicitly typed. The type must have a 'static lifetime: any references it contains must have 'static lifetimes.

Constants may refer to the address of other constants, in which case the address will have elided lifetimes where applicable, otherwise – in most cases – defaulting to the static lifetime. (See static lifetime elision.) The compiler is, however, still at liberty to translate the constant many times, so the address referred to may not be stable.

# #![allow(unused_variables)]
#fn main() {
const BIT1: u32 = 1 << 0;
const BIT2: u32 = 1 << 1;

const BITS: [u32; 2] = [BIT1, BIT2];
const STRING: &'static str = "bitstring";

struct BitsNStrings<'a> {
    mybits: [u32; 2],
    mystring: &'a str,
}

const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
    mybits: BITS,
    mystring: STRING,
};
#}

Constants can contain destructors. Destructors are run when the value goes out of scope.

# #![allow(unused_variables)]
#fn main() {
struct TypeWithDestructor(i32);

impl Drop for TypeWithDestructor {
    fn drop(&mut self) {
        println!("Dropped. Held {}.", self.0);
    }
}

const ZERO_WITH_DESTRUCTOR: TypeWithDestructor = TypeWithDestructor(0);

fn create_and_drop_zero_with_destructor() {
    let x = ZERO_WITH_DESTRUCTOR;
    // x gets dropped at end of function, calling drop.
    // prints "Dropped. Held 0.".
}
#}

^Syntax
StaticItem :
   static mut^? IDENTIFIER : Type = Expression ;

A static item is similar to a constant, except that it represents a precise memory location in the program. All references to the static refer to the same memory location. Static items have the static lifetime, which outlives all other lifetimes in a Rust program. Non-mut static items that contain a type that is not interior mutable may be placed in read-only memory. Static items do not call drop at the end of the program.

All access to a static is safe, but there are a number of restrictions on statics:

• The type must have the Sync trait bound to allow thread-safe access.
• Statics allow using paths to statics in the constant expression used to initialize them, but statics may not refer to other statics by value, only through a reference.
• Constants cannot refer to statics.

If a static item is declared with the mut keyword, then it is allowed to be modified by the program. One of Rust's goals is to make concurrency bugs hard to run into, and this is obviously a very large source of race conditions or other bugs. For this reason, an unsafe block is required when either reading or writing a mutable static variable. Care should be taken to ensure that modifications to a mutable static are safe with respect to other threads running in the same process.

Mutable statics are still very useful, however. They can be used with C libraries and can also be bound from C libraries in an extern block.

# #![allow(unused_variables)]
#fn main() {
# fn atomic_add(_: &mut u32, _: u32) -> u32 { 2 }

static mut LEVELS: u32 = 0;

// This violates the idea of no shared state, and this doesn't internally
// protect against races, so this function is `unsafe`
unsafe fn bump_levels_unsafe1() -> u32 {
    let ret = LEVELS;
    LEVELS += 1;
    return ret;
}

// Assuming that we have an atomic_add function which returns the old value,
// this function is "safe" but the meaning of the return value may not be what
// callers expect, so it's still marked as `unsafe`
unsafe fn bump_levels_unsafe2() -> u32 {
    return atomic_add(&mut LEVELS, 1);
}
#}

Mutable statics have the same restrictions as normal statics, except that the type does not have to implement the Sync trait.

It can be confusing whether or not you should use a constant item or a static item. Constants should, in general, be preferred over statics unless one of the following are true:

• Large amounts of data are being stored
• The single-address property of statics is required.
• Interior mutability is required.

^Syntax
Trait :
   unsafe^? trait IDENTIFIER  Generics^? ( : TypeParamBounds^? )^? WhereClause^? {
     TraitItem^*
   }

TraitItem :
   OuterAttribute^* (
         TraitFunc
      | TraitMethod
      | TraitConst
      | TraitType
      | MacroInvocationSemi
   )

TraitFunc :
      TraitFunctionDecl ( ; | BlockExpression )

TraitMethod :
      TraitMethodDecl ( ; | BlockExpression )

TraitFunctionDecl :
   FunctionQualifiers fn IDENTIFIER Generics^?
      ( TraitFunctionParameters^? )
      FunctionReturnType^? WhereClause^?

TraitMethodDecl :
   FunctionQualifiers fn IDENTIFIER Generics^?
      ( SelfParam (, TraitFunctionParam)^* ,^? )
      FunctionReturnType^? WhereClause^?

TraitFunctionParameters :
   TraitFunctionParam (, TraitFunctionParam)^* ,^?

TraitFunctionParam^† :
   ( Pattern : )^? Type

TraitConst :
   const IDENTIFIER : Type ( = Expression )^? ;

TraitType :
   type IDENTIFIER ( : TypeParamBounds^? )^? ;

A trait describes an abstract interface that types can implement. This interface consists of associated items, which come in three varieties:

• functions
• types
• constants

All traits define an implicit type parameter Self that refers to "the type that is implementing this interface". Traits may also contain additional type parameters. These type parameters, including Self, may be constrained by other traits and so forth as usual.

Traits are implemented for specific types through separate implementations.

Items associated with a trait do not need to be defined in the trait, but they may be. If the trait provides a definition, then this definition acts as a default for any implementation which does not override it. If it does not, then any implementation must provide a definition.

Generic items may use traits as bounds on their type parameters.

Type parameters can be specified for a trait to make it generic. These appear after the trait name, using the same syntax used in generic functions.

# #![allow(unused_variables)]
#fn main() {
trait Seq<T> {
    fn len(&self) -> u32;
    fn elt_at(&self, n: u32) -> T;
    fn iter<F>(&self, f: F) where F: Fn(T);
}
#}

Object safe traits can be the base trait of a trait object. A trait is object safe if it has the following qualities (defined in RFC 255):

• It must not require Self: Sized
• All associated functions must either have a where Self: Sized bound, or
  • Not have any type parameters (although lifetime parameters are allowed), and
  • Be a method that does not use Self except in the type of the receiver.
• It must not have any associated constants.
• All supertraits must also be object safe.

Supertraits are traits that are required to be implemented for a type to implement a specific trait. Furthermore, anywhere a generic or trait object is bounded by a trait, it has access to the associated items of its supertraits.

Supertraits are declared by trait bounds on the Self type of a trait and transitively the supertraits of the traits declared in those trait bounds. It is an error for a trait to be its own supertrait.

The trait with a supertrait is called a subtrait of its supertrait.

The following is an example of declaring Shape to be a supertrait of Circle.

# #![allow(unused_variables)]
#fn main() {
trait Shape { fn area(&self) -> f64; }
trait Circle : Shape { fn radius(&self) -> f64; }
#}

And the following is the same example, except using where clauses.

# #![allow(unused_variables)]
#fn main() {
trait Shape { fn area(&self) -> f64; }
trait Circle where Self: Shape { fn radius(&self) -> f64; }
#}

This next example gives radius a default implementation using the area function from Shape.

# #![allow(unused_variables)]
#fn main() {
# trait Shape { fn area(&self) -> f64; }
trait Circle where Self: Shape {
    fn radius(&self) -> f64 {
        // A = pi * r^2
        // so algebraically,
        // r = sqrt(A / pi)
        (self.area() /std::f64::consts::PI).sqrt()
    }
}
#}

This next example calls a supertrait method on a generic parameter.

# #![allow(unused_variables)]
#fn main() {
# trait Shape { fn area(&self) -> f64; }
# trait Circle : Shape { fn radius(&self) -> f64; }
fn print_area_and_radius<C: Circle>(c: C) {
    // Here we call the area method from the supertrait `Shape` of `Circle`.
    println!("Area: {}", c.area());
    println!("Radius: {}", c.radius());
}
#}

Similarly, here is an example of calling supertrait methods on trait objects.

# #![allow(unused_variables)]
#fn main() {
# trait Shape { fn area(&self) -> f64; }
# trait Circle : Shape { fn radius(&self) -> f64; }
# struct UnitCircle;
# impl Shape for UnitCircle { fn area(&self) -> f64 { std::f64::consts::PI } }
# impl Circle for UnitCircle { fn radius(&self) -> f64 { 1.0 } }
# let circle = UnitCircle;
let circle = Box::new(circle) as Box<dyn Circle>;
let nonsense = circle.radius() * circle.area();
#}

Traits items that begin with the unsafe keyword indicate that implementing the trait may be unsafe. It is safe to use a correctly implemented unsafe trait. The trait implementation must also begin with the unsafe keyword.

Sync and Send are examples of unsafe traits.

Function or method declarations without a body only allow IDENTIFIER or _ wild card patterns. mut IDENTIFIER is currently allowed, but it is deprecated and will become a hard error in the future.

In the 2015 edition, the pattern for a trait function or method parameter is optional:

# #![allow(unused_variables)]
#fn main() {
trait T {
    fn f(i32);  // Parameter identifiers are not required.
}
#}

The kinds of patterns for parameters is limited to one of the following:

• IDENTIFIER
• mut IDENTIFIER
• _
• & IDENTIFIER
• && IDENTIFIER

Beginning in the 2018 edition, function or method parameter patterns are no longer optional. Also, all irrefutable patterns are allowed as long as there is a body. Without a body, the limitations listed above are still in effect.

# #![allow(unused_variables)]
#fn main() {
trait T {
    fn f1((a, b): (i32, i32)) {}
    fn f2(_: (i32, i32));  // Cannot use tuple pattern without a body.
}
#}

^Syntax
Implementation :
   InherentImpl | TraitImpl

InherentImpl :
   impl Generics^? Type WhereClause^? {
      InnerAttribute^*
      InherentImplItem^*
   }

InherentImplItem :
   OuterAttribute^* (
         MacroInvocationSemi
      | ( Visibility^? ( ConstantItem | Function | Method ) )
   )

TraitImpl :
   unsafe^? impl Generics^? !^? TypePath for Type
   WhereClause^?
   {
      InnerAttribute^*
      TraitImplItem^*
   }

TraitImplItem :
   OuterAttribute^* (
         MacroInvocationSemi
      | ( Visibility^? ( TypeAlias | ConstantItem | Function | Method ) )
   )

An implementation is an item that associates items with an implementing type. Implementations are defined with the keyword impl and contain functions that belong to an instance of the type that is being implemented or to the type statically.

There are two types of implementations:

• inherent implementations
• trait implementations

An inherent implementation is defined as the sequence of the impl keyword, generic type declarations, a path to a nominal type, a where clause, and a bracketed set of associable items.

The nominal type is called the implementing type and the associable items are the associated items to the implementing type.

Inherent implementations associate the contained items to the implementing type. Inherent implementations can contain associated functions (including methods) and associated constants. They cannot contain associated type aliases.

The path to an associated item is any path to the implementing type, followed by the associated item's identifier as the final path component.

A type can also have multiple inherent implementations. An implementing type must be defined within the same crate as the original type definition.

pub mod color {
    pub struct Color(pub u8, pub u8, pub u8);

    impl Color {
        pub const WHITE: Color = Color(255, 255, 255);
    }
}

mod values {
    use super::color::Color;
    impl Color {
        pub fn red() -> Color {
            Color(255, 0, 0)
        }
    }
}

pub use self::color::Color;
fn main() {
    // Actual path to the implementing type and impl in the same module.
    color::Color::WHITE;

    // Impl blocks in different modules are still accessed through a path to the type.
    color::Color::red();

    // Re-exported paths to the implementing type also work.
    Color::red();

    // Does not work, because use in `values` is not pub.
    // values::Color::red();
}

A trait implementation is defined like an inherent implementation except that the optional generic type declarations is followed by a trait followed by the keyword for. Followed by a path to a nominal type.

The trait is known as the implemented trait. The implementing type implements the implemented trait.

A trait implementation must define all non-default associated items declared by the implemented trait, may redefine default associated items defined by the implemented trait, and cannot define any other items.

The path to the associated items is < followed by a path to the implementing type followed by as followed by a path to the trait followed by > as a path component followed by the associated item's path component.

Unsafe traits require the trait implementation to begin with the unsafe keyword.

# #![allow(unused_variables)]
#fn main() {
# #[derive(Copy, Clone)]
# struct Point {x: f64, y: f64};
# type Surface = i32;
# struct BoundingBox {x: f64, y: f64, width: f64, height: f64};
# trait Shape { fn draw(&self, Surface); fn bounding_box(&self) -> BoundingBox; }
# fn do_draw_circle(s: Surface, c: Circle) { }
struct Circle {
    radius: f64,
    center: Point,
}

impl Copy for Circle {}

impl Clone for Circle {
    fn clone(&self) -> Circle { *self }
}

impl Shape for Circle {
    fn draw(&self, s: Surface) { do_draw_circle(s, *self); }
    fn bounding_box(&self) -> BoundingBox {
        let r = self.radius;
        BoundingBox {
            x: self.center.x - r,
            y: self.center.y - r,
            width: 2.0 * r,
            height: 2.0 * r,
        }
    }
}
#}

A trait implementation is considered incoherent if either the orphan check fails or there are overlapping implementation instances.

Two trait implementations overlap when there is a non-empty intersection of the traits the implementation is for, the implementations can be instantiated with the same type.

The Orphan Check states that every trait implementation must meet either of the following conditions:

1. The trait being implemented is defined in the same crate.

2. At least one of either Self or a generic type parameter of the trait must meet the following grammar, where C is a nominal type defined within the containing crate:

    T = C
      | &C
      | &mut C
      | Box<C>
   

An implementation can take type and lifetime parameters, which can be used in the rest of the implementation. Type parameters declared for an implementation must be used at least once in either the trait or the implementing type of an implementation. Implementation parameters are written directly after the impl keyword.

# #![allow(unused_variables)]
#fn main() {
# trait Seq<T> { fn dummy(&self, _: T) { } }
impl<T> Seq<T> for Vec<T> {
    /* ... */
}
impl Seq<bool> for u32 {
    /* Treat the integer as a sequence of bits */
}
#}

Implementations may contain outer attributes before the impl keyword and inner attributes inside the brackets that contain the associated items. Inner attributes must come before any associated items. That attributes that have meaning here are cfg, deprecated, doc, and the lint check attributes.

^Syntax
ExternBlock :
   extern Abi^? {
      InnerAttribute^*
      ExternalItem^*
   }

ExternalItem :
   OuterAttribute^*
   Visibility^?
   ( ExternalStaticItem | ExternalFunctionItem )

ExternalStaticItem :
   static mut^? IDENTIFIER : Type ;

ExternalFunctionItem :
   fn IDENTIFIER Generics^?
   ( ( NamedFunctionParameters | NamedFunctionParametersWithVariadics )^? )
   FunctionReturnType^? WhereClause^? ;

NamedFunctionParameters :
   NamedFunctionParam ( , NamedFunctionParam )^* ,^?

NamedFunctionParam :
   ( IDENTIFIER | _ ) : Type

NamedFunctionParametersWithVariadics :
   ( NamedFunctionParam , )^* NamedFunctionParam , ...

External blocks form the basis for Rust's foreign function interface. Declarations in an external block describe symbols in external, non-Rust libraries.

Functions within external blocks are declared in the same way as other Rust functions, with the exception that they may not have a body and are instead terminated by a semicolon. Patterns are not allowed in parameters, only IDENTIFIER or _ may be used.

Functions within external blocks may be called by Rust code, just like functions defined in Rust. The Rust compiler automatically translates between the Rust ABI and the foreign ABI.

A function declared in an extern block is implicitly unsafe. When coerced to a function pointer, a function declared in an extern block has type unsafe extern "abi" for<'l1, ..., 'lm> fn(A1, ..., An) -> R, where 'l1, ... 'lm are its lifetime parameters, A1, ..., An are the declared types of its parameters and R is the declared return type.

It is unsafe to access a static item declared in an extern block, whether or not it's mutable.

By default external blocks assume that the library they are calling uses the standard C ABI on the specific platform. Other ABIs may be specified using an abi string, as shown here:

// Interface to the Windows API
extern "stdcall" { }

There are three ABI strings which are cross-platform, and which all compilers are guaranteed to support:

• extern "Rust" -- The default ABI when you write a normal fn foo() in any Rust code.
• extern "C" -- This is the same as extern fn foo(); whatever the default your C compiler supports.
• extern "system" -- Usually the same as extern "C", except on Win32, in which case it's "stdcall", or what you should use to link to the Windows API itself

There are also some platform-specific ABI strings:

• extern "cdecl" -- The default for x86_32 C code.
• extern "stdcall" -- The default for the Win32 API on x86_32.
• extern "win64" -- The default for C code on x86_64 Windows.
• extern "sysv64" -- The default for C code on non-Windows x86_64.
• extern "aapcs" -- The default for ARM.
• extern "fastcall" -- The fastcall ABI -- corresponds to MSVC's __fastcall and GCC and clang's __attribute__((fastcall))
• extern "vectorcall" -- The vectorcall ABI -- corresponds to MSVC's __vectorcall and clang's __attribute__((vectorcall))

Finally, there are some rustc-specific ABI strings:

• extern "rust-intrinsic" -- The ABI of rustc intrinsics.
• extern "rust-call" -- The ABI of the Fn::call trait functions.
• extern "platform-intrinsic" -- Specific platform intrinsics -- like, for example, sqrt -- have this ABI. You should never have to deal with it.

Functions within external blocks may be variadic by specifying ... after one or more named arguments in the argument list:

extern {
    fn foo(x: i32, ...);
}

The following attributes control the behavior of external blocks.

The link attribute specifies the name of a native library that the compiler should link with for the items within an extern block. It uses the MetaListNameValueStr syntax to specify its inputs. The name key is the name of the native library to link. The kind key is an optional value which specifies the kind of library with the following possible values:

• dylib — Indicates a dynamic library. This is the default if kind is not specified.
• static — Indicates a static library.
• framework — Indicates a macOS framework. This is only valid for macOS targets.

The name key must be included if kind is specified.

The wasm_import_module key may be used to specify the WebAssembly module name for the items within an extern block when importing symbols from the host environment. The default module name is env if wasm_import_module is not specified.

#[link(name = "crypto")]
extern {
    // …
}

#[link(name = "CoreFoundation", kind = "framework")]
extern {
    // …
}

#[link(wasm_import_module = "foo")]
extern {
    // …
}

It is valid to add the link attribute on an empty extern block. You can use this to satisfy the linking requirements of extern blocks elsewhere in your code (including upstream crates) instead of adding the attribute to each extern block.

The link_name attribute may be specified on declarations inside an extern block to indicate the symbol to import for the given function or static. It uses the MetaNameValueStr syntax to specify the name of the symbol.

extern {
    #[link_name = "actual_symbol_name"]
    fn name_in_rust();
}

^Syntax
Generics :
   < GenericParams >

GenericParams :
      LifetimeParams
   | ( LifetimeParam , )^* TypeParams

LifetimeParams :
   ( LifetimeParam , )^* LifetimeParam^?

LifetimeParam :
   OuterAttribute^? LIFETIME_OR_LABEL ( : LifetimeBounds )^?

TypeParams:
   ( TypeParam , )^* TypeParam^?

TypeParam :
   OuterAttribute^? IDENTIFIER ( : TypeParamBounds^? )^? ( = Type )^?

Functions, type aliases, structs, enumerations, unions, traits and implementations may be parameterized by types and lifetimes. These parameters are listed in angle brackets (<...>), usually immediately after the name of the item and before its definition. For implementations, which don't have a name, they come directly after impl. Lifetime parameters must be declared before type parameters. Some examples of items with type and lifetime parameters:

# #![allow(unused_variables)]
#fn main() {
fn foo<'a, T>() {}
trait A<U> {}
struct Ref<'a, T> where T: 'a { r: &'a T }
#}

References, raw pointers, arrays, slices, tuples and function pointers have lifetime or type parameters as well, but are not referred to with path syntax.

^Syntax
WhereClause :
   where ( WhereClauseItem , )^* WhereClauseItem ^?

WhereClauseItem :
      LifetimeWhereClauseItem
   | TypeBoundWhereClauseItem

LifetimeWhereClauseItem :
   Lifetime : LifetimeBounds

TypeBoundWhereClauseItem :
   ForLifetimes^? Type : TypeParamBounds^?

ForLifetimes :
   for < LifetimeParams >

Where clauses provide another way to specify bounds on type and lifetime parameters as well as a way to specify bounds on types that aren't type parameters.

Bounds that don't use the item's parameters or higher-ranked lifetimes are checked when the item is defined. It is an error for such a bound to be false.

Copy, Clone and Sized bounds are also checked for certain generic types when defining the item. It is an error to have Copy or Cloneas a bound on a mutable reference, trait object or slice or Sized as a bound on a trait object or slice.

struct A<T>
where
    T: Iterator,            // Could use A<T: Iterator> instead
    T::Item: Copy,
    String: PartialEq<T>,
    i32: Default,           // Allowed, but not useful
    i32: Iterator,          // Error: the trait bound is not satisfied
    [T]: Copy,              // Error: the trait bound is not satisfied
{
    f: T,
}

Generic lifetime and type parameters allow attributes on them. There are no built-in attributes that do anything in this position, although custom derive attributes may give meaning to it.

This example shows using a custom derive attribute to modify the meaning of a generic parameter.

// Assume that the derive for MyFlexibleClone declared `my_flexible_clone` as
// an attribute it understands.
#[derive(MyFlexibleClone)] struct Foo<#[my_flexible_clone(unbounded)] H> {
    a: *const H
}

Associated Items are the items declared in traits or defined in implementations. They are called this because they are defined on an associate type — the type in the implementation. They are a subset of the kinds of items you can declare in a module. Specifically, there are associated functions (including methods), associated types, and associated constants.

Associated items are useful when the associated item logically is related to the associating item. For example, the is_some method on Option is intrinsically related to Options, so should be associated.

Every associated item kind comes in two varieties: definitions that contain the actual implementation and declarations that declare signatures for definitions.

It is the declarations that make up the contract of traits and what it available on generic types.

Associated functions are functions associated with a type.

An associated function declaration declares a signature for an associated function definition. It is written as a function item, except the function body is replaced with a ;.

The identifier is the name of the function. The generics, parameter list, return type, and where clause of the associated function must be the same as the associated function declarations's.

An associated function definition defines a function associated with another type. It is written the same as a function item.

An example of a common associated function is a new function that returns a value of the type the associated function is associated with.

struct Struct {
    field: i32
}

impl Struct {
    fn new() -> Struct {
        Struct {
            field: 0i32
        }
    }
}

fn main () {
    let _struct = Struct::new();
}

When the associated function is declared on a trait, the function can also be called with a path that is a path to the trait appended by the name of the trait. When this happens, it is substituted for <_ as Trait>::function_name.

# #![allow(unused_variables)]
#fn main() {
trait Num {
    fn from_i32(n: i32) -> Self;
}

impl Num for f64 {
    fn from_i32(n: i32) -> f64 { n as f64 }
}

// These 4 are all equivalent in this case.
let _: f64 = Num::from_i32(42);
let _: f64 = <_ as Num>::from_i32(42);
let _: f64 = <f64 as Num>::from_i32(42);
let _: f64 = f64::from_i32(42);
#}

Method :
   FunctionQualifiers fn IDENTIFIER Generics^?
      ( SelfParam (, FunctionParam)^* ,^? )
      FunctionReturnType^? WhereClause^?
      BlockExpression

SelfParam :
      (& | & Lifetime)^? mut^? self
   | mut^? self (: Type)^?

Associated functions whose first parameter is named self are called methods and may be invoked using the method call operator, for example, x.foo(), as well as the usual function call notation.

If the type of the self parameter is specified, it is limited to one of the following types:

• Self
• &Self
• &mut Self
• Box<Self>
• Rc<Self>
• Arc<Self>
• Pin<P> where P is one of the above types except Self.

The Self term can be replaced with the type being implemented.

# #![allow(unused_variables)]
#fn main() {
# use std::rc::Rc;
# use std::sync::Arc;
# use std::pin::Pin;
struct Example;
impl Example {
    fn by_value(self: Self) {}
    fn by_ref(self: &Self) {}
    fn by_ref_mut(self: &mut Self) {}
    fn by_box(self: Box<Self>) {}
    fn by_rc(self: Rc<Self>) {}
    fn by_arc(self: Arc<Self>) {}
    fn by_pin(self: Pin<&Self>) {}
    fn explicit_type(self: Arc<Example>) {}
    fn with_lifetime<'a>(self: &'a Self) {}
}
#}

Shorthand syntax can be used without specifying a type, which have the following equivalents:

Note: Lifetimes can be and usually are elided with this shorthand.

If the self parameter is prefixed with mut, it becomes a mutable variable, similar to regular parameters using a mut identifier pattern. For example:

# #![allow(unused_variables)]
#fn main() {
trait Changer: Sized {
    fn change(mut self) {}
    fn modify(mut self: Box<Self>) {}
}
#}

As an example of methods on a trait, consider the following:

# #![allow(unused_variables)]
#fn main() {
# type Surface = i32;
# type BoundingBox = i32;
trait Shape {
    fn draw(&self, surface: Surface);
    fn bounding_box(&self) -> BoundingBox;
}
#}

This defines a trait with two methods. All values that have implementations of this trait while the trait is in scope can have their draw and bounding_box methods called.

# #![allow(unused_variables)]
#fn main() {
# type Surface = i32;
# type BoundingBox = i32;
# trait Shape {
#     fn draw(&self, surface: Surface);
#     fn bounding_box(&self) -> BoundingBox;
# }
#
struct Circle {
    // ...
}

impl Shape for Circle {
    // ...
#   fn draw(&self, _: Surface) {}
#   fn bounding_box(&self) -> BoundingBox { 0i32 }
}

# impl Circle {
#     fn new() -> Circle { Circle{} }
# }
#
let circle_shape = Circle::new();
let bounding_box = circle_shape.bounding_box();
#}

Edition Differences: In the 2015 edition, it is possible to declare trait methods with anonymous parameters (e.g. fn foo(u8)). This is deprecated and an error as of the 2018 edition. All parameters must have an argument name.

Associated types are type aliases associated with another type. Associated types cannot be defined in inherent implementations nor can they be given a default implementation in traits.

An associated type declaration declares a signature for associated type definitions. It is written as type, then an identifier, and finally an optional list of trait bounds.

The identifier is the name of the declared type alias. The optional trait bounds must be fulfilled by the implementations of the type alias.

An associated type definition defines a type alias on another type. It is written as type, then an identifier, then an =, and finally a type.

If a type Item has an associated type Assoc from a trait Trait, then <Item as Trait>::Assoc is a type that is an alias of the type specified in the associated type definition. Furthermore, if Item is a type parameter, then Item::Assoc can be used in type parameters.

trait AssociatedType {
    // Associated type declaration
    type Assoc;
}

struct Struct;

struct OtherStruct;

impl AssociatedType for Struct {
    // Associated type definition
    type Assoc = OtherStruct;
}

impl OtherStruct {
    fn new() -> OtherStruct {
        OtherStruct
    }
}

fn main() {
    // Usage of the associated type to refer to OtherStruct as <Struct as AssociatedType>::Assoc
    let _other_struct: OtherStruct = <Struct as AssociatedType>::Assoc::new();
}

Consider the following example of a Container trait. Notice that the type is available for use in the method signatures:

# #![allow(unused_variables)]
#fn main() {
trait Container {
    type E;
    fn empty() -> Self;
    fn insert(&mut self, elem: Self::E);
}
#}

In order for a type to implement this trait, it must not only provide implementations for every method, but it must specify the type E. Here's an implementation of Container for the standard library type Vec:

# #![allow(unused_variables)]
#fn main() {
# trait Container {
#     type E;
#     fn empty() -> Self;
#     fn insert(&mut self, elem: Self::E);
# }
impl<T> Container for Vec<T> {
    type E = T;
    fn empty() -> Vec<T> { Vec::new() }
    fn insert(&mut self, x: T) { self.push(x); }
}
#}

Associated constants are constants associated with a type.

An associated constant declaration declares a signature for associated constant definitions. It is written as const, then an identifier, then :, then a type, finished by a ;.

The identifier is the name of the constant used in the path. The type is the type that the definition has to implement.

An associated constant definition defines a constant associated with a type. It is written the same as a constant item.

A basic example:

trait ConstantId {
    const ID: i32;
}

struct Struct;

impl ConstantId for Struct {
    const ID: i32 = 1;
}

fn main() {
    assert_eq!(1, Struct::ID);
}

Using default values:

trait ConstantIdDefault {
    const ID: i32 = 1;
}

struct Struct;
struct OtherStruct;

impl ConstantIdDefault for Struct {}

impl ConstantIdDefault for OtherStruct {
    const ID: i32 = 5;
}

fn main() {
    assert_eq!(1, Struct::ID);
    assert_eq!(5, OtherStruct::ID);
}

^Syntax
Visibility :
      pub
   | pub ( crate )
   | pub ( self )
   | pub ( super )
   | pub ( in SimplePath )

These two terms are often used interchangeably, and what they are attempting to convey is the answer to the question "Can this item be used at this location?"

Rust's name resolution operates on a global hierarchy of namespaces. Each level in the hierarchy can be thought of as some item. The items are one of those mentioned above, but also include external crates. Declaring or defining a new module can be thought of as inserting a new tree into the hierarchy at the location of the definition.

To control whether interfaces can be used across modules, Rust checks each use of an item to see whether it should be allowed or not. This is where privacy warnings are generated, or otherwise "you used a private item of another module and weren't allowed to."

By default, everything in Rust is private, with two exceptions: Associated items in a pub Trait are public by default; Enum variants in a pub enum are also public by default. When an item is declared as pub, it can be thought of as being accessible to the outside world. For example:

# fn main() {}
// Declare a private struct
struct Foo;

// Declare a public struct with a private field
pub struct Bar {
    field: i32,
}

// Declare a public enum with two public variants
pub enum State {
    PubliclyAccessibleState,
    PubliclyAccessibleState2,
}

With the notion of an item being either public or private, Rust allows item accesses in two cases:

1. If an item is public, then it can be accessed externally from some module m if you can access all the item's parent modules from m. You can also potentially be able to name the item through re-exports. See below.
2. If an item is private, it may be accessed by the current module and its descendants.

These two cases are surprisingly powerful for creating module hierarchies exposing public APIs while hiding internal implementation details. To help explain, here's a few use cases and what they would entail:

• A library developer needs to expose functionality to crates which link against their library. As a consequence of the first case, this means that anything which is usable externally must be pub from the root down to the destination item. Any private item in the chain will disallow external accesses.

• A crate needs a global available "helper module" to itself, but it doesn't want to expose the helper module as a public API. To accomplish this, the root of the crate's hierarchy would have a private module which then internally has a "public API". Because the entire crate is a descendant of the root, then the entire local crate can access this private module through the second case.

• When writing unit tests for a module, it's often a common idiom to have an immediate child of the module to-be-tested named mod test. This module could access any items of the parent module through the second case, meaning that internal implementation details could also be seamlessly tested from the child module.

In the second case, it mentions that a private item "can be accessed" by the current module and its descendants, but the exact meaning of accessing an item depends on what the item is. Accessing a module, for example, would mean looking inside of it (to import more items). On the other hand, accessing a function would mean that it is invoked. Additionally, path expressions and import statements are considered to access an item in the sense that the import/expression is only valid if the destination is in the current visibility scope.

Here's an example of a program which exemplifies the three cases outlined above:

// This module is private, meaning that no external crate can access this
// module. Because it is private at the root of this current crate, however, any
// module in the crate may access any publicly visible item in this module.
mod crate_helper_module {

    // This function can be used by anything in the current crate
    pub fn crate_helper() {}

    // This function *cannot* be used by anything else in the crate. It is not
    // publicly visible outside of the `crate_helper_module`, so only this
    // current module and its descendants may access it.
    fn implementation_detail() {}
}

// This function is "public to the root" meaning that it's available to external
// crates linking against this one.
pub fn public_api() {}

// Similarly to 'public_api', this module is public so external crates may look
// inside of it.
pub mod submodule {
    use crate_helper_module;

    pub fn my_method() {
        // Any item in the local crate may invoke the helper module's public
        // interface through a combination of the two rules above.
        crate_helper_module::crate_helper();
    }

    // This function is hidden to any module which is not a descendant of
    // `submodule`
    fn my_implementation() {}

    #[cfg(test)]
    mod test {

        #[test]
        fn test_my_implementation() {
            // Because this module is a descendant of `submodule`, it's allowed
            // to access private items inside of `submodule` without a privacy
            // violation.
            super::my_implementation();
        }
    }
}

# fn main() {}

For a Rust program to pass the privacy checking pass, all paths must be valid accesses given the two rules above. This includes all use statements, expressions, types, etc.

In addition to public and private, Rust allows users to declare an item as visible within a given scope. The rules for pub restrictions are as follows:

• pub(in path) makes an item visible within the provided path. path must be a parent module of the item whose visibility is being declared.
• pub(crate) makes an item visible within the current crate.
• pub(super) makes an item visible to the parent module. This is equivalent to pub(in super).
• pub(self) makes an item visible to the current module. This is equivalent to pub(in self).

Edition Differences: Starting with the 2018 edition, paths for pub(in path) must start with crate, self, or super. The 2015 edition may also use paths starting with :: or modules from the crate root.

Here's an example:

pub mod outer_mod {
    pub mod inner_mod {
        // This function is visible within `outer_mod`
        pub(in crate::outer_mod) fn outer_mod_visible_fn() {}
        // Same as above, this is only valid in the 2015 edition.
        pub(in outer_mod) fn outer_mod_visible_fn_2015() {}

        // This function is visible to the entire crate
        pub(crate) fn crate_visible_fn() {}

        // This function is visible within `outer_mod`
        pub(super) fn super_mod_visible_fn() {
            // This function is visible since we're in the same `mod`
            inner_mod_visible_fn();
        }

        // This function is visible
        pub(self) fn inner_mod_visible_fn() {}
    }
    pub fn foo() {
        inner_mod::outer_mod_visible_fn();
        inner_mod::crate_visible_fn();
        inner_mod::super_mod_visible_fn();

        // This function is no longer visible since we're outside of `inner_mod`
        // Error! `inner_mod_visible_fn` is private
        //inner_mod::inner_mod_visible_fn();
    }
}

fn bar() {
    // This function is still visible since we're in the same crate
    outer_mod::inner_mod::crate_visible_fn();

    // This function is no longer visible since we're outside of `outer_mod`
    // Error! `super_mod_visible_fn` is private
    //outer_mod::inner_mod::super_mod_visible_fn();

    // This function is no longer visible since we're outside of `outer_mod`
    // Error! `outer_mod_visible_fn` is private
    //outer_mod::inner_mod::outer_mod_visible_fn();

    outer_mod::foo();
}

fn main() { bar() }

Rust allows publicly re-exporting items through a pub use directive. Because this is a public directive, this allows the item to be used in the current module through the rules above. It essentially allows public access into the re-exported item. For example, this program is valid:

pub use self::implementation::api;

mod implementation {
    pub mod api {
        pub fn f() {}
    }
}

# fn main() {}

This means that any external crate referencing implementation::api::f would receive a privacy violation, while the path api::f would be allowed.

When re-exporting a private item, it can be thought of as allowing the "privacy chain" being short-circuited through the reexport instead of passing through the namespace hierarchy as it normally would.

^Syntax
InnerAttribute :
   # ! [ Attr ]

OuterAttribute :
   # [ Attr ]

Attr :
   SimplePath AttrInput^?

AttrInput :
      DelimTokenTree
   | = LiteralExpression_{without suffix}

An attribute is a general, free-form metadatum that is interpreted according to name, convention, and language and compiler version. Attributes are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334 (C#).

Inner attributes, written with a bang (!) after the hash (#), apply to the item that the attribute is declared within. Outer attributes, written without the bang after the hash, apply to the thing that follows the attribute.

The attribute consists of a path to the attribute, followed by an optional delimited token tree whose interpretation is defined by the attribute. Attributes other than macro attributes also allow the input to be an equals sign (=) followed by a literal expression. See the meta item syntax below for more details.

Attributes can be classified into the following kinds:

• Built-in attributes
• Macro attributes
• Derive macro helper attributes
• Tool attributes

Attributes may be applied to many things in the language:

• All item declarations accept outer attributes while external blocks, functions, implementations, and modules accept inner attributes.
• Most statements accept outer attributes (see Expression Attributes for limitations on expression statements).
• Block expressions accept outer and inner attributes, but only when they are the outer expression of an expression statement or the final expression of another block expression.
• Enum variants and struct and union fields accept outer attributes.
• Match expression arms accept outer attributes.
• Generic lifetime or type parameter accept outer attributes.
• Expressions accept outer attributes in limited situations, see Expression Attributes for details.

Some examples of attributes:

# #![allow(unused_variables)]
#fn main() {
// General metadata applied to the enclosing module or crate.
#![crate_type = "lib"]

// A function marked as a unit test
#[test]
fn test_foo() {
    /* ... */
}

// A conditionally-compiled module
#[cfg(target_os = "linux")]
mod bar {
    /* ... */
}

// A lint attribute used to suppress a warning/error
#[allow(non_camel_case_types)]
type int8_t = i8;

// Inner attribute applies to the entire function.
fn some_unused_variables() {
  #![allow(unused_variables)]

  let x = ();
  let y = ();
  let z = ();
}
#}

A "meta item" is the syntax used for the Attr rule by most built-in attributes and the meta macro fragment specifier. It has the following grammar:

^Syntax
MetaItem :
      SimplePath
   | SimplePath = LiteralExpression_{without suffix}
   | SimplePath ( MetaSeq^? )

MetaSeq :
   MetaItemInner ( , MetaItemInner )^* ,^?

MetaItemInner :
      MetaItem
   | LiteralExpression_{without suffix}

Literal expressions in meta items must not include integer or float type suffixes.

Various built-in attributes use different subsets of the meta item syntax to specify their inputs. The following grammar rules show some commonly used forms:

^Syntax
MetaWord:
   IDENTIFIER

MetaNameValueStr:
   IDENTIFIER = (STRING_LITERAL | RAW_STRING_LITERAL)

MetaListPaths:
   IDENTIFIER ( ( SimplePath (, SimplePath)* ,^? )^? )

MetaListIdents:
   IDENTIFIER ( ( IDENTIFIER (, IDENTIFIER)* ,^? )^? )

MetaListNameValueStr:
   IDENTIFIER ( ( MetaNameValueStr (, MetaNameValueStr)* ,^? )^? )

Some examples of meta items are:

An attribute is either active or inert. During attribute processing, active attributes remove themselves from the thing they are on while inert attributes stay on.

The cfg and cfg_attr attributes are active. The test attribute is inert when compiling for tests and active otherwise. Attribute macros are active. All other attributes are inert.

The compiler may allow attributes for external tools where each tool resides in its own namespace. The first segment of the attribute path is the name of the tool, with one or more additional segments whose interpretation is up to the tool.

When a tool is not in use, the tool's attributes are accepted without a warning. When the tool is in use, the tool is responsible for processing and interpretation of its attributes.

Tool attributes are not available if the no_implicit_prelude attribute is used.

# #![allow(unused_variables)]
#fn main() {
// Tells the rustfmt tool to not format the following element.
#[rustfmt::skip]
struct S {
}

// Controls the "cyclomatic complexity" threshold for the clippy tool.
#[clippy::cyclomatic_complexity = "100"]
pub fn f() {}
#}

Note: rustc currently recognizes the tools "clippy" and "rustfmt".

The following is an index of all built-in attributes.

• Conditional compilation
  • cfg — Controls conditional compilation.
  • cfg_attr — Conditionally includes attributes.
• Testing
  • test — Marks a function as a test.
  • ignore — Disables a test function.
  • should_panic — Indicates a test should generate a panic.
• Derive
  • derive — Automatic trait implementations.
• Macros
  • macro_export — Exports a macro_rules macro for cross-crate usage.
  • macro_use — Expands macro visibility, or imports macros from other crates.
  • proc_macro — Defines a function-like macro.
  • proc_macro_derive — Defines a derive macro.
  • proc_macro_attribute — Defines an attribute macro.
• Diagnostics
  • allow, warn, deny, forbid — Alters the default lint level.
  • deprecated — Generates deprecation notices.
  • must_use — Generates a lint for unused values.
• ABI, linking, symbols, and FFI
  • link — Specifies a native library to link with an extern block.
  • link_name — Specifies the name of the symbol for functions or statics in an extern block.
  • no_link — Prevents linking an extern crate.
  • repr — Controls type layout.
  • crate_type — Specifies the type of crate (library, executable, etc.).
  • no_main — Disables emitting the main symbol.
  • export_name — Specifies the exported symbol name for a function or static.
  • link_section — Specifies the section of an object file to use for a function or static.
  • no_mangle — Disables symbol name encoding.
  • used — Forces the compiler to keep a static item in the output object file.
  • crate_name — Specifies the crate name.
• Code generation
  • inline — Hint to inline code.
  • cold — Hint that a function is unlikely to be called.
  • no_builtins — Disables use of certain built-in functions.
  • target_feature — Configure platform-specific code generation.
• Documentation
  • doc — Specifies documentation. See The Rustdoc Book for more information. Doc comments are transformed into doc attributes.
• Preludes
  • no_std — Removes std from the prelude.
  • no_implicit_prelude — Disables prelude lookups within a module.
• Modules
  • path — Specifies the filename for a module.
• Limits
  • recursion_limit — Sets the maximum recursion limit for certain compile-time operations.
  • type_length_limit — Sets the maximum size of a polymorphic type.
• Runtime
  • panic_handler — Sets the function to handle panics.
  • global_allocator — Sets the global memory allocator.
  • windows_subsystem — Specifies the windows subsystem to link with.
• Features
  • feature — Used to enable unstable or experimental compiler features. See The Unstable Book for features implemented in rustc.

The following attributes are used for specifying functions for performing tests. Compiling a crate in "test" mode enables building the test functions along with a test harness for executing the tests. Enabling the test mode also enables the test conditional compilation option.

The test attribute marks a function to be executed as a test. These functions are only compiled when in test mode. Test functions must be free, monomorphic functions that take no arguments, and the return type must be one of the following:

• ()
• Result<(), E> where E: Error

Note: The implementation of which return types are allowed is determined by the unstable Termination trait.

Note: The test mode is enabled by passing the --test argument to rustc or using cargo test.

Tests that return () pass as long as they terminate and do not panic. Tests that return a Result<(), E> pass as long as they return Ok(()). Tests that do not terminate neither pass nor fail.

# #![allow(unused_variables)]
#fn main() {
# use std::io;
# fn setup_the_thing() -> io::Result<i32> { Ok(1) }
# fn do_the_thing(s: &i32) -> io::Result<()> { Ok(()) }
#[test]
fn test_the_thing() -> io::Result<()> {
    let state = setup_the_thing()?; // expected to succeed
    do_the_thing(&state)?;          // expected to succeed
    Ok(())
}
#}

A function annotated with the test attribute can also be annotated with the ignore attribute. The ignore attribute tells the test harness to not execute that function as a test. It will still be compiled when in test mode.

The ignore attribute may optionally be written with the MetaNameValueStr syntax to specify a reason why the test is ignored.

# #![allow(unused_variables)]
#fn main() {
#[test]
#[ignore = "not yet implemented"]
fn mytest() {
    // …
}
#}

Note: The rustc test harness supports the --include-ignored flag to force ignored tests to be run.

A function annotated with the test attribute that returns () can also be annotated with the should_panic attribute. The should_panic attribute makes the test only pass if it actually panics.

The should_panic attribute may optionally take an input string that must appear within the panic message. If the string is not found in the message, then the test will fail. The string may be passed using the MetaNameValueStr syntax or the MetaListNameValueStr syntax with an expected field.

# #![allow(unused_variables)]
#fn main() {
#[test]
#[should_panic(expected = "values don't match")]
fn mytest() {
    assert_eq!(1, 2, "values don't match");
}
#}

The derive attribute allows new items to be automatically generated for data structures. It uses the MetaListPaths syntax to specify a list of traits to implement or paths to derive macros to process.

For example, the following will create an impl item for the PartialEq and Clone traits for Foo, and the type parameter T will be given the PartialEq or Clone constraints for the appropriate impl:

# #![allow(unused_variables)]
#fn main() {
#[derive(PartialEq, Clone)]
struct Foo<T> {
    a: i32,
    b: T,
}
#}

The generated impl for PartialEq is equivalent to

# #![allow(unused_variables)]
#fn main() {
# struct Foo<T> { a: i32, b: T }
impl<T: PartialEq> PartialEq for Foo<T> {
    fn eq(&self, other: &Foo<T>) -> bool {
        self.a == other.a && self.b == other.b
    }

    fn ne(&self, other: &Foo<T>) -> bool {
        self.a != other.a || self.b != other.b
    }
}
#}

You can implement derive for your own traits through procedural macros.

The following attributes are used for controlling or generating diagnostic messages during compilation.

A lint check names a potentially undesirable coding pattern, such as unreachable code or omitted documentation. The lint attributes allow, warn, deny, and forbid use the MetaListPaths syntax to specify a list of lint names to change the lint level for the entity to which the attribute applies.

For any lint check C:

• allow(C) overrides the check for C so that violations will go unreported,
• warn(C) warns about violations of C but continues compilation.
• deny(C) signals an error after encountering a violation of C,
• forbid(C) is the same as deny(C), but also forbids changing the lint level afterwards,

Note: The lint checks supported by rustc can be found via rustc -W help, along with their default settings and are documented in the rustc book.

# #![allow(unused_variables)]
#fn main() {
pub mod m1 {
    // Missing documentation is ignored here
    #[allow(missing_docs)]
    pub fn undocumented_one() -> i32 { 1 }

    // Missing documentation signals a warning here
    #[warn(missing_docs)]
    pub fn undocumented_too() -> i32 { 2 }

    // Missing documentation signals an error here
    #[deny(missing_docs)]
    pub fn undocumented_end() -> i32 { 3 }
}
#}

This example shows how one can use allow and warn to toggle a particular check on and off:

# #![allow(unused_variables)]
#fn main() {
#[warn(missing_docs)]
pub mod m2{
    #[allow(missing_docs)]
    pub mod nested {
        // Missing documentation is ignored here
        pub fn undocumented_one() -> i32 { 1 }

        // Missing documentation signals a warning here,
        // despite the allow above.
        #[warn(missing_docs)]
        pub fn undocumented_two() -> i32 { 2 }
    }

    // Missing documentation signals a warning here
    pub fn undocumented_too() -> i32 { 3 }
}
#}

This example shows how one can use forbid to disallow uses of allow for that lint check:

# #![allow(unused_variables)]
#fn main() {
#[forbid(missing_docs)]
pub mod m3 {
    // Attempting to toggle warning signals an error here
    #[allow(missing_docs)]
    /// Returns 2.
    pub fn undocumented_too() -> i32 { 2 }
}
#}

Tool lints allows using scoped lints, to allow, warn, deny or forbid lints of certain tools.

Currently clippy is the only available lint tool.

Tool lints only get checked when the associated tool is active. If a lint attribute, such as allow, references a nonexistent tool lint, the compiler will not warn about the nonexistent lint until you use the tool.

Otherwise, they work just like regular lint attributes:

// set the entire `pedantic` clippy lint group to warn
#![warn(clippy::pedantic)]
// silence warnings from the `filter_map` clippy lint
#![allow(clippy::filter_map)]

fn main() {
    // ...
}

// silence the `cmp_nan` clippy lint just for this function
#[allow(clippy::cmp_nan)]
fn foo() {
    // ...
}

The deprecated attribute marks an item as deprecated. rustc will issue warnings on usage of #[deprecated] items. rustdoc will show item deprecation, including the since version and note, if available.

The deprecated attribute has several forms:

• deprecated — Issues a generic message.
• deprecated = "message" — Includes the given string in the deprecation message.
• MetaListNameValueStr syntax with two optional fields:
  • since — Specifies a version number when the item was deprecated. rustc does not currently interpret the string, but external tools like Clippy may check the validity of the value.
  • note — Specifies a string that should be included in the deprecation message. This is typically used to provide an explanation about the deprecation and preferred alternatives.

The deprecated attribute may be applied to any item, trait item, enum variant, struct field, or external block item. It cannot be applied to trait implementation items. When applied to an item containing other items, such as a module or implementation, all child items inherit the deprecation attribute.

Here is an example:

# #![allow(unused_variables)]
#fn main() {
#[deprecated(since = "5.2", note = "foo was rarely used. Users should instead use bar")]
pub fn foo() {}

pub fn bar() {}
#}

The RFC contains motivations and more details.

The must_use attribute is used to issue a diagnostic warning when a value is not "used". It can be applied to user-defined composite types (structs, enums, and unions), functions, and traits.

The must_use attribute may include a message by using the MetaNameValueStr syntax such as #[must_use = "example message"]. The message will be given alongside the warning.

When used on user-defined composite types, if the expression of an expression statement has that type, then the unused_must_use lint is violated.

# #![allow(unused_variables)]
#fn main() {
#[must_use]
struct MustUse {
    // some fields
}

# impl MustUse {
#   fn new() -> MustUse { MustUse {} }
# }
#
// Violates the `unused_must_use` lint.
MustUse::new();
#}

When used on a function, if the expression of an expression statement is a call expression to that function, then the unused_must_use lint is violated.

# #![allow(unused_variables)]
#fn main() {
#[must_use]
fn five() -> i32 { 5i32 }

// Violates the unused_must_use lint.
five();
#}

When used on a trait declaration, a call expression of an expression statement to a function that returns an impl trait of that trait violates the unsued_must_use lint.

# #![allow(unused_variables)]
#fn main() {
#[must_use]
trait Critical {}
impl Critical for i32 {}

fn get_critical() -> impl Critical {
    4i32
}

// Violates the `unused_must_use` lint.
get_critical();
#}

When used on a function in a trait declaration, then the behavior also applies when the call expression is a function from an implementation of the trait.

# #![allow(unused_variables)]
#fn main() {
trait Trait {
    #[must_use]
    fn use_me(&self) -> i32;
}

impl Trait for i32 {
    fn use_me(&self) -> i32 { 0i32 }
}

// Violates the `unused_must_use` lint.
5i32.use_me();
#}

When used on a function in a trait implementation, the attribute does nothing.

Note: Trivial no-op expressions containing the value will not violate the lint. Examples include wrapping the value in a type that does not implement Drop and then not using that type and being the final expression of a block expression that is not used.

# #![allow(unused_variables)]
#fn main() {
#[must_use]
fn five() -> i32 { 5i32 }

// None of these violate the unused_must_use lint.
(five(),);
Some(five());
{ five() };
if true { five() } else { 0i32 };
match true {
    _ => five()
};
#}

Note: It is idiomatic to use a let statement with a pattern of _ when a must-used value is purposely discarded.

# #![allow(unused_variables)]
#fn main() {
#[must_use]
fn five() -> i32 { 5i32 }

// Does not violate the unused_must_use lint.
let _ = five();
#}

The following attributes are used for controlling code generation.

The cold and inline attributes give suggestions to generate code in a way that may be faster than what it would do without the hint. The attributes are only hints, and may be ignored.

Both attributes can be used on functions. When applied to a function in a trait, they apply only to that function when used as a default function for a trait implementation and not to all trait implementations. The attributes have no effect on a trait function without a body.

The inline attribute suggests that a copy of the attributed function should be placed in the caller, rather than generating code to call the function where it is defined.

Note: The rustc compiler automatically inlines functions based on internal heuristics. Incorrectly inlining functions can make the program slower, so this attribute should be used with care.

There are three ways to use the inline attribute:

• #[inline] suggests performing an inline expansion.
• #[inline(always)] suggests that an inline expansion should always be performed.
• #[inline(never)] suggests that an inline expansion should never be performed.

The cold attribute suggests that the attributed function is unlikely to be called.

The no_builtins attribute may be applied at the crate level to disable optimizing certain code patterns to invocations of library functions that are assumed to exist.

The target_feature attribute may be applied to an unsafe function to enable code generation of that function for specific platform architecture features. It uses the MetaListNameValueStr syntax with a single key of enable whose value is a string of comma-separated feature names to enable.

#[target_feature(enable = "avx2")]
unsafe fn foo_avx2() {}

Each target architecture has a set of features that may be enabled. It is an error to specify a feature for a target architecture that the crate is not being compiled for.

It is undefined behavior to call a function that is compiled with a feature that is not supported on the current platform the code is running on.

Functions marked with target_feature are not inlined into a context that does not support the given features. The #[inline(always)] attribute may not be used with a target_feature attribute.

The following is a list of the available feature names.

See the target_feature conditional compilation option for selectively enabling or disabling compilation of code based on compile-time settings. Note that this option is not affected by the target_feature attribute, and is only driven by the features enabled for the entire crate.

See the is_x86_feature_detected macro in the standard library for runtime feature detection on the x86 platforms.

Note: rustc has a default set of features enabled for each target and CPU. The CPU may be chosen with the -C target-cpu flag. Individual features may be enabled or disabled for an entire crate with the -C target-feature flag.

The following attributes affect compile-time limits.

The recursion_limit attribute may be applied at the crate level to set the maximum depth for potentially infinitely-recursive compile-time operations like macro expansion or auto-dereference. It uses the MetaNameValueStr syntax to specify the recursion depth.

Note: The default in rustc is 64.

# #![allow(unused_variables)]
#![recursion_limit = "4"]

#fn main() {
macro_rules! a {
    () => { a!(1) };
    (1) => { a!(2) };
    (2) => { a!(3) };
    (3) => { a!(4) };
    (4) => { };
}

// This fails to expand because it requires a recursion depth greater than 4.
a!{}
#}

# #![allow(unused_variables)]
#![recursion_limit = "1"]

#fn main() {
// This fails because it requires two recursive steps to auto-derefence.
(|_: &u8| {})(&&1);
#}

The type_length_limit attribute limits the maximum number of type substitutions made when constructing a concrete type during monomorphization. It is applied at the crate level, and uses the MetaNameValueStr syntax to set the limit based on the number of type substitutions.

Note: The default in rustc is 1048576.

# #![allow(unused_variables)]
#![type_length_limit = "8"]

#fn main() {
fn f<T>(x: T) {}

// This fails to compile because monomorphizing to
// `f::<(i32, i32, i32, i32, i32, i32, i32, i32, i32)>>` requires more
// than 8 type elements.
f(((1, 2, 3, 4, 5, 6, 7, 8, 9));
#}

Rust is primarily an expression language. This means that most forms of value-producing or effect-causing evaluation are directed by the uniform syntax category of expressions. Each kind of expression can typically nest within each other kind of expression, and rules for evaluation of expressions involve specifying both the value produced by the expression and the order in which its sub-expressions are themselves evaluated.

In contrast, statements in Rust serve mostly to contain and explicitly sequence expression evaluation.

^Syntax
Statement :
      ;
   | Item
   | LetStatement
   | ExpressionStatement
   | MacroInvocationSemi

A statement is a component of a block, which is in turn a component of an outer expression or function.

Rust has two kinds of statement: declaration statements and expression statements.

A declaration statement is one that introduces one or more names into the enclosing statement block. The declared names may denote new variables or new items.

The two kinds of declaration statements are item declarations and let statements.

An item declaration statement has a syntactic form identical to an item declaration within a module. Declaring an item within a statement block restricts its scope to the block containing the statement. The item is not given a canonical path nor are any sub-items it may declare. The exception to this is that associated items defined by implementations are still accessible in outer scopes as long as the item and, if applicable, trait are accessible. It is otherwise identical in meaning to declaring the item inside a module.

There is no implicit capture of the containing function's generic parameters, parameters, and local variables. For example, inner may not access outer_var.

# #![allow(unused_variables)]
#fn main() {
fn outer() {
  let outer_var = true;

  fn inner() { /* outer_var is not in scope here */ }

  inner();
}
#}

^Syntax
LetStatement :
   OuterAttribute^* let Pattern ( : Type )^? (= Expression )^? ;

A let statement introduces a new set of variables, given by a pattern. The pattern is followed optionally by a type annotation and then optionally by an initializer expression. When no type annotation is given, the compiler will infer the type, or signal an error if insufficient type information is available for definite inference. Any variables introduced by a variable declaration are visible from the point of declaration until the end of the enclosing block scope.

^Syntax
ExpressionStatement :
      ExpressionWithoutBlock ;
   | ExpressionWithBlock

An expression statement is one that evaluates an expression and ignores its result. As a rule, an expression statement's purpose is to trigger the effects of evaluating its expression.

An expression that consists of only a block expression or control flow expression, if used in a context where a statement is permitted, can omit the trailing semicolon. This can cause an ambiguity between it being parsed as a standalone statement and as a part of another expression; in this case, it is parsed as a statement. The type of ExpressionWithBlock expressions when used as statements must be the unit type.

# #![allow(unused_variables)]
#fn main() {
# let mut v = vec![1, 2, 3];
v.pop();          // Ignore the element returned from pop
if v.is_empty() {
    v.push(5);
} else {
    v.remove(0);
}                 // Semicolon can be omitted.
[1];              // Separate expression statement, not an indexing expression.
#}

When the trailing semicolon is omitted, the result must be type ().

# #![allow(unused_variables)]
#fn main() {
// bad: the block's type is i32, not ()
// Error: expected `()` because of default return type
// if true {
//   1
// }

// good: the block's type is i32
if true {
  1
} else {
  2
};
#}

Statements accept outer attributes. The attributes that have meaning on a statement are cfg, and the lint check attributes.

^Syntax
Expression :
      ExpressionWithoutBlock
   | ExpressionWithBlock

ExpressionWithoutBlock :
   OuterAttribute^*†
   (
         LiteralExpression
      | PathExpression
      | OperatorExpression
      | GroupedExpression
      | ArrayExpression
      | IndexExpression
      | TupleExpression
      | TupleIndexingExpression
      | StructExpression
      | EnumerationVariantExpression
      | CallExpression
      | MethodCallExpression
      | FieldExpression
      | ClosureExpression
      | ContinueExpression
      | BreakExpression
      | RangeExpression
      | ReturnExpression
      | MacroInvocation
   )

ExpressionWithBlock :
   OuterAttribute^*†
   (
         BlockExpression
      | UnsafeBlockExpression
      | LoopExpression
      | IfExpression
      | IfLetExpression
      | MatchExpression
   )

An expression may have two roles: it always produces a value, and it may have effects (otherwise known as "side effects"). An expression evaluates to a value, and has effects during evaluation. Many expressions contain sub-expressions (operands). The meaning of each kind of expression dictates several things:

• Whether or not to evaluate the sub-expressions when evaluating the expression
• The order in which to evaluate the sub-expressions
• How to combine the sub-expressions' values to obtain the value of the expression

In this way, the structure of expressions dictates the structure of execution. Blocks are just another kind of expression, so blocks, statements, expressions, and blocks again can recursively nest inside each other to an arbitrary depth.

The precedence of Rust operators and expressions is ordered as follows, going from strong to weak. Binary Operators at the same precedence level are grouped in the order given by their associativity.

Expressions are divided into two main categories: place expressions and value expressions. Likewise within each expression, sub-expressions may occur in either place context or value context. The evaluation of an expression depends both on its own category and the context it occurs within.

A place expression is an expression that represents a memory location. These expressions are paths which refer to local variables, static variables, dereferences (*expr), array indexing expressions (expr[expr]), field references (expr.f) and parenthesized place expressions. All other expressions are value expressions.

A value expression is an expression that represents an actual value.

The following contexts are place expression contexts:

• The left operand of an assignment or compound assignment expression.
• The operand of a unary borrow or dereference operator.
• The operand of a field expression.
• The indexed operand of an array indexing expression.
• The operand of any implicit borrow.
• The initializer of a let statement.
• The scrutinee of an if let, match, or while let expression.
• The base of a functional update struct expression.

Note: Historically, place expressions were called lvalues and value expressions were called rvalues.

When a place expression is evaluated in a value expression context, or is bound by value in a pattern, it denotes the value held in that memory location. If the type of that value implements Copy, then the value will be copied. In the remaining situations if that type is Sized, then it may be possible to move the value. Only the following place expressions may be moved out of:

• Variables which are not currently borrowed.
• Temporary values.
• Fields of a place expression which can be moved out of and doesn't implement Drop.
• The result of dereferencing an expression with type Box<T> and that can also be moved out of.

Moving out of a place expression that evaluates to a local variable, the location is deinitialized and cannot be read from again until it is reinitialized. In all other cases, trying to use a place expression in a value expression context is an error.

For a place expression to be assigned to, mutably borrowed, implicitly mutably borrowed, or bound to a pattern containing ref mut it must be mutable. We call these mutable place expressions. In contrast, other place expressions are called immutable place expressions.

The following expressions can be mutable place expression contexts:

• Mutable variables, which are not currently borrowed.
• Mutable static items.
• Temporary values.
• Fields, this evaluates the subexpression in a mutable place expression context.
• Dereferences of a *mut T pointer.
• Dereference of a variable, or field of a variable, with type &mut T. Note: This is an exception to the requirement of the next rule.
• Dereferences of a type that implements DerefMut, this then requires that the value being dereferenced is evaluated is a mutable place expression context.
• Array indexing of a type that implements DerefMut, this then evaluates the value being indexed, but not the index, in mutable place expression context.

When using a value expression in most place expression contexts, a temporary unnamed memory location is created initialized to that value and the expression evaluates to that location instead, except if promoted to 'static. Promotion of a value expression to a 'static slot occurs when the expression could be written in a constant, borrowed, and dereferencing that borrow where the expression was originally written, without changing the runtime behavior. That is, the promoted expression can be evaluated at compile-time and the resulting value does not contain interior mutability or destructors (these properties are determined based on the value where possible, e.g. &None always has the type &'static Option<_>, as it contains nothing disallowed). Otherwise, the lifetime of temporary values is typically

• the innermost enclosing statement; the tail expression of a block is considered part of the statement that encloses the block, or
• the condition expression or the loop conditional expression if the temporary is created in the condition expression of an if or in the loop conditional expression of a while expression.

When a temporary value expression is being created that is assigned into a let declaration, however, the temporary is created with the lifetime of the enclosing block instead, as using the enclosing let declaration would be a guaranteed error (since a pointer to the temporary would be stored into a variable, but the temporary would be freed before the variable could be used). The compiler uses simple syntactic rules to decide which values are being assigned into a let binding, and therefore deserve a longer temporary lifetime.

Here are some examples:

• let x = foo(&temp()). The expression temp() is a value expression. As it is being borrowed, a temporary is created which will be freed after the innermost enclosing statement; in this case, the let declaration.
• let x = temp().foo(). This is the same as the previous example, except that the value of temp() is being borrowed via autoref on a method-call. Here we are assuming that foo() is an &self method defined in some trait, say Foo. In other words, the expression temp().foo() is equivalent to Foo::foo(&temp()).
• let x = if foo(&temp()) {bar()} else {baz()};. The expression temp() is a value expression. As the temporary is created in the condition expression of an if, it will be freed at the end of the condition expression; in this example before the call to bar or baz is made.
• let x = if temp().must_run_bar {bar()} else {baz()};. Here we assume the type of temp() is a struct with a boolean field must_run_bar. As the previous example, the temporary corresponding to temp() will be freed at the end of the condition expression.
• while foo(&temp()) {bar();}. The temporary containing the return value from the call to temp() is created in the loop conditional expression. Hence it will be freed at the end of the loop conditional expression; in this example before the call to bar if the loop body is executed.
• let x = &temp(). Here, the same temporary is being assigned into x, rather than being passed as a parameter, and hence the temporary's lifetime is considered to be the enclosing block.
• let x = SomeStruct { foo: &temp() }. As in the previous case, the temporary is assigned into a struct which is then assigned into a binding, and hence it is given the lifetime of the enclosing block.
• let x = [ &temp() ]. As in the previous case, the temporary is assigned into an array which is then assigned into a binding, and hence it is given the lifetime of the enclosing block.
• let ref x = temp(). In this case, the temporary is created using a ref binding, but the result is the same: the lifetime is extended to the enclosing block.

Certain expressions will treat an expression as a place expression by implicitly borrowing it. For example, it is possible to compare two unsized slices for equality directly, because the == operator implicitly borrows it's operands:

# #![allow(unused_variables)]
#fn main() {
# let c = [1, 2, 3];
# let d = vec![1, 2, 3];
let a: &[i32];
let b: &[i32];
# a = &c;
# b = &d;
// ...
*a == *b;
// Equivalent form:
::std::cmp::PartialEq::eq(&*a, &*b);
#}

Implicit borrows may be taken in the following expressions:

• Left operand in method-call expressions.
• Left operand in field expressions.
• Left operand in call expressions.
• Left operand in array indexing expressions.
• Operand of the dereference operator (*).
• Operands of comparison.
• Left operands of the compound assignment.

Many of the following operators and expressions can also be overloaded for other types using traits in std::ops or std::cmp. These traits also exist in core::ops and core::cmp with the same names.

Outer attributes before an expression are allowed only in a few specific cases:

• Before an expression used as a statement.
• Elements of array expressions, tuple expressions, call expressions, tuple-style struct and enum variant expressions.
• The tail expression of block expressions.

They are never allowed before:

• if and if let expressions.
• Range expressions.
• Binary operator expressions (ArithmeticOrLogicalExpression, ComparisonExpression, LazyBooleanExpression, TypeCastExpression, AssignmentExpression, CompoundAssignmentExpression).

^Syntax
LiteralExpression :
      CHAR_LITERAL
   | STRING_LITERAL
   | RAW_STRING_LITERAL
   | BYTE_LITERAL
   | BYTE_STRING_LITERAL
   | RAW_BYTE_STRING_LITERAL
   | INTEGER_LITERAL
   | FLOAT_LITERAL
   | BOOLEAN_LITERAL

A literal expression consists of one of the literal forms described earlier. It directly describes a number, character, string, or boolean value.

# #![allow(unused_variables)]
#fn main() {
"hello";   // string type
'5';       // character type
5;         // integer type
#}

^Syntax
PathExpression :
      PathInExpression
   | QualifiedPathInExpression

A path used as an expression context denotes either a local variable or an item. Path expressions that resolve to local or static variables are place expressions, other paths are value expressions. Using a static mut variable requires an unsafe block.

# #![allow(unused_variables)]
#fn main() {
# mod globals {
#     pub static STATIC_VAR: i32 = 5;
#     pub static mut STATIC_MUT_VAR: i32 = 7;
# }
# let local_var = 3;
local_var;
globals::STATIC_VAR;
unsafe { globals::STATIC_MUT_VAR };
let some_constructor = Some::<i32>;
let push_integer = Vec::<i32>::push;
let slice_reverse = <[i32]>::reverse;
#}

^Syntax
BlockExpression :
   {
      InnerAttribute^*
      Statements^?
   }

Statements :
      Statement⁺
   | Statement⁺ ExpressionWithoutBlock
   | ExpressionWithoutBlock

A block expression, or block, is a control flow expression and anonymous namespace scope for items and variable declarations. As a control flow expression, a block sequentially executes its component non-item declaration statements and then its final optional expression. As an anonymous namespace scope, item declarations are only in scope inside the block itself and variables declared by let statements are in scope from the next statement until the end of the block.

Blocks are written as {, then any inner attributes, then statements, then an optional expression, and finally a }. Statements are usually required to be followed a semicolon, with two exceptions. Item declaration statements do not need to be followed by a semicolon. Expression statements usually require a following semicolon except if its outer expression is a flow control expression. Furthermore, extra semicolons between statements are allowed, but these semicolons do not affect semantics.

Note: The semicolon following a statement is not a part of the statement itself. They are invalid when using the stmt macro matcher.

When evaluating a block expression, each statement, except for item declaration statements, is executed sequentially. Then the final expression is executed, if given.

The type of a block is the type of the final expression, or () if the final expression is omitted.

# #![allow(unused_variables)]
#fn main() {
# fn fn_call() {}
let _: () = {
    fn_call();
};

let five: i32 = {
    fn_call();
    5
};

assert_eq!(5, five);
#}

Note: As a control flow expression, if a block expression is the outer expression of an expression statement, the expected type is () unless it is followed immediately by a semicolon.

Blocks are always value expressions and evaluate the last expression in value expression context. This can be used to force moving a value if really needed. For example, the following example fails on the call to consume_self because the struct was moved out of s in the block expression.

# #![allow(unused_variables)]
#fn main() {
struct Struct;

impl Struct {
    fn consume_self(self) {}
    fn borrow_self(&self) {}
}

fn move_by_block_expression() {
    let s = Struct;

    // Move the value out of `s` in the block expression.
    (&{ s }).borrow_self();

    // Fails to execute because `s` is moved out of.
    s.consume_self();
}
#}

^Syntax
UnsafeBlockExpression :
   unsafe BlockExpression

See unsafe block for more information on when to use unsafe

A block of code can be prefixed with the unsafe keyword to permit unsafe operations. Examples:

# #![allow(unused_variables)]
#fn main() {
unsafe {
    let b = [13u8, 17u8];
    let a = &b[0] as *const u8;
    assert_eq!(*a, 13);
    assert_eq!(*a.offset(1), 17);
}

# unsafe fn an_unsafe_fn() -> i32 { 10 }
let a = unsafe { an_unsafe_fn() };
#}

Inner attributes are allowed directly after the opening brace of a block expression in the following situations:

• Function and method bodies.
• Loop bodies (loop, while, while let, and for).
• Block expressions used as a statement.
• Block expressions as elements of array expressions, tuple expressions, call expressions, tuple-style struct and enum variant expressions.
• A block expression as the tail expression of another block expression.

The attributes that have meaning on a block expression are cfg and the lint check attributes.

For example, this function returns true on unix platforms and false on other platforms.

# #![allow(unused_variables)]
#fn main() {
fn is_unix_platform() -> bool {
    #[cfg(unix)] { true }
    #[cfg(not(unix))] { false }
}
#}

^Syntax
OperatorExpression :
      BorrowExpression
   | DereferenceExpression
   | ErrorPropagationExpression
   | NegationExpression
   | ArithmeticOrLogicalExpression
   | ComparisonExpression
   | LazyBooleanExpression
   | TypeCastExpression
   | AssignmentExpression
   | CompoundAssignmentExpression

Operators are defined for built in types by the Rust language. Many of the following operators can also be overloaded using traits in std::ops or std::cmp.

Integer operators will panic when they overflow when compiled in debug mode. The -C debug-assertions and -C overflow-checks compiler flags can be used to control this more directly. The following things are considered to be overflow:

• When +, * or - create a value greater than the maximum value, or less than the minimum value that can be stored. This includes unary - on the smallest value of any signed integer type.
• Using / or %, where the left-hand argument is the smallest integer of a signed integer type and the right-hand argument is -1.
• Using << or >> where the right-hand argument is greater than or equal to the number of bits in the type of the left-hand argument, or is negative.

^Syntax
BorrowExpression :
      (&|&&) Expression
   | (&|&&) mut Expression

The & (shared borrow) and &mut (mutable borrow) operators are unary prefix operators. When applied to a place expression, this expressions produces a reference (pointer) to the location that the value refers to. The memory location is also placed into a borrowed state for the duration of the reference. For a shared borrow (&), this implies that the place may not be mutated, but it may be read or shared again. For a mutable borrow (&mut), the place may not be accessed in any way until the borrow expires. &mut evaluates its operand in a mutable place expression context. If the & or &mut operators are applied to a value expression, then a temporary value is created.

These operators cannot be overloaded.

# #![allow(unused_variables)]
#fn main() {
{
    // a temporary with value 7 is created that lasts for this scope.
    let shared_reference = &7;
}
let mut array = [-2, 3, 9];
{
    // Mutably borrows `array` for this scope.
    // `array` may only be used through `mutable_reference`.
    let mutable_reference = &mut array;
}
#}

Even though && is a single token (the lazy 'and' operator), when used in the context of borrow expressions it works as two borrows:

# #![allow(unused_variables)]
#fn main() {
// same meanings:
let a = &&  10;
let a = & & 10;

// same meanings:
let a = &&&&  mut 10;
let a = && && mut 10;
let a = & & & & mut 10;
#}

^Syntax
DereferenceExpression :
   * Expression

The * (dereference) operator is also a unary prefix operator. When applied to a pointer it denotes the pointed-to location. If the expression is of type &mut T and *mut T, and is either a local variable, a (nested) field of a local variable or is a mutable place expression, then the resulting memory location can be assigned to. Dereferencing a raw pointer requires unsafe.

On non-pointer types *x is equivalent to *std::ops::Deref::deref(&x) in an immutable place expression context and *std::ops::DerefMut::deref_mut(&mut x) in a mutable place expression context.

# #![allow(unused_variables)]
#fn main() {
let x = &7;
assert_eq!(*x, 7);
let y = &mut 9;
*y = 11;
assert_eq!(*y, 11);
#}

^Syntax
ErrorPropagationExpression :
   Expression ?

The question mark operator (?) unwraps valid values or returns erroneous values, propagating them to the calling function. It is a unary postfix operator that can only be applied to the types Result<T, E> and Option<T>.

When applied to values of the Result<T, E> type, it propagates errors. If the value is Err(e), then it will return Err(From::from(e)) from the enclosing function or closure. If applied to Ok(x), then it will unwrap the value to evaluate to x.

# #![allow(unused_variables)]
#fn main() {
# use std::num::ParseIntError;
fn try_to_parse() -> Result<i32, ParseIntError> {
    let x: i32 = "123".parse()?; // x = 123
    let y: i32 = "24a".parse()?; // returns an Err() immediately
    Ok(x + y)                    // Doesn't run.
}

let res = try_to_parse();
println!("{:?}", res);
# assert!(res.is_err())
#}

When applied to values of the Option<T> type, it propagates Nones. If the value is None, then it will return None. If applied to Some(x), then it will unwrap the value to evaluate to x.

# #![allow(unused_variables)]
#fn main() {
fn try_option_some() -> Option<u8> {
    let val = Some(1)?;
    Some(val)
}
assert_eq!(try_option_some(), Some(1));

fn try_option_none() -> Option<u8> {
    let val = None?;
    Some(val)
}
assert_eq!(try_option_none(), None);
#}

? cannot be overloaded.

^Syntax
NegationExpression :
      - Expression
   | ! Expression

These are the last two unary operators. This table summarizes the behavior of them on primitive types and which traits are used to overload these operators for other types. Remember that signed integers are always represented using two's complement. The operands of all of these operators are evaluated in value expression context so are moved or copied.

* Only for signed integer types.

Here are some example of these operators

# #![allow(unused_variables)]
#fn main() {
let x = 6;
assert_eq!(-x, -6);
assert_eq!(!x, -7);
assert_eq!(true, !false);
#}

^Syntax
ArithmeticOrLogicalExpression :
      Expression + Expression
   | Expression - Expression
   | Expression * Expression
   | Expression / Expression
   | Expression % Expression
   | Expression & Expression
   | Expression | Expression
   | Expression ^ Expression
   | Expression << Expression
   | Expression >> Expression

Binary operators expressions are all written with infix notation. This table summarizes the behavior of arithmetic and logical binary operators on primitive types and which traits are used to overload these operators for other types. Remember that signed integers are always represented using two's complement. The operands of all of these operators are evaluated in value expression context so are moved or copied.

* Integer division rounds towards zero.

** Arithmetic right shift on signed integer types, logical right shift on unsigned integer types.

Here are examples of these operators being used.

# #![allow(unused_variables)]
#fn main() {
assert_eq!(3 + 6, 9);
assert_eq!(5.5 - 1.25, 4.25);
assert_eq!(-5 * 14, -70);
assert_eq!(14 / 3, 4);
assert_eq!(100 % 7, 2);
assert_eq!(0b1010 & 0b1100, 0b1000);
assert_eq!(0b1010 | 0b1100, 0b1110);
assert_eq!(0b1010 ^ 0b1100, 0b110);
assert_eq!(13 << 3, 104);
assert_eq!(-10 >> 2, -3);
#}

^Syntax
ComparisonExpression :
      Expression == Expression
   | Expression != Expression
   | Expression > Expression
   | Expression < Expression
   | Expression >= Expression
   | Expression <= Expression

Comparison operators are also defined both for primitive types and many type in the standard library. Parentheses are required when chaining comparison operators. For example, the expression a == b == c is invalid and may be written as (a == b) == c.

Unlike arithmetic and logical operators, the traits for overloading the operators the traits for these operators are used more generally to show how a type may be compared and will likely be assumed to define actual comparisons by functions that use these traits as bounds. Many functions and macros in the standard library can then use that assumption (although not to ensure safety). Unlike the arithmetic and logical operators above, these operators implicitly take shared borrows of their operands, evaluating them in place expression context:

a == b;
// is equivalent to
::std::cmp::PartialEq::eq(&a, &b);

This means that the operands don't have to be moved out of.

Here are examples of the comparison operators being used.

# #![allow(unused_variables)]
#fn main() {
assert!(123 == 123);
assert!(23 != -12);
assert!(12.5 > 12.2);
assert!([1, 2, 3] < [1, 3, 4]);
assert!('A' <= 'B');
assert!("World" >= "Hello");
#}

^Syntax
LazyBooleanExpression :
      Expression || Expression
   | Expression && Expression

The operators || and && may be applied to operands of boolean type. The || operator denotes logical 'or', and the && operator denotes logical 'and'. They differ from | and & in that the right-hand operand is only evaluated when the left-hand operand does not already determine the result of the expression. That is, || only evaluates its right-hand operand when the left-hand operand evaluates to false, and && only when it evaluates to true.

# #![allow(unused_variables)]
#fn main() {
let x = false || true; // true
let y = false && panic!(); // false, doesn't evaluate `panic!()`
#}

^Syntax
TypeCastExpression :
   Expression as TypeNoBounds

A type cast expression is denoted with the binary operator as.

Executing an as expression casts the value on the left-hand side to the type on the right-hand side.

An example of an as expression:

# #![allow(unused_variables)]
#fn main() {
# fn sum(values: &[f64]) -> f64 { 0.0 }
# fn len(values: &[f64]) -> i32 { 0 }
fn average(values: &[f64]) -> f64 {
    let sum: f64 = sum(values);
    let size: f64 = len(values) as f64;
    sum / size
}
#}

as can be used to explicitly perform coercions, as well as the following additional casts. Here *T means either *const T or *mut T.