Crate std

The Rust Standard Library

The Rust Standard Library is the foundation of portable Rust software, a set of minimal and battle-tested shared abstractions for the broader Rust ecosystem. It offers core types, like Vec<T> and Option<T>, library-defined operations on language primitives, standard macros, I/O and multithreading, among many other things.

std is available to all Rust crates by default. Therefore, the standard library can be accessed in use statements through the path std, as in use std::env.

How to read this documentation

If you already know the name of what you are looking for, the fastest way to find it is to use the search bar at the top of the page.

Otherwise, you may want to jump to one of these useful sections:

• std::* modules
• Primitive types
• Standard macros
• The Rust Prelude

If this is your first time, the documentation for the standard library is written to be casually perused. Clicking on interesting things should generally lead you to interesting places. Still, there are important bits you don't want to miss, so read on for a tour of the standard library and its documentation!

Once you are familiar with the contents of the standard library you may begin to find the verbosity of the prose distracting. At this stage in your development you may want to press the [-] button near the top of the page to collapse it into a more skimmable view.

While you are looking at that [-] button also notice the [src] button. Rust's API documentation comes with the source code and you are encouraged to read it. The standard library source is generally high quality and a peek behind the curtains is often enlightening.

What is in the standard library documentation?

First of all, The Rust Standard Library is divided into a number of focused modules, all listed further down this page. These modules are the bedrock upon which all of Rust is forged, and they have mighty names like std::slice and std::cmp. Modules' documentation typically includes an overview of the module along with examples, and are a smart place to start familiarizing yourself with the library.

Second, implicit methods on primitive types are documented here. This can be a source of confusion for two reasons:

1. While primitives are implemented by the compiler, the standard library implements methods directly on the primitive types (and it is the only library that does so), which are documented in the section on primitives.
2. The standard library exports many modules with the same name as primitive types. These define additional items related to the primitive type, but not the all-important methods.

So for example there is a page for the primitive type i32 that lists all the methods that can be called on 32-bit integers (very useful), and there is a page for the module std::i32 that documents the constant values MIN and MAX (rarely useful).

Note the documentation for the primitives str and [T] (also called 'slice'). Many method calls on String and Vec<T> are actually calls to methods on str and [T] respectively, via deref coercions.

Third, the standard library defines The Rust Prelude, a small collection of items - mostly traits - that are imported into every module of every crate. The traits in the prelude are pervasive, making the prelude documentation a good entry point to learning about the library.

And finally, the standard library exports a number of standard macros, and lists them on this page (technically, not all of the standard macros are defined by the standard library - some are defined by the compiler - but they are documented here the same). Like the prelude, the standard macros are imported by default into all crates.

Contributing changes to the documentation

Check out the rust contribution guidelines here. The source for this documentation can be found on Github. To contribute changes, make sure you read the guidelines first, then submit pull-requests for your suggested changes.

Contributions are appreciated! If you see a part of the docs that can be improved, submit a PR, or chat with us first on irc.mozilla.org #rust-docs.

A Tour of The Rust Standard Library

The rest of this crate documentation is dedicated to pointing out notable features of The Rust Standard Library.

Containers and collections

The option and result modules define optional and error-handling types, Option<T> and Result<T, E>. The iter module defines Rust's iterator trait, Iterator, which works with the for loop to access collections.

The standard library exposes three common ways to deal with contiguous regions of memory:

• Vec<T> - A heap-allocated vector that is resizable at runtime.
• [T; n] - An inline array with a fixed size at compile time.
• [T] - A dynamically sized slice into any other kind of contiguous storage, whether heap-allocated or not.

Slices can only be handled through some kind of pointer, and as such come in many flavors such as:

• &[T] - shared slice
• &mut [T] - mutable slice
• Box<[T]> - owned slice

str, a UTF-8 string slice, is a primitive type, and the standard library defines many methods for it. Rust strs are typically accessed as immutable references: &str. Use the owned String for building and mutating strings.

For converting to strings use the format! macro, and for converting from strings use the FromStr trait.

Data may be shared by placing it in a reference-counted box or the Rc type, and if further contained in a Cell or RefCell, may be mutated as well as shared. Likewise, in a concurrent setting it is common to pair an atomically-reference-counted box, Arc, with a Mutex to get the same effect.

The collections module defines maps, sets, linked lists and other typical collection types, including the common HashMap<K, V>.

Platform abstractions and I/O

Besides basic data types, the standard library is largely concerned with abstracting over differences in common platforms, most notably Windows and Unix derivatives.

Common types of I/O, including files, TCP, UDP, are defined in the io, fs, and net modules.

The thread module contains Rust's threading abstractions. sync contains further primitive shared memory types, including atomic and mpsc, which contains the channel types for message passing.

Re-exports

pub use core::arch;

Primitive Types

array	A fixed-size array, denoted [T; N], for the element type, T, and the non-negative compile-time constant size, N.
bool	The boolean type.
char	A character type.
f32	The 32-bit floating point type.
f64	The 64-bit floating point type.
fn	Function pointers, like fn(usize) -> bool.
i8	The 8-bit signed integer type.
i16	The 16-bit signed integer type.
i32	The 32-bit signed integer type.
i64	The 64-bit signed integer type.
i128	The 128-bit signed integer type.
isize	The pointer-sized signed integer type.
pointer	Raw, unsafe pointers, *const T, and *mut T.
reference	References, both shared and mutable.
slice	A dynamically-sized view into a contiguous sequence, [T].
str	String slices.
tuple	A finite heterogeneous sequence, (T, U, ..).
u8	The 8-bit unsigned integer type.
u16	The 16-bit unsigned integer type.
u32	The 32-bit unsigned integer type.
u64	The 64-bit unsigned integer type.
u128	The 128-bit unsigned integer type.
unit	The () type, sometimes called "unit" or "nil".
usize	The pointer-sized unsigned integer type.
never	Experimental The ! type, also called "never".

Modules

alloc	Memory allocation APIs
any	This module implements the Any trait, which enables dynamic typing of any 'static type through runtime reflection.
array	Implementations of things like Eq for fixed-length arrays up to a certain length. Eventually we should able to generalize to all lengths.
ascii	Operations on ASCII strings and characters.
borrow	A module for working with borrowed data.
boxed	A pointer type for heap allocation.
cell	Shareable mutable containers.
char	A character type.
clone	The Clone trait for types that cannot be 'implicitly copied'.
cmp	Functionality for ordering and comparison.
collections	Collection types.
convert	Traits for conversions between types.
default	The Default trait for types which may have meaningful default values.
env	Inspection and manipulation of the process's environment.
error	Traits for working with Errors.
f32	This module provides constants which are specific to the implementation of the f32 floating point data type.
f64	This module provides constants which are specific to the implementation of the f64 floating point data type.
ffi	Utilities related to FFI bindings.
fmt	Utilities for formatting and printing Strings.
fs	Filesystem manipulation operations.
future	Asynchronous values.
hash	Generic hashing support.
hint	Hints to compiler that affects how code should be emitted or optimized.
i8	The 8-bit signed integer type.
i16	The 16-bit signed integer type.
i32	The 32-bit signed integer type.
i64	The 64-bit signed integer type.
i128	The 128-bit signed integer type.
io	Traits, helpers, and type definitions for core I/O functionality.
isize	The pointer-sized signed integer type.
iter	Composable external iteration.
marker	Primitive traits and types representing basic properties of types.
mem	Basic functions for dealing with memory.
net	Networking primitives for TCP/UDP communication.
num	Additional functionality for numerics.
ops	Overloadable operators.
option	Optional values.
os	OS-specific functionality.
panic	Panic support in the standard library.
path	Cross-platform path manipulation.
pin	Types that pin data to its location in memory.
prelude	The Rust Prelude.
process	A module for working with processes.
ptr	Manually manage memory through raw pointers.
rc	Single-threaded reference-counting pointers. 'Rc' stands for 'Reference Counted'.
result	Error handling with the Result type.
slice	A dynamically-sized view into a contiguous sequence, [T].
str	Unicode string slices.
string	A UTF-8 encoded, growable string.
sync	Useful synchronization primitives.
task	Types and Traits for working with asynchronous tasks.
thread	Native threads.
time	Temporal quantification.
u8	The 8-bit unsigned integer type.
u16	The 16-bit unsigned integer type.
u32	The 32-bit unsigned integer type.
u64	The 64-bit unsigned integer type.
u128	The 128-bit unsigned integer type.
usize	The pointer-sized unsigned integer type.
vec	A contiguous growable array type with heap-allocated contents, written Vec<T>.
intrinsics	Experimental Compiler intrinsics.
raw	Experimental Contains struct definitions for the layout of compiler built-in types.

Macros

assert	Asserts that a boolean expression is true at runtime.
assert_eq	Asserts that two expressions are equal to each other (using PartialEq).
assert_ne	Asserts that two expressions are not equal to each other (using PartialEq).
cfg	Evaluates boolean combinations of configuration flags at compile-time.
column	Expands to the column number at which it was invoked.
compile_error	Causes compilation to fail with the given error message when encountered.
concat	Concatenates literals into a static string slice.
dbg	Prints and returns the value of a given expression for quick and dirty debugging.
debug_assert	Asserts that a boolean expression is true at runtime.
debug_assert_eq	Asserts that two expressions are equal to each other.
debug_assert_ne	Asserts that two expressions are not equal to each other.
env	Inspects an environment variable at compile time.
eprint	Prints to the standard error.
eprintln	Prints to the standard error, with a newline.
file	Expands to the file name in which it was invoked.
format	Creates a String using interpolation of runtime expressions.
format_args	Constructs parameters for the other string-formatting macros.
include	Parses a file as an expression or an item according to the context.
include_bytes	Includes a file as a reference to a byte array.
include_str	Includes a utf8-encoded file as a string.
is_x86_feature_detected	A macro to test at runtime whether a CPU feature is available on x86/x86-64 platforms.
line	Expands to the line number on which it was invoked.
module_path	Expands to a string that represents the current module path.
option_env	Optionally inspects an environment variable at compile time.
panic	Panics the current thread.
print	Prints to the standard output.
println	Prints to the standard output, with a newline.
stringify	Stringifies its arguments.
thread_local	Declare a new thread local storage key of type std::thread::LocalKey.
try	Unwraps a result or propagates its error.
unimplemented	Indicates unfinished code.
unreachable	Indicates unreachable code.
vec	Creates a Vec containing the arguments.
write	Writes formatted data into a buffer.
writeln	Write formatted data into a buffer, with a newline appended.
concat_idents	Experimental Concatenates identifiers into one identifier.
is_aarch64_feature_detected	Experimental Prevents compilation if is_aarch64_feature_detected is used somewhere else than aarch64 targets.
is_arm_feature_detected	Experimental Prevents compilation if is_arm_feature_detected is used somewhere else than ARM targets.
is_mips64_feature_detected	Experimental Prevents compilation if is_mips64_feature_detected is used somewhere else than MIPS64 targets.
is_mips_feature_detected	Experimental Prevents compilation if is_mips_feature_detected is used somewhere else than MIPS targets.
is_powerpc64_feature_detected	Experimental Prevents compilation if is_powerpc64_feature_detected is used somewhere else than PowerPC64 targets.
is_powerpc_feature_detected	Experimental Prevents compilation if is_powerpc_feature_detected is used somewhere else than PowerPC targets.
todo	Experimental Indicates unfinished code.

Keywords

Self	The implementing type within a trait or impl block, or the current type within a type definition.
as	Cast between types, or rename an import.
async	Experimental Return a Future instead of blocking the current thread.
await	Experimental Suspend execution until the result of a Future is ready.
break	Exit early from a loop.
const	Compile-time constants and deterministic functions.
continue	Skip to the next iteration of a loop.
crate	A Rust binary or library.
dyn	Name the type of a trait object.
else	What to do when an if condition does not hold.
enum	A type that can be any one of several variants.
extern	Link to or import external code.
false	A value of type bool representing logical false.
fn	A function or function pointer.
for	Iteration with in, trait implementation with impl, or higher-ranked trait bounds (for<'a>).
if	Evaluate a block if a condition holds.
impl	Implement some functionality for a type.
in	Iterate over a series of values with for.
let	Bind a value to a variable.
loop	Loop indefinitely.
match	Control flow based on pattern matching.
mod	Organize code into modules.
move	Capture a closure's environment by value.
mut	A mutable binding, reference, or pointer.
pub	Make an item visible to others.
ref	Bind by reference during pattern matching.
return	Return a value from a function.
self	The receiver of a method, or the current module.
static	A place that is valid for the duration of a program.
struct	A type that is composed of other types.
super	The parent of the current module.
trait	A common interface for a class of types.
true	A value of type bool representing logical true.
type	Define an alias for an existing type.
union	The Rust equivalent of a C-style union.
unsafe	Code or interfaces whose memory safety cannot be verified by the type system.
use	Import or rename items from other crates or modules.
where	Add constraints that must be upheld to use an item.
while	Loop while a condition is upheld.