Welcome to the Unstable Book! This book consists of a number of chapters, each one organized by a "feature flag." That is, when using an unstable feature of Rust, you must use a flag, like this:

#![feature(box_syntax)]

fn main() {
    let five = box 5;
}

The box_syntax feature has a chapter describing how to use it.

Because this documentation relates to unstable features, we make no guarantees that what is contained here is accurate or up to date. It's developed on a best-effort basis. Each page will have a link to its tracking issue with the latest developments; you might want to check those as well.

The tracking issue for this feature is: #54192

The rustc flag -Z emit-stack-sizes makes LLVM emit stack size metadata.

NOTE: This LLVM feature only supports the ELF object format as of LLVM 8.0. Using this flag with targets that use other object formats (e.g. macOS and Windows) will result in it being ignored.

Consider this crate:

#![crate_type = "lib"]

use std::ptr;

pub fn foo() {
    // this function doesn't use the stack
}

pub fn bar() {
    let xs = [0u32; 2];

    // force LLVM to allocate `xs` on the stack
    unsafe { ptr::read_volatile(&xs.as_ptr()); }
}

Using the -Z emit-stack-sizes flag produces extra linker sections in the output object file.

$ rustc -C opt-level=3 --emit=obj foo.rs

$ size -A foo.o
foo.o  :
section                                 size   addr
.text                                      0      0
.text._ZN3foo3foo17he211d7b4a3a0c16eE      1      0
.text._ZN3foo3bar17h1acb594305f70c2eE     22      0
.note.GNU-stack                            0      0
.eh_frame                                 72      0
Total                                     95

$ rustc -C opt-level=3 --emit=obj -Z emit-stack-sizes foo.rs

$ size -A foo.o
foo.o  :
section                                 size   addr
.text                                      0      0
.text._ZN3foo3foo17he211d7b4a3a0c16eE      1      0
.stack_sizes                               9      0
.text._ZN3foo3bar17h1acb594305f70c2eE     22      0
.stack_sizes                               9      0
.note.GNU-stack                            0      0
.eh_frame                                 72      0
Total                                    113

As of LLVM 7.0 the data will be written into a section named .stack_sizes and the format is "an array of pairs of function symbol values (pointer size) and stack sizes (unsigned LEB128)".

$ objdump -d foo.o

foo.o:     file format elf64-x86-64

Disassembly of section .text._ZN3foo3foo17he211d7b4a3a0c16eE:

0000000000000000 <_ZN3foo3foo17he211d7b4a3a0c16eE>:
   0:   c3                      retq

Disassembly of section .text._ZN3foo3bar17h1acb594305f70c2eE:

0000000000000000 <_ZN3foo3bar17h1acb594305f70c2eE>:
   0:   48 83 ec 10             sub    $0x10,%rsp
   4:   48 8d 44 24 08          lea    0x8(%rsp),%rax
   9:   48 89 04 24             mov    %rax,(%rsp)
   d:   48 8b 04 24             mov    (%rsp),%rax
  11:   48 83 c4 10             add    $0x10,%rsp
  15:   c3                      retq

$ objdump -s -j .stack_sizes foo.o

foo.o:     file format elf64-x86-64

Contents of section .stack_sizes:
 0000 00000000 00000000 00                 .........
Contents of section .stack_sizes:
 0000 00000000 00000000 10                 .........

It's important to note that linkers will discard this linker section by default. To preserve the section you can use a linker script like the one shown below.

/* file: keep-stack-sizes.x */
SECTIONS
{
  /* `INFO` makes the section not allocatable so it won't be loaded into memory */
  .stack_sizes (INFO) :
  {
    KEEP(*(.stack_sizes));
  }
}

The linker script must be passed to the linker using a rustc flag like -C link-arg.

// file: src/main.rs
use std::ptr;

#[inline(never)]
fn main() {
    let xs = [0u32; 2];

    // force LLVM to allocate `xs` on the stack
    unsafe { ptr::read_volatile(&xs.as_ptr()); }
}

$ RUSTFLAGS="-Z emit-stack-sizes" cargo build --release

$ size -A target/release/hello | grep stack_sizes || echo section was not found
section was not found

$ RUSTFLAGS="-Z emit-stack-sizes" cargo rustc --release -- \
    -C link-arg=-Wl,-Tkeep-stack-sizes.x \
    -C link-arg=-N

$ size -A target/release/hello | grep stack_sizes
.stack_sizes                               90   176272

$ # non-allocatable section (flags don't contain the "A" (alloc) flag)
$ readelf -S target/release/hello
Section Headers:
  [Nr]   Name              Type             Address           Offset
       Size              EntSize            Flags  Link  Info  Align
(..)
  [1031] .stack_sizes      PROGBITS         000000000002b090  0002b0f0
       000000000000005a  0000000000000000   L       5     0     1

$ objdump -s -j .stack_sizes target/release/hello

target/release/hello:     file format elf64-x86-64

Contents of section .stack_sizes:
 2b090 c0040000 00000000 08f00400 00000000  ................
 2b0a0 00080005 00000000 00000810 05000000  ................
 2b0b0 00000000 20050000 00000000 10400500  .... ........@..
 2b0c0 00000000 00087005 00000000 00000080  ......p.........
 2b0d0 05000000 00000000 90050000 00000000  ................
 2b0e0 00a00500 00000000 0000               ..........

Author note: I'm not entirely sure why, in this case, -N is required in addition to -Tkeep-stack-sizes.x. For example, it's not required when producing statically linked files for the ARM Cortex-M architecture.

The tracking issue for this feature is: #42524.

This feature allows the generation of code coverage reports.

Set the -Zprofile compiler flag in order to enable gcov profiling.

For example:

cargo new testgcov --bin
cd testgcov
export RUSTFLAGS="-Zprofile"
cargo build
cargo run

Once you've built and run your program, files with the gcno (after build) and gcda (after execution) extensions will be created. You can parse them with llvm-cov gcov or grcov.

The tracking issue for this feature is: #44839

The tracking issue for this feature is: #51575

The tracking issue for this feature is: #38487

In the MSP430 architecture, interrupt handlers have a special calling convention. You can use the "msp430-interrupt" ABI to make the compiler apply the right calling convention to the interrupt handlers you define.

#![feature(abi_msp430_interrupt)]
#![no_std]

// Place the interrupt handler at the appropriate memory address
// (Alternatively, you can use `#[used]` and remove `pub` and `#[no_mangle]`)
#[link_section = "__interrupt_vector_10"]
#[no_mangle]
pub static TIM0_VECTOR: extern "msp430-interrupt" fn() = tim0;

// The interrupt handler
extern "msp430-interrupt" fn tim0() {
    // ..
}

$ msp430-elf-objdump -CD ./target/msp430/release/app
Disassembly of section __interrupt_vector_10:

0000fff2 <TIM0_VECTOR>:
    fff2:       00 c0           interrupt service routine at 0xc000

Disassembly of section .text:

0000c000 <int::tim0>:
    c000:       00 13           reti

The tracking issue for this feature is: #38788

When emitting PTX code, all vanilla Rust functions (fn) get translated to "device" functions. These functions are not callable from the host via the CUDA API so a crate with only device functions is not too useful!

OTOH, "global" functions can be called by the host; you can think of them as the real public API of your crate. To produce a global function use the "ptx-kernel" ABI.

#![feature(abi_ptx)]
#![no_std]

pub unsafe extern "ptx-kernel" fn global_function() {
    device_function();
}

pub fn device_function() {
    // ..
}

$ xargo rustc --target nvptx64-nvidia-cuda --release -- --emit=asm

$ cat $(find -name '*.s')
//
// Generated by LLVM NVPTX Back-End
//

.version 3.2
.target sm_20
.address_size 64

        // .globl       _ZN6kernel15global_function17h46111ebe6516b382E

.visible .entry _ZN6kernel15global_function17h46111ebe6516b382E()
{


        ret;
}

        // .globl       _ZN6kernel15device_function17hd6a0e4993bbf3f78E
.visible .func _ZN6kernel15device_function17hd6a0e4993bbf3f78E()
{


        ret;
}

The tracking issue for this feature is: #42202

The MSVC ABI on x86 Windows uses the thiscall calling convention for C++ instance methods by default; it is identical to the usual (C) calling convention on x86 Windows except that the first parameter of the method, the this pointer, is passed in the ECX register.

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #40180

The tracking issue for this feature is: #44839

The tracking issue for this feature is: #51540

This feature does not have a tracking issue, it is an unstable implementation detail of the global_allocator feature not intended for use outside the compiler.

The tracking issue for this feature is: #46488

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #44874

The tracking issue for this feature is: #44839

The tracking issue for this feature is: #29722

For extremely low-level manipulations and performance reasons, one might wish to control the CPU directly. Rust supports using inline assembly to do this via the asm! macro.

asm!(assembly template
   : output operands
   : input operands
   : clobbers
   : options
   );

Any use of asm is feature gated (requires #![feature(asm)] on the crate to allow) and of course requires an unsafe block.

Note: the examples here are given in x86/x86-64 assembly, but all platforms are supported.

The assembly template is the only required parameter and must be a literal string (i.e. "")

#![feature(asm)]

#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
fn foo() {
    unsafe {
        asm!("NOP");
    }
}

// Other platforms:
#[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
fn foo() { /* ... */ }

fn main() {
    // ...
    foo();
    // ...
}

(The feature(asm) and #[cfg]s are omitted from now on.)

Output operands, input operands, clobbers and options are all optional but you must add the right number of : if you skip them:

# #![feature(asm)]
# #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
# fn main() { unsafe {
asm!("xor %eax, %eax"
    :
    :
    : "eax"
   );
# } }
# #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
# fn main() {}

Whitespace also doesn't matter:

# #![feature(asm)]
# #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
# fn main() { unsafe {
asm!("xor %eax, %eax" ::: "eax");
# } }
# #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
# fn main() {}

Input and output operands follow the same format: : "constraints1"(expr1), "constraints2"(expr2), ...". Output operand expressions must be mutable lvalues, or not yet assigned:

# #![feature(asm)]
# #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
fn add(a: i32, b: i32) -> i32 {
    let c: i32;
    unsafe {
        asm!("add $2, $0"
             : "=r"(c)
             : "0"(a), "r"(b)
             );
    }
    c
}
# #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
# fn add(a: i32, b: i32) -> i32 { a + b }

fn main() {
    assert_eq!(add(3, 14159), 14162)
}

If you would like to use real operands in this position, however, you are required to put curly braces {} around the register that you want, and you are required to put the specific size of the operand. This is useful for very low level programming, where which register you use is important:


# #![allow(unused_variables)]
#fn main() {
# #![feature(asm)]
# #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
# unsafe fn read_byte_in(port: u16) -> u8 {
let result: u8;
asm!("in %dx, %al" : "={al}"(result) : "{dx}"(port));
result
# }
#}

Some instructions modify registers which might otherwise have held different values so we use the clobbers list to indicate to the compiler not to assume any values loaded into those registers will stay valid.

# #![feature(asm)]
# #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
# fn main() { unsafe {
// Put the value 0x200 in eax:
asm!("mov $$0x200, %eax" : /* no outputs */ : /* no inputs */ : "eax");
# } }
# #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
# fn main() {}

Input and output registers need not be listed since that information is already communicated by the given constraints. Otherwise, any other registers used either implicitly or explicitly should be listed.

If the assembly changes the condition code register cc should be specified as one of the clobbers. Similarly, if the assembly modifies memory, memory should also be specified.

The last section, options is specific to Rust. The format is comma separated literal strings (i.e. :"foo", "bar", "baz"). It's used to specify some extra info about the inline assembly:

Current valid options are:

volatile - specifying this is analogous to __asm__ __volatile__ (...) in gcc/clang.
alignstack - certain instructions expect the stack to be aligned a certain way (i.e. SSE) and specifying this indicates to the compiler to insert its usual stack alignment code
intel - use intel syntax instead of the default AT&T.

# #![feature(asm)]
# #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
# fn main() {
let result: i32;
unsafe {
   asm!("mov eax, 2" : "={eax}"(result) : : : "intel")
}
println!("eax is currently {}", result);
# }
# #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
# fn main() {}

The current implementation of the asm! macro is a direct binding to LLVM's inline assembler expressions, so be sure to check out their documentation as well for more information about clobbers, constraints, etc.

If you need more power and don't mind losing some of the niceties of asm!, check out global_asm.

The tracking issue for this feature is: #29661

The tracking issue for this feature is: #50547

The tracking issue for this feature is: #44839

The tracking issue for this feature is: #50547

The tracking issue for this feature is: #15287

The tracking issue for this feature is: #29641

`cfg_target_thread_local`

The tracking issue for this feature is: #29594

The tracking issue for this feature is: #44839

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #29599

The concat_idents feature adds a macro for concatenating multiple identifiers into one identifier.

#![feature(concat_idents)]

fn main() {
    fn foobar() -> u32 { 23 }
    let f = concat_idents!(foo, bar);
    assert_eq!(f(), 23);
}

The tracking issue for this feature is: #53020

The tracking issue for this feature is: #57563

The const_fn feature allows marking free functions and inherent methods as const, enabling them to be called in constants contexts, with constant arguments.

#![feature(const_fn)]

const fn double(x: i32) -> i32 {
    x * 2
}

const FIVE: i32 = 5;
const TEN: i32 = double(FIVE);

fn main() {
    assert_eq!(5, FIVE);
    assert_eq!(10, TEN);
}

The tracking issue for this feature is: #51909

The tracking issue for this feature is: #44580

The tracking issue for this feature is: #51999

The tracking issue for this feature is: #51911

The tracking issue for this feature is: #51910

The tracking issue for this feature is: #53605

The tracking issue for this feature is: #53120

The crate_visibility_modifier feature allows the crate keyword to be used as a visibility modifier synonymous to pub(crate), indicating that a type (function, &c.) is to be visible to the entire enclosing crate, but not to other crates.


# #![allow(unused_variables)]
#![feature(crate_visibility_modifier)]

#fn main() {
crate struct Foo {
    bar: usize,
}
#}

The tracking issue for this feature is: #29642

The tracking issue for this feature is: #54726

The tracking issue for this feature is: #50297

The custom_test_frameworks feature allows the use of #[test_case] and #![test_runner]. Any function, const, or static can be annotated with #[test_case] causing it to be aggregated (like #[test]) and be passed to the test runner determined by the #![test_runner] crate attribute.


# #![allow(unused_variables)]
#![feature(custom_test_frameworks)]
#![test_runner(my_runner)]

#fn main() {
fn my_runner(tests: &[&i32]) {
    for t in tests {
        if **t == 0 {
            println!("PASSED");
        } else {
            println!("FAILED");
        }
    }
}

#[test_case]
const WILL_PASS: i32 = 0;

#[test_case]
const WILL_FAIL: i32 = 4;
#}

The tracking issue for this feature is: #39412

The tracking issue for this feature is: #27336

The tracking issue for this feature is: #50146

You can add alias(es) to an item when using the rustdoc search through the doc(alias) attribute. Example:


# #![allow(unused_variables)]
#![feature(doc_alias)]

#fn main() {
#[doc(alias = "x")]
#[doc(alias = "big")]
pub struct BigX;
#}

Then, when looking for it through the rustdoc search, if you enter "x" or "big", search will show the BigX struct first.

Note that this feature is currently hidden behind the feature(doc_alias) gate.

The tracking issue for this feature is: #43781

The doc_cfg feature allows an API be documented as only available in some specific platforms. This attribute has two effects:

In the annotated item's documentation, there will be a message saying "This is supported on (platform) only".
The item's doc-tests will only run on the specific platform.

In addition to allowing the use of the #[doc(cfg)] attribute, this feature enables the use of a special conditional compilation flag, #[cfg(rustdoc)], set whenever building documentation on your crate.

This feature was introduced as part of PR #43348 to allow the platform-specific parts of the standard library be documented.


# #![allow(unused_variables)]
#![feature(doc_cfg)]

#fn main() {
#[cfg(any(windows, rustdoc))]
#[doc(cfg(windows))]
/// The application's icon in the notification area (a.k.a. system tray).
///
/// # Examples
///
/// ```no_run
/// extern crate my_awesome_ui_library;
/// use my_awesome_ui_library::current_app;
/// use my_awesome_ui_library::windows::notification;
///
/// let icon = current_app().get::<notification::Icon>();
/// icon.show();
/// icon.show_message("Hello");
/// ```
pub struct Icon {
    // ...
}
#}

The tracking issue for this feature is: #51315

The tracking issue for this feature is: #44027

The doc_masked feature allows a crate to exclude types from a given crate from appearing in lists of trait implementations. The specifics of the feature are as follows:

When rustdoc encounters an extern crate statement annotated with a #[doc(masked)] attribute, it marks the crate as being masked.
When listing traits a given type implements, rustdoc ensures that traits from masked crates are not emitted into the documentation.
When listing types that implement a given trait, rustdoc ensures that types from masked crates are not emitted into the documentation.

This feature was introduced in PR #44026 to ensure that compiler-internal and implementation-specific types and traits were not included in the standard library's documentation. Such types would introduce broken links into the documentation.

The tracking issue for this feature is: #45040

The doc_spotlight feature allows the use of the spotlight parameter to the #[doc] attribute, to "spotlight" a specific trait on the return values of functions. Adding a #[doc(spotlight)] attribute to a trait definition will make rustdoc print extra information for functions which return a type that implements that trait. This attribute is applied to the Iterator, io::Read, and io::Write traits in the standard library.

You can do this on your own traits, like this:

#![feature(doc_spotlight)]

#[doc(spotlight)]
pub trait MyTrait {}

pub struct MyStruct;
impl MyTrait for MyStruct {}

/// The docs for this function will have an extra line about `MyStruct` implementing `MyTrait`,
/// without having to write that yourself!
pub fn my_fn() -> MyStruct { MyStruct }

This feature was originally implemented in PR #45039.

The tracking issue for this feature is: #34761

The tracking issue for this feature is: #28498

The tracking issue for this feature is: #37854

The tracking issue for this feature is: #51085

The tracking issue for this feature is: #34511

The tracking issue for this feature is: #43467

The tracking issue for this feature is: #44732

The external_doc feature allows the use of the include parameter to the #[doc] attribute, to include external files in documentation. Use the attribute in place of, or in addition to, regular doc comments and #[doc] attributes, and rustdoc will load the given file when it renders documentation for your crate.

With the following files in the same directory:

external-doc.md:

# My Awesome Type

This is the documentation for this spectacular type.

lib.rs:

#![feature(external_doc)]

#[doc(include = "external-doc.md")]
pub struct MyAwesomeType;

rustdoc will load the file external-doc.md and use it as the documentation for the MyAwesomeType struct.

When locating files, rustdoc will base paths in the src/ directory, as if they were alongside the lib.rs for your crate. So if you want a docs/ folder to live alongside the src/ directory, start your paths with ../docs/ for rustdoc to properly find the file.

This feature was proposed in RFC #1990 and initially implemented in PR #44781.

The tracking issue for this feature is: #44839

The tracking issue for this feature is: #58314

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #29635

The tracking issue for this feature is: #43122

The generators feature gate in Rust allows you to define generator or coroutine literals. A generator is a "resumable function" that syntactically resembles a closure but compiles to much different semantics in the compiler itself. The primary feature of a generator is that it can be suspended during execution to be resumed at a later date. Generators use the yield keyword to "return", and then the caller can resume a generator to resume execution just after the yield keyword.

Generators are an extra-unstable feature in the compiler right now. Added in RFC 2033 they're mostly intended right now as a information/constraint gathering phase. The intent is that experimentation can happen on the nightly compiler before actual stabilization. A further RFC will be required to stabilize generators/coroutines and will likely contain at least a few small tweaks to the overall design.

A syntactical example of a generator is:

#![feature(generators, generator_trait)]

use std::ops::{Generator, GeneratorState};
use std::pin::Pin;

fn main() {
    let mut generator = || {
        yield 1;
        return "foo"
    };

    match Pin::new(&mut generator).resume() {
        GeneratorState::Yielded(1) => {}
        _ => panic!("unexpected value from resume"),
    }
    match Pin::new(&mut generator).resume() {
        GeneratorState::Complete("foo") => {}
        _ => panic!("unexpected value from resume"),
    }
}

Generators are closure-like literals which can contain a yield statement. The yield statement takes an optional expression of a value to yield out of the generator. All generator literals implement the Generator trait in the std::ops module. The Generator trait has one main method, resume, which resumes execution of the generator at the previous suspension point.

An example of the control flow of generators is that the following example prints all numbers in order:

#![feature(generators, generator_trait)]

use std::ops::Generator;
use std::pin::Pin;

fn main() {
    let mut generator = || {
        println!("2");
        yield;
        println!("4");
    };

    println!("1");
    Pin::new(&mut generator).resume();
    println!("3");
    Pin::new(&mut generator).resume();
    println!("5");
}

At this time the main intended use case of generators is an implementation primitive for async/await syntax, but generators will likely be extended to ergonomic implementations of iterators and other primitives in the future. Feedback on the design and usage is always appreciated!

The Generator trait in std::ops currently looks like:

# #![feature(arbitrary_self_types, generator_trait)]
# use std::ops::GeneratorState;
# use std::pin::Pin;

pub trait Generator {
    type Yield;
    type Return;
    fn resume(self: Pin<&mut Self>) -> GeneratorState<Self::Yield, Self::Return>;
}

The Generator::Yield type is the type of values that can be yielded with the yield statement. The Generator::Return type is the returned type of the generator. This is typically the last expression in a generator's definition or any value passed to return in a generator. The resume function is the entry point for executing the Generator itself.

The return value of resume, GeneratorState, looks like:

pub enum GeneratorState<Y, R> {
    Yielded(Y),
    Complete(R),
}

The Yielded variant indicates that the generator can later be resumed. This corresponds to a yield point in a generator. The Complete variant indicates that the generator is complete and cannot be resumed again. Calling resume after a generator has returned Complete will likely result in a panic of the program.

The closure-like syntax for generators alludes to the fact that they also have closure-like semantics. Namely:

When created, a generator executes no code. A closure literal does not actually execute any of the closure's code on construction, and similarly a generator literal does not execute any code inside the generator when constructed.
Generators can capture outer variables by reference or by move, and this can be tweaked with the move keyword at the beginning of the closure. Like closures all generators will have an implicit environment which is inferred by the compiler. Outer variables can be moved into a generator for use as the generator progresses.
Generator literals produce a value with a unique type which implements the std::ops::Generator trait. This allows actual execution of the generator through the Generator::resume method as well as also naming it in return types and such.
Traits like Send and Sync are automatically implemented for a Generator depending on the captured variables of the environment. Unlike closures, generators also depend on variables live across suspension points. This means that although the ambient environment may be Send or Sync, the generator itself may not be due to internal variables live across yield points being not-Send or not-Sync. Note that generators, like closures, do not implement traits like Copy or Clone automatically.
Whenever a generator is dropped it will drop all captured environment variables.

Note that unlike closures, generators at this time cannot take any arguments. That is, generators must always look like || { ... }. This restriction may be lifted at a future date, the design is ongoing!

In the compiler, generators are currently compiled as state machines. Each yield expression will correspond to a different state that stores all live variables over that suspension point. Resumption of a generator will dispatch on the current state and then execute internally until a yield is reached, at which point all state is saved off in the generator and a value is returned.

Let's take a look at an example to see what's going on here:

#![feature(generators, generator_trait)]

use std::ops::Generator;
use std::pin::Pin;

fn main() {
    let ret = "foo";
    let mut generator = move || {
        yield 1;
        return ret
    };

    Pin::new(&mut generator).resume();
    Pin::new(&mut generator).resume();
}

This generator literal will compile down to something similar to:

#![feature(arbitrary_self_types, generators, generator_trait)]

use std::ops::{Generator, GeneratorState};
use std::pin::Pin;

fn main() {
    let ret = "foo";
    let mut generator = {
        enum __Generator {
            Start(&'static str),
            Yield1(&'static str),
            Done,
        }

        impl Generator for __Generator {
            type Yield = i32;
            type Return = &'static str;

            fn resume(mut self: Pin<&mut Self>) -> GeneratorState<i32, &'static str> {
                use std::mem;
                match mem::replace(&mut *self, __Generator::Done) {
                    __Generator::Start(s) => {
                        *self = __Generator::Yield1(s);
                        GeneratorState::Yielded(1)
                    }

                    __Generator::Yield1(s) => {
                        *self = __Generator::Done;
                        GeneratorState::Complete(s)
                    }

                    __Generator::Done => {
                        panic!("generator resumed after completion")
                    }
                }
            }
        }

        __Generator::Start(ret)
    };

    Pin::new(&mut generator).resume();
    Pin::new(&mut generator).resume();
}

Notably here we can see that the compiler is generating a fresh type, __Generator in this case. This type has a number of states (represented here as an enum) corresponding to each of the conceptual states of the generator. At the beginning we're closing over our outer variable foo and then that variable is also live over the yield point, so it's stored in both states.

When the generator starts it'll immediately yield 1, but it saves off its state just before it does so indicating that it has reached the yield point. Upon resuming again we'll execute the return ret which returns the Complete state.

Here we can also note that the Done state, if resumed, panics immediately as it's invalid to resume a completed generator. It's also worth noting that this is just a rough desugaring, not a normative specification for what the compiler does.

The tracking issue for this feature is: #44265

The tracking issue for this feature is: #35119

The global_asm! macro allows the programmer to write arbitrary assembly outside the scope of a function body, passing it through rustc and llvm to the assembler. The macro is a no-frills interface to LLVM's concept of module-level inline assembly. That is, all caveats applicable to LLVM's module-level inline assembly apply to global_asm!.

global_asm! fills a role not currently satisfied by either asm! or #[naked] functions. The programmer has all features of the assembler at their disposal. The linker will expect to resolve any symbols defined in the inline assembly, modulo any symbols marked as external. It also means syntax for directives and assembly follow the conventions of the assembler in your toolchain.

A simple usage looks like this:

# #![feature(global_asm)]
# you also need relevant target_arch cfgs
global_asm!(include_str!("something_neato.s"));

And a more complicated usage looks like this:

# #![feature(global_asm)]
# #![cfg(any(target_arch = "x86", target_arch = "x86_64"))]

pub mod sally {
    global_asm!(r#"
        .global foo
      foo:
        jmp baz
    "#);

    #[no_mangle]
    pub unsafe extern "C" fn baz() {}
}

// the symbols `foo` and `bar` are global, no matter where
// `global_asm!` was used.
extern "C" {
    fn foo();
    fn bar();
}

pub mod harry {
    global_asm!(r#"
        .global bar
      bar:
        jmp quux
    "#);

    #[no_mangle]
    pub unsafe extern "C" fn quux() {}
}

You may use global_asm! multiple times, anywhere in your crate, in whatever way suits you. The effect is as if you concatenated all usages and placed the larger, single usage in the crate root.

If you don't need quite as much power and flexibility as global_asm! provides, and you don't mind restricting your inline assembly to fn bodies only, you might try the asm feature instead.

The tracking issue for this feature is: #44839

The tracking issue for this feature is: #34511

The impl_trait_in_bindings feature gate lets you use impl Trait syntax in let, static, and const bindings.

A simple example is:

#![feature(impl_trait_in_bindings)]

use std::fmt::Debug;

fn main() {
    let a: impl Debug + Clone = 42;
    let b = a.clone();
    println!("{:?}", b); // prints `42`
}

Note however that because the types of a and b are opaque in the above example, calling inherent methods or methods outside of the specified traits (e.g., a.abs() or b.abs()) is not allowed, and yields an error.

The tracking issue for this feature is: #44524

The tracking issue for this feature is: #44493

The infer_static_outlives_requirements feature indicates that certain 'static outlives requirements can be inferred by the compiler rather than stating them explicitly.

Note: It is an accompanying feature to infer_outlives_requirements, which must be enabled to infer outlives requirements.

For example, currently generic struct definitions that contain references, require where-clauses of the form T: 'static. By using this feature the outlives predicates will be inferred, although they may still be written explicitly.

struct Foo<U> where U: 'static { // <-- currently required
    bar: Bar<U>
}
struct Bar<T: 'static> {
    x: T,
}

#![feature(infer_outlives_requirements)]
#![feature(infer_static_outlives_requirements)]

#[rustc_outlives]
// Implicitly infer U: 'static
struct Foo<U> {
    bar: Bar<U>
}
struct Bar<T: 'static> {
    x: T,
}

The tracking issue for this feature is: None.

Intrinsics are never intended to be stable directly, but intrinsics are often exported in some sort of stable manner. Prefer using the stable interfaces to the intrinsic directly when you can.

These are imported as if they were FFI functions, with the special rust-intrinsic ABI. For example, if one was in a freestanding context, but wished to be able to transmute between types, and perform efficient pointer arithmetic, one would import those functions via a declaration like

#![feature(intrinsics)]
# fn main() {}

extern "rust-intrinsic" {
    fn transmute<T, U>(x: T) -> U;

    fn offset<T>(dst: *const T, offset: isize) -> *const T;
}

As with any other FFI functions, these are always unsafe to call.

The tracking issue for this feature is: #48594

The tracking issue for this feature is: None.

The rustc compiler has certain pluggable operations, that is, functionality that isn't hard-coded into the language, but is implemented in libraries, with a special marker to tell the compiler it exists. The marker is the attribute #[lang = "..."] and there are various different values of ..., i.e. various different 'lang items'.

For example, Box pointers require two lang items, one for allocation and one for deallocation. A freestanding program that uses the Box sugar for dynamic allocations via malloc and free:

#![feature(lang_items, box_syntax, start, libc, core_intrinsics)]
#![no_std]
use core::intrinsics;
use core::panic::PanicInfo;

extern crate libc;

#[lang = "owned_box"]
pub struct Box<T>(*mut T);

#[lang = "exchange_malloc"]
unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
    let p = libc::malloc(size as libc::size_t) as *mut u8;

    // Check if `malloc` failed:
    if p as usize == 0 {
        intrinsics::abort();
    }

    p
}

#[lang = "box_free"]
unsafe fn box_free<T: ?Sized>(ptr: *mut T) {
    libc::free(ptr as *mut libc::c_void)
}

#[start]
fn main(_argc: isize, _argv: *const *const u8) -> isize {
    let _x = box 1;

    0
}

#[lang = "eh_personality"] extern fn rust_eh_personality() {}
#[lang = "panic_impl"] extern fn rust_begin_panic(info: &PanicInfo) -> ! { unsafe { intrinsics::abort() } }
#[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
#[no_mangle] pub extern fn rust_eh_register_frames () {}
#[no_mangle] pub extern fn rust_eh_unregister_frames () {}

Note the use of abort: the exchange_malloc lang item is assumed to return a valid pointer, and so needs to do the check internally.

Other features provided by lang items include:

overloadable operators via traits: the traits corresponding to the ==, <, dereferencing (*) and + (etc.) operators are all marked with lang items; those specific four are eq, ord, deref, and add respectively.
stack unwinding and general failure; the eh_personality, eh_unwind_resume, fail and fail_bounds_checks lang items.
the traits in std::marker used to indicate types of various kinds; lang items send, sync and copy.
the marker types and variance indicators found in std::marker; lang items covariant_type, contravariant_lifetime, etc.

Lang items are loaded lazily by the compiler; e.g. if one never uses Box then there is no need to define functions for exchange_malloc and box_free. rustc will emit an error when an item is needed but not found in the current crate or any that it depends on.

Most lang items are defined by libcore, but if you're trying to build an executable without the standard library, you'll run into the need for lang items. The rest of this page focuses on this use-case, even though lang items are a bit broader than that.

In order to build a #[no_std] executable we will need libc as a dependency. We can specify this using our Cargo.toml file:

[dependencies]
libc = { version = "0.2.14", default-features = false }

Note that the default features have been disabled. This is a critical step - the default features of libc include the standard library and so must be disabled.

Controlling the entry point is possible in two ways: the #[start] attribute, or overriding the default shim for the C main function with your own.

The function marked #[start] is passed the command line parameters in the same format as C:

#![feature(lang_items, core_intrinsics)]
#![feature(start)]
#![no_std]
use core::intrinsics;
use core::panic::PanicInfo;

// Pull in the system libc library for what crt0.o likely requires.
extern crate libc;

// Entry point for this program.
#[start]
fn start(_argc: isize, _argv: *const *const u8) -> isize {
    0
}

// These functions are used by the compiler, but not
// for a bare-bones hello world. These are normally
// provided by libstd.
#[lang = "eh_personality"]
#[no_mangle]
pub extern fn rust_eh_personality() {
}

// This function may be needed based on the compilation target.
#[lang = "eh_unwind_resume"]
#[no_mangle]
pub extern fn rust_eh_unwind_resume() {
}

#[lang = "panic_impl"]
#[no_mangle]
pub extern fn rust_begin_panic(info: &PanicInfo) -> ! {
    unsafe { intrinsics::abort() }
}

To override the compiler-inserted main shim, one has to disable it with #![no_main] and then create the appropriate symbol with the correct ABI and the correct name, which requires overriding the compiler's name mangling too:

#![feature(lang_items, core_intrinsics)]
#![feature(start)]
#![no_std]
#![no_main]
use core::intrinsics;
use core::panic::PanicInfo;

// Pull in the system libc library for what crt0.o likely requires.
extern crate libc;

// Entry point for this program.
#[no_mangle] // ensure that this symbol is called `main` in the output
pub extern fn main(_argc: i32, _argv: *const *const u8) -> i32 {
    0
}

// These functions are used by the compiler, but not
// for a bare-bones hello world. These are normally
// provided by libstd.
#[lang = "eh_personality"]
#[no_mangle]
pub extern fn rust_eh_personality() {
}

// This function may be needed based on the compilation target.
#[lang = "eh_unwind_resume"]
#[no_mangle]
pub extern fn rust_eh_unwind_resume() {
}

#[lang = "panic_impl"]
#[no_mangle]
pub extern fn rust_begin_panic(info: &PanicInfo) -> ! {
    unsafe { intrinsics::abort() }
}

In many cases, you may need to manually link to the compiler_builtins crate when building a no_std binary. You may observe this via linker error messages such as "undefined reference to `__rust_probestack'". Using this crate also requires enabling the library feature compiler_builtins_lib. You can read more about this here.

The compiler currently makes a few assumptions about symbols which are available in the executable to call. Normally these functions are provided by the standard library, but without it you must define your own. These symbols are called "language items", and they each have an internal name, and then a signature that an implementation must conform to.

The first of these functions, rust_eh_personality, is used by the failure mechanisms of the compiler. This is often mapped to GCC's personality function (see the libstd implementation for more information), but crates which do not trigger a panic can be assured that this function is never called. The language item's name is eh_personality.

The second function, rust_begin_panic, is also used by the failure mechanisms of the compiler. When a panic happens, this controls the message that's displayed on the screen. While the language item's name is panic_impl, the symbol name is rust_begin_panic.

A third function, rust_eh_unwind_resume, is also needed if the custom_unwind_resume flag is set in the options of the compilation target. It allows customizing the process of resuming unwind at the end of the landing pads. The language item's name is eh_unwind_resume.

This is a list of all language items in Rust along with where they are located in the source code.

Primitives
- i8: libcore/num/mod.rs
- i16: libcore/num/mod.rs
- i32: libcore/num/mod.rs
- i64: libcore/num/mod.rs
- i128: libcore/num/mod.rs
- isize: libcore/num/mod.rs
- u8: libcore/num/mod.rs
- u16: libcore/num/mod.rs
- u32: libcore/num/mod.rs
- u64: libcore/num/mod.rs
- u128: libcore/num/mod.rs
- usize: libcore/num/mod.rs
- f32: libstd/f32.rs
- f64: libstd/f64.rs
- char: libcore/char.rs
- slice: liballoc/slice.rs
- str: liballoc/str.rs
- const_ptr: libcore/ptr.rs
- mut_ptr: libcore/ptr.rs
- unsafe_cell: libcore/cell.rs
Runtime
- start: libstd/rt.rs
- eh_personality: libpanic_unwind/emcc.rs (EMCC)
- eh_personality: libpanic_unwind/seh64_gnu.rs (SEH64 GNU)
- eh_personality: libpanic_unwind/seh.rs (SEH)
- eh_unwind_resume: libpanic_unwind/seh64_gnu.rs (SEH64 GNU)
- eh_unwind_resume: libpanic_unwind/gcc.rs (GCC)
- msvc_try_filter: libpanic_unwind/seh.rs (SEH)
- panic: libcore/panicking.rs
- panic_bounds_check: libcore/panicking.rs
- panic_impl: libcore/panicking.rs
- panic_impl: libstd/panicking.rs
Allocations
- owned_box: liballoc/boxed.rs
- exchange_malloc: liballoc/heap.rs
- box_free: liballoc/heap.rs
Operands
- not: libcore/ops/bit.rs
- bitand: libcore/ops/bit.rs
- bitor: libcore/ops/bit.rs
- bitxor: libcore/ops/bit.rs
- shl: libcore/ops/bit.rs
- shr: libcore/ops/bit.rs
- bitand_assign: libcore/ops/bit.rs
- bitor_assign: libcore/ops/bit.rs
- bitxor_assign: libcore/ops/bit.rs
- shl_assign: libcore/ops/bit.rs
- shr_assign: libcore/ops/bit.rs
- deref: libcore/ops/deref.rs
- deref_mut: libcore/ops/deref.rs
- index: libcore/ops/index.rs
- index_mut: libcore/ops/index.rs
- add: libcore/ops/arith.rs
- sub: libcore/ops/arith.rs
- mul: libcore/ops/arith.rs
- div: libcore/ops/arith.rs
- rem: libcore/ops/arith.rs
- neg: libcore/ops/arith.rs
- add_assign: libcore/ops/arith.rs
- sub_assign: libcore/ops/arith.rs
- mul_assign: libcore/ops/arith.rs
- div_assign: libcore/ops/arith.rs
- rem_assign: libcore/ops/arith.rs
- eq: libcore/cmp.rs
- ord: libcore/cmp.rs
Functions
- fn: libcore/ops/function.rs
- fn_mut: libcore/ops/function.rs
- fn_once: libcore/ops/function.rs
- generator_state: libcore/ops/generator.rs
- generator: libcore/ops/generator.rs
Other
- coerce_unsized: libcore/ops/unsize.rs
- drop: libcore/ops/drop.rs
- drop_in_place: libcore/ptr.rs
- clone: libcore/clone.rs
- copy: libcore/marker.rs
- send: libcore/marker.rs
- sized: libcore/marker.rs
- unsize: libcore/marker.rs
- sync: libcore/marker.rs
- phantom_data: libcore/marker.rs
- freeze: libcore/marker.rs
- debug_trait: libcore/fmt/mod.rs
- non_zero: libcore/nonzero.rs
- arc: liballoc/sync.rs
- rc: liballoc/rc.rs

The tracking issue for this feature is: #29596

You can tell rustc how to customize linking, and that is via the link_args attribute. This attribute is applied to extern blocks and specifies raw flags which need to get passed to the linker when producing an artifact. An example usage would be:

#![feature(link_args)]

#[link_args = "-foo -bar -baz"]
extern {}
# fn main() {}

Note that this feature is currently hidden behind the feature(link_args) gate because this is not a sanctioned way of performing linking. Right now rustc shells out to the system linker (gcc on most systems, link.exe on MSVC), so it makes sense to provide extra command line arguments, but this will not always be the case. In the future rustc may use LLVM directly to link native libraries, in which case link_args will have no meaning. You can achieve the same effect as the link_args attribute with the -C link-args argument to rustc.

It is highly recommended to not use this attribute, and rather use the more formal #[link(...)] attribute on extern blocks instead.

The tracking issue for this feature is: #37406

The tracking issue for this feature is: #29602

The tracking issue for this feature is: #29603

The tracking issue for this feature is: #54503

The tracking issue for this feature is: #29598

The tracking issue for this feature is: #49476

`main`

The tracking issue for this feature is: #29634

The tracking issue for this feature is: #29864

Normally, Rust keeps you from adding trait implementations that could overlap with each other, as it would be ambiguous which to use. This feature, however, carves out an exception to that rule: a trait can opt-in to having overlapping implementations, at the cost that those implementations are not allowed to override anything (and thus the trait itself cannot have any associated items, as they're pointless when they'd need to do the same thing for every type anyway).


# #![allow(unused_variables)]
#![feature(marker_trait_attr)]

#fn main() {
#[marker] trait CheapToClone: Clone {}

impl<T: Copy> CheapToClone for T {}

// These could potentally overlap with the blanket implementation above,
// so are only allowed because CheapToClone is a marker trait.
impl<T: CheapToClone, U: CheapToClone> CheapToClone for (T, U) {}
impl<T: CheapToClone> CheapToClone for std::ops::Range<T> {}

fn cheap_clone<T: CheapToClone>(t: T) -> T {
    t.clone()
}
#}

This is expected to replace the unstable overlapping_marker_traits feature, which applied to all empty traits (without needing an opt-in).

The tracking issue for this feature is: #44839

The tracking issue for this feature is: #32408

The tracking issue for this feature is: #32837

The tracking issue for this feature is: #35121

The tracking issue for this feature is: #43234

The tracking issue for this feature is: #29639

The tracking issue for this feature is: #29721

The tracking issue for this feature is: #55467

The non_ascii_idents feature adds support for non-ASCII identifiers.


# #![allow(unused_variables)]
#![feature(non_ascii_idents)]

#fn main() {
const ε: f64 = 0.00001f64;
const Π: f64 = 3.14f64;
#}

^Lexer:
IDENTIFIER :
XID_start XID_continue^*
| _ XID_continue⁺

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

Either

The first character has property XID_start
The remaining characters have property XID_continue

The first character is _
The identifier is more than one character, _ alone is not an identifier
The remaining characters have property XID_continue

that does not occur in the set of strict keywords.

Note: XID_start and XID_continue as character properties cover the character ranges used to form the more familiar C and Java language-family identifiers.

The tracking issue for this feature is: #44109

The non_exhaustive gate allows you to use the #[non_exhaustive] attribute on structs, enums and enum variants. When applied within a crate, users of the crate will need to use the _ pattern when matching enums and use the .. pattern when matching structs. Enum variants cannot be matched against. Structs and enum variants marked as non_exhaustive will not be able to be created normally outside of the defining crate. This is demonstrated below:

use std::error::Error as StdError;

#[non_exhaustive]
pub enum Error {
    Message(String),
    Other,
}
impl StdError for Error {
    fn description(&self) -> &str {
        // This will not error, despite being marked as non_exhaustive, as this
        // enum is defined within the current crate, it can be matched
        // exhaustively.
        match *self {
            Message(ref s) => s,
            Other => "other or unknown error",
        }
    }
}

use mycrate::Error;

// This will not error as the non_exhaustive Error enum has been matched with
// a wildcard.
match error {
    Message(ref s) => ...,
    Other => ...,
    _ => ...,
}

#[non_exhaustive]
pub struct Config {
    pub window_width: u16,
    pub window_height: u16,
}

// We can create structs as normal within the defining crate when marked as
// non_exhaustive.
let config = Config { window_width: 640, window_height: 480 };

// We can match structs exhaustively when within the defining crate.
if let Ok(Config { window_width, window_height }) = load_config() {
    // ...
}

use mycrate::Config;

// We cannot create a struct like normal if it has been marked as
// non_exhaustive.
let config = Config { window_width: 640, window_height: 480 };
// By adding the `..` we can match the config as below outside of the crate
// when marked non_exhaustive.
let &Config { window_width, window_height, .. } = config;

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #29628

The on_unimplemented feature provides the #[rustc_on_unimplemented] attribute, which allows trait definitions to add specialized notes to error messages when an implementation was expected but not found. You can refer to the trait's generic arguments by name and to the resolved type using Self.

For example:

#![feature(on_unimplemented)]

#[rustc_on_unimplemented="an iterator over elements of type `{A}` \
    cannot be built from a collection of type `{Self}`"]
trait MyIterator<A> {
    fn next(&mut self) -> A;
}

fn iterate_chars<I: MyIterator<char>>(i: I) {
    // ...
}

fn main() {
    iterate_chars(&[1, 2, 3][..]);
}

When the user compiles this, they will see the following;

error[E0277]: the trait bound `&[{integer}]: MyIterator<char>` is not satisfied
  --> <anon>:14:5
   |
14 |     iterate_chars(&[1, 2, 3][..]);
   |     ^^^^^^^^^^^^^ an iterator over elements of type `char` cannot be built from a collection of type `&[{integer}]`
   |
   = help: the trait `MyIterator<char>` is not implemented for `&[{integer}]`
   = note: required by `iterate_chars`

on_unimplemented also supports advanced filtering for better targeting of messages, as well as modifying specific parts of the error message. You target the text of:

the main error message (message)
the label (label)
an extra note (note)

For example, the following attribute


# #![allow(unused_variables)]
#fn main() {
#[rustc_on_unimplemented(
    message="message",
    label="label",
    note="note"
)]
trait MyIterator<A> {
    fn next(&mut self) -> A;
}
#}

Would generate the following output:

error[E0277]: message
  --> <anon>:14:5
   |
14 |     iterate_chars(&[1, 2, 3][..]);
   |     ^^^^^^^^^^^^^ label
   |
   = note: note
   = help: the trait `MyIterator<char>` is not implemented for `&[{integer}]`
   = note: required by `iterate_chars`

To allow more targeted error messages, it is possible to filter the application of these fields based on a variety of attributes when using on:

crate_local: whether the code causing the trait bound to not be fulfilled is part of the user's crate. This is used to avoid suggesting code changes that would require modifying a dependency.
Any of the generic arguments that can be substituted in the text can be referred by name as well for filtering, like Rhs="i32", except for Self.
_Self: to filter only on a particular calculated trait resolution, like Self="std::iter::Iterator<char>". This is needed because Self is a keyword which cannot appear in attributes.
direct: user-specified rather than derived obligation.
from_method: usable both as boolean (whether the flag is present, like crate_local) or matching against a particular method. Currently used for try.
from_desugaring: usable both as boolean (whether the flag is present) or matching against a particular desugaring.

For example, the Iterator trait can be annotated in the following way:


# #![allow(unused_variables)]
#fn main() {
#[rustc_on_unimplemented(
    on(
        _Self="&str",
        note="call `.chars()` or `.as_bytes()` on `{Self}"
    ),
    message="`{Self}` is not an iterator",
    label="`{Self}` is not an iterator",
    note="maybe try calling `.iter()` or a similar method"
)]
pub trait Iterator {}
#}

Which would produce the following outputs:

error[E0277]: `Foo` is not an iterator
 --> src/main.rs:4:16
  |
4 |     for foo in Foo {}
  |                ^^^ `Foo` is not an iterator
  |
  = note: maybe try calling `.iter()` or a similar method
  = help: the trait `std::iter::Iterator` is not implemented for `Foo`
  = note: required by `std::iter::IntoIterator::into_iter`

error[E0277]: `&str` is not an iterator
 --> src/main.rs:5:16
  |
5 |     for foo in "" {}
  |                ^^ `&str` is not an iterator
  |
  = note: call `.chars()` or `.bytes() on `&str`
  = help: the trait `std::iter::Iterator` is not implemented for `&str`
  = note: required by `std::iter::IntoIterator::into_iter`

If you need to filter on multiple attributes, you can use all, any or not in the following way:


# #![allow(unused_variables)]
#fn main() {
#[rustc_on_unimplemented(
    on(
        all(_Self="&str", T="std::string::String"),
        note="you can coerce a `{T}` into a `{Self}` by writing `&*variable`"
    )
)]
pub trait From<T>: Sized { /* ... */ }
#}

The tracking issue for this feature is: #54882

The tracking issue for this feature is #13231

The optin_builtin_traits feature gate allows you to define auto traits.

Auto traits, like Send or Sync in the standard library, are marker traits that are automatically implemented for every type, unless the type, or a type it contains, has explicitly opted out via a negative impl.

impl !Type for Trait

Example:

#![feature(optin_builtin_traits)]

auto trait Valid {}

struct True;
struct False;

impl !Valid for False {}

struct MaybeValid<T>(T);

fn must_be_valid<T: Valid>(_t: T) { }

fn main() {
    // works
    must_be_valid( MaybeValid(True) );
                
    // compiler error - trait bound not satisfied
    // must_be_valid( MaybeValid(False) );
}

The tracking issue for this feature is: #29864

The tracking issue for this feature is: #32837

The tracking issue for this feature is: #27731

The tracking issue for this feature is: #29597

This feature is part of "compiler plugins." It will often be used with the plugin_registrar and rustc_private features.

rustc can load compiler plugins, which are user-provided libraries that extend the compiler's behavior with new syntax extensions, lint checks, etc.

A plugin is a dynamic library crate with a designated registrar function that registers extensions with rustc. Other crates can load these extensions using the crate attribute #![plugin(...)]. See the rustc_plugin documentation for more about the mechanics of defining and loading a plugin.

If present, arguments passed as #![plugin(foo(... args ...))] are not interpreted by rustc itself. They are provided to the plugin through the Registry's args method.

In the vast majority of cases, a plugin should only be used through #![plugin] and not through an extern crate item. Linking a plugin would pull in all of libsyntax and librustc as dependencies of your crate. This is generally unwanted unless you are building another plugin. The plugin_as_library lint checks these guidelines.

The usual practice is to put compiler plugins in their own crate, separate from any macro_rules! macros or ordinary Rust code meant to be used by consumers of a library.

Plugins can extend Rust's syntax in various ways. One kind of syntax extension is the procedural macro. These are invoked the same way as ordinary macros, but the expansion is performed by arbitrary Rust code that manipulates syntax trees at compile time.

Let's write a plugin roman_numerals.rs that implements Roman numeral integer literals.

#![crate_type="dylib"]
#![feature(plugin_registrar, rustc_private)]

extern crate syntax;
extern crate syntax_pos;
extern crate rustc;
extern crate rustc_plugin;

use syntax::parse::token;
use syntax::tokenstream::TokenTree;
use syntax::ext::base::{ExtCtxt, MacResult, DummyResult, MacEager};
use syntax::ext::build::AstBuilder;  // A trait for expr_usize.
use syntax_pos::Span;
use rustc_plugin::Registry;

fn expand_rn(cx: &mut ExtCtxt, sp: Span, args: &[TokenTree])
        -> Box<MacResult + 'static> {

    static NUMERALS: &'static [(&'static str, usize)] = &[
        ("M", 1000), ("CM", 900), ("D", 500), ("CD", 400),
        ("C",  100), ("XC",  90), ("L",  50), ("XL",  40),
        ("X",   10), ("IX",   9), ("V",   5), ("IV",   4),
        ("I",    1)];

    if args.len() != 1 {
        cx.span_err(
            sp,
            &format!("argument should be a single identifier, but got {} arguments", args.len()));
        return DummyResult::any(sp);
    }

    let text = match args[0] {
        TokenTree::Token(_, token::Ident(s)) => s.to_string(),
        _ => {
            cx.span_err(sp, "argument should be a single identifier");
            return DummyResult::any(sp);
        }
    };

    let mut text = &*text;
    let mut total = 0;
    while !text.is_empty() {
        match NUMERALS.iter().find(|&&(rn, _)| text.starts_with(rn)) {
            Some(&(rn, val)) => {
                total += val;
                text = &text[rn.len()..];
            }
            None => {
                cx.span_err(sp, "invalid Roman numeral");
                return DummyResult::any(sp);
            }
        }
    }

    MacEager::expr(cx.expr_usize(sp, total))
}

#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
    reg.register_macro("rn", expand_rn);
}

Then we can use rn!() like any other macro:

#![feature(plugin)]
#![plugin(roman_numerals)]

fn main() {
    assert_eq!(rn!(MMXV), 2015);
}

The advantages over a simple fn(&str) -> u32 are:

The (arbitrarily complex) conversion is done at compile time.
Input validation is also performed at compile time.
It can be extended to allow use in patterns, which effectively gives a way to define new literal syntax for any data type.

In addition to procedural macros, you can define new derive-like attributes and other kinds of extensions. See Registry::register_syntax_extension and the SyntaxExtension enum. For a more involved macro example, see regex_macros.

You can use syntax::parse to turn token trees into higher-level syntax elements like expressions:

fn expand_foo(cx: &mut ExtCtxt, sp: Span, args: &[TokenTree])
        -> Box<MacResult+'static> {

    let mut parser = cx.new_parser_from_tts(args);

    let expr: P<Expr> = parser.parse_expr();

Looking through libsyntax parser code will give you a feel for how the parsing infrastructure works.

Keep the Spans of everything you parse, for better error reporting. You can wrap Spanned around your custom data structures.

Calling ExtCtxt::span_fatal will immediately abort compilation. It's better to instead call ExtCtxt::span_err and return DummyResult so that the compiler can continue and find further errors.

To print syntax fragments for debugging, you can use span_note together with syntax::print::pprust::*_to_string.

The example above produced an integer literal using AstBuilder::expr_usize. As an alternative to the AstBuilder trait, libsyntax provides a set of quasiquote macros. They are undocumented and very rough around the edges. However, the implementation may be a good starting point for an improved quasiquote as an ordinary plugin library.

Plugins can extend Rust's lint infrastructure with additional checks for code style, safety, etc. Now let's write a plugin lint_plugin_test.rs that warns about any item named lintme.

#![feature(plugin_registrar)]
#![feature(box_syntax, rustc_private)]

extern crate syntax;

// Load rustc as a plugin to get macros
#[macro_use]
extern crate rustc;
extern crate rustc_plugin;

use rustc::lint::{EarlyContext, LintContext, LintPass, EarlyLintPass,
                  EarlyLintPassObject, LintArray};
use rustc_plugin::Registry;
use syntax::ast;

declare_lint!(TEST_LINT, Warn, "Warn about items named 'lintme'");

struct Pass;

impl LintPass for Pass {
    fn get_lints(&self) -> LintArray {
        lint_array!(TEST_LINT)
    }
}

impl EarlyLintPass for Pass {
    fn check_item(&mut self, cx: &EarlyContext, it: &ast::Item) {
        if it.ident.as_str() == "lintme" {
            cx.span_lint(TEST_LINT, it.span, "item is named 'lintme'");
        }
    }
}

#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
    reg.register_early_lint_pass(box Pass as EarlyLintPassObject);
}

Then code like

#![plugin(lint_plugin_test)]

fn lintme() { }

will produce a compiler warning:

foo.rs:4:1: 4:16 warning: item is named 'lintme', #[warn(test_lint)] on by default
foo.rs:4 fn lintme() { }
         ^~~~~~~~~~~~~~~

The components of a lint plugin are:

one or more declare_lint! invocations, which define static Lint structs;
a struct holding any state needed by the lint pass (here, none);
a LintPass implementation defining how to check each syntax element. A single LintPass may call span_lint for several different Lints, but should register them all through the get_lints method.

Lint passes are syntax traversals, but they run at a late stage of compilation where type information is available. rustc's built-in lints mostly use the same infrastructure as lint plugins, and provide examples of how to access type information.

Lints defined by plugins are controlled by the usual attributes and compiler flags, e.g. #[allow(test_lint)] or -A test-lint. These identifiers are derived from the first argument to declare_lint!, with appropriate case and punctuation conversion.

You can run rustc -W help foo.rs to see a list of lints known to rustc, including those provided by plugins loaded by foo.rs.

The tracking issue for this feature is: #29597

This feature is part of "compiler plugins." It will often be used with the plugin and rustc_private features as well. For more details, see their docs.

The tracking issue for this feature is: #44839

The tracking issue for this feature is: #56354

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #54727

The tracking issue for this feature is: #42524.

The tracking issue for this feature is: #55437

The re_rebalance_coherence feature tweaks the rules regarding which trait impls are allowed in crates. The following rule is used:

Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only if at least one of the following is true:

Trait is a local trait
All of
- At least one of the types T0..=Tn must be a local type. Let Ti be the first such type.
- No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)

See the RFC for details.

The tracking issue for this feature is: #57996

The repr_align_enum feature allows using the #[repr(align(x))] attribute on enums, similarly to structs.

#![feature(repr_align_enum)]

#[repr(align(8))]
enum Aligned {
    Foo,
    Bar { value: u32 },
}

fn main() {
    assert_eq!(std::mem::align_of::<Aligned>(), 8);
}

This is equivalent to using an aligned wrapper struct everywhere:

#[repr(align(8))]
struct Aligned(Unaligned);

enum Unaligned {
    Foo,
    Bar { value: u32 },
}

fn main() {
    assert_eq!(std::mem::align_of::<Aligned>(), 8);
}

The tracking issue for this feature is: #27731

The tracking issue for this feature is: #35118

The repr128 feature adds support for #[repr(u128)] on enums.


# #![allow(unused_variables)]
#![feature(repr128)]

#fn main() {
#[repr(u128)]
enum Foo {
    Bar(u64),
}
#}

The tracking issue for this feature is: #44839

The tracking issue for this feature is: #29642

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #27731

The tracking issue for this feature is: #23121

The slice_patterns feature gate lets you use .. to indicate any number of elements inside a pattern matching a slice. This wildcard can only be used once for a given array. If there's an pattern before the .., the subslice will be matched against that pattern. For example:

#![feature(slice_patterns)]

fn is_symmetric(list: &[u32]) -> bool {
    match list {
        &[] | &[_] => true,
        &[x, ref inside.., y] if x == y => is_symmetric(inside),
        &[..] => false,
    }
}

fn main() {
    let sym = &[0, 1, 4, 2, 4, 1, 0];
    assert!(is_symmetric(sym));

    let not_sym = &[0, 1, 7, 2, 4, 1, 0];
    assert!(!is_symmetric(not_sym));
}

The tracking issue for this feature is: #31844

The tracking issue for this feature is: #44839

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #29633

The tracking issue for this feature is: #37403

The tracking issue for this feature is: #15701

The tracking issue for this feature is: #31434

The tracking issue for this feature is: #44839

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

`thread_local`

The tracking issue for this feature is: #29594

The tracking issue for this feature is #29598.

With trace_macros you can trace the expansion of macros in your code.

#![feature(trace_macros)]

fn main() {
    trace_macros!(true);
    println!("Hello, Rust!");
    trace_macros!(false);
}

The cargo build output:

note: trace_macro
 --> src/main.rs:5:5
  |
5 |     println!("Hello, Rust!");
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: expanding `println! { "Hello, Rust!" }`
  = note: to `print ! ( concat ! ( "Hello, Rust!" , "\n" ) )`
  = note: expanding `print! { concat ! ( "Hello, Rust!" , "\n" ) }`
  = note: to `$crate :: io :: _print ( format_args ! ( concat ! ( "Hello, Rust!" , "\n" ) )
          )`

    Finished dev [unoptimized + debuginfo] target(s) in 0.60 secs

The tracking issue for this feature is: [#41517]

The trait_alias feature adds support for trait aliases. These allow aliases to be created for one or more traits (currently just a single regular trait plus any number of auto-traits), and used wherever traits would normally be used as either bounds or trait objects.

#![feature(trait_alias)]

trait Foo = std::fmt::Debug + Send;
trait Bar = Foo + Sync;

// Use trait alias as bound on type parameter.
fn foo<T: Foo>(v: &T) {
    println!("{:?}", v);
}

pub fn main() {
    foo(&1);

    // Use trait alias for trait objects.
    let a: &Bar = &123;
    println!("{:?}", a);
    let b = Box::new(456) as Box<dyn Foo>;
    println!("{:?}", b);
}

The tracking issue for this feature is: #48214

The tracking issue for this feature is: #31436

The try_blocks feature adds support for try blocks. A try block creates a new scope one can use the ? operator in.


# #![allow(unused_variables)]
#![feature(try_blocks)]

#fn main() {
use std::num::ParseIntError;

let result: Result<i32, ParseIntError> = try {
    "1".parse::<i32>()?
        + "2".parse::<i32>()?
        + "3".parse::<i32>()?
};
assert_eq!(result, Ok(6));

let result: Result<i32, ParseIntError> = try {
    "1".parse::<i32>()?
        + "foo".parse::<i32>()?
        + "3".parse::<i32>()?
};
assert!(result.is_err());
#}

The tracking issue for this feature is: #49683

The type_alias_enum_variants feature enables the use of variants on type aliases that refer to enums, as both a constructor and a pattern. That is, it allows for the syntax EnumAlias::Variant, which behaves exactly the same as Enum::Variant (assuming that EnumAlias is an alias for some enum type Enum).

Note that since Self exists as a type alias, this feature also enables the use of the syntax Self::Variant within an impl block for an enum type.

#![feature(type_alias_enum_variants)]

enum Foo {
    Bar(i32),
    Baz { i: i32 },
}

type Alias = Foo;

fn main() {
    let t = Alias::Bar(0);
    let t = Alias::Baz { i: 0 };
    match t {
        Alias::Bar(_i) => {}
        Alias::Baz { i: _i } => {}
    }
}

The tracking issue for this feature is: #23416

The tracking issue for this feature is #29625

`hash_raw_entry`

The tracking issue for this feature is: #56167

`hash_set_entry`

The tracking issue for this feature is: #60896

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #50264

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #22639

The tracking issue for this feature is: #32976

The tracking issue for this feature is: #27709

The tracking issue for this feature is: #53485

Add the methods is_sorted, is_sorted_by and is_sorted_by_key to [T]; add the methods is_sorted, is_sorted_by and is_sorted_by_key to Iterator.

The tracking issue for this feature is: #57581

This feature is internal to the Rust compiler and is not intended for general use.

`libstd_thread_internals`

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #27794

The tracking issue for this feature is: #55422

`map_entry_replace`

The tracking issue for this feature is: #44286

The tracking issue for this feature is: #49347

The tracking issue for this feature is: #53491

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #48655

The tracking issue for this feature is: #54890

The tracking issue for this feature is: #33577

The tracking issue for this feature is: #60258

The tracking issue for this feature is: #50512

The tracking issue for this feature is: #32837

`panic_info_message`

The tracking issue for this feature is: #44489

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #32837

The tracking issue for this feature is: #58234

The tracking issue for this feature is: #27721

The tracking issue for this feature is: #60245

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #54724

The tracking issue for this feature is: #54140

The tracking issue for this feature is: #27812

The tracking issue for this feature is: #54722

The tracking issue for this feature is: #54723

The tracking issue for this feature is: #54725

The tracking issue for this feature is: #48711

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #60602

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #41079

The tracking issue for this feature is: #48111

The tracking issue for this feature is: #27751

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #47336

The tracking issue for this feature is: [#42788]

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #55002

The tracking issue for this feature is: #48784

The tracking issue for this feature is: #53268

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #27812

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #59983

The tracking issue for this feature is: #59359

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #56975

The tracking issue for this feature is: #56431

The tracking issue for this feature is: #27747

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #58957

The tracking issue for this feature is: #55300

The tracking issue for this feature is: #54279

This feature is internal to the Rust compiler and is not intended for general use.

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #48556

The tracking issue for this feature is: #42168

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #43301

The tracking issue for this feature is: None.

The internals of the test crate are unstable, behind the test flag. The most widely used part of the test crate are benchmark tests, which can test the performance of your code. Let's make our src/lib.rs look like this (comments elided):

#![feature(test)]

extern crate test;

pub fn add_two(a: i32) -> i32 {
    a + 2
}

#[cfg(test)]
mod tests {
    use super::*;
    use test::Bencher;

    #[test]
    fn it_works() {
        assert_eq!(4, add_two(2));
    }

    #[bench]
    fn bench_add_two(b: &mut Bencher) {
        b.iter(|| add_two(2));
    }
}

Note the test feature gate, which enables this unstable feature.

We've imported the test crate, which contains our benchmarking support. We have a new function as well, with the bench attribute. Unlike regular tests, which take no arguments, benchmark tests take a &mut Bencher. This Bencher provides an iter method, which takes a closure. This closure contains the code we'd like to benchmark.

We can run benchmark tests with cargo bench:

$ cargo bench
   Compiling adder v0.0.1 (file:///home/steve/tmp/adder)
     Running target/release/adder-91b3e234d4ed382a

running 2 tests
test tests::it_works ... ignored
test tests::bench_add_two ... bench:         1 ns/iter (+/- 0)

test result: ok. 0 passed; 0 failed; 1 ignored; 1 measured

Our non-benchmark test was ignored. You may have noticed that cargo bench takes a bit longer than cargo test. This is because Rust runs our benchmark a number of times, and then takes the average. Because we're doing so little work in this example, we have a 1 ns/iter (+/- 0), but this would show the variance if there was one.

Advice on writing benchmarks:

Move setup code outside the iter loop; only put the part you want to measure inside
Make the code do "the same thing" on each iteration; do not accumulate or change state
Make the outer function idempotent too; the benchmark runner is likely to run it many times
Make the inner iter loop short and fast so benchmark runs are fast and the calibrator can adjust the run-length at fine resolution
Make the code in the iter loop do something simple, to assist in pinpointing performance improvements (or regressions)

There's another tricky part to writing benchmarks: benchmarks compiled with optimizations activated can be dramatically changed by the optimizer so that the benchmark is no longer benchmarking what one expects. For example, the compiler might recognize that some calculation has no external effects and remove it entirely.

#![feature(test)]

extern crate test;
use test::Bencher;

#[bench]
fn bench_xor_1000_ints(b: &mut Bencher) {
    b.iter(|| {
        (0..1000).fold(0, |old, new| old ^ new);
    });
}

gives the following results

running 1 test
test bench_xor_1000_ints ... bench:         0 ns/iter (+/- 0)

test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

The benchmarking runner offers two ways to avoid this. Either, the closure that the iter method receives can return an arbitrary value which forces the optimizer to consider the result used and ensures it cannot remove the computation entirely. This could be done for the example above by adjusting the b.iter call to


# #![allow(unused_variables)]
#fn main() {
# struct X;
# impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
b.iter(|| {
    // Note lack of `;` (could also use an explicit `return`).
    (0..1000).fold(0, |old, new| old ^ new)
});
#}

Or, the other option is to call the generic test::black_box function, which is an opaque "black box" to the optimizer and so forces it to consider any argument as used.

#![feature(test)]

extern crate test;

# fn main() {
# struct X;
# impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
b.iter(|| {
    let n = test::black_box(1000);

    (0..n).fold(0, |a, b| a ^ b)
})
# }

Neither of these read or modify the value, and are very cheap for small values. Larger values can be passed indirectly to reduce overhead (e.g. black_box(&huge_struct)).

Performing either of the above changes gives the following benchmarking results

running 1 test
test bench_xor_1000_ints ... bench:       131 ns/iter (+/- 3)

test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

However, the optimizer can still modify a testcase in an undesirable manner even when using either of the above.

`thread_local_internals`

This feature is internal to the Rust compiler and is not intended for general use.

`thread_spawn_unchecked`

The tracking issue for this feature is: #55132

The tracking issue for this feature is: #59277

The tracking issue for this feature is: #41263

The tracking issue for this feature is: #37572

The tracking issue for this feature is: #48043

The tracking issue for this feature is: #42327

This introduces a new trait Try for extending the ? operator to types other than Result (a part of RFC 1859). The trait provides the canonical way to view a type in terms of a success/failure dichotomy. This will allow ? to supplant the try_opt! macro on Option and the try_ready! macro on Poll, among other things.

Here's an example implementation of the trait:

/// A distinct type to represent the `None` value of an `Option`.
///
/// This enables using the `?` operator on `Option`; it's rarely useful alone.
#[derive(Debug)]
#[unstable(feature = "try_trait", issue = "42327")]
pub struct None { _priv: () }

#[unstable(feature = "try_trait", issue = "42327")]
impl<T> ops::Try for Option<T>  {
    type Ok = T;
    type Error = None;

    fn into_result(self) -> Result<T, None> {
        self.ok_or(None { _priv: () })
    }

    fn from_ok(v: T) -> Self {
        Some(v)
    }

    fn from_error(_: None) -> Self {
        None
    }
}

Note the Error associated type here is a new marker. The ? operator allows interconversion between different Try implementers only when the error type can be converted Into the error type of the enclosing function (or catch block). Having a distinct error type (as opposed to just (), or similar) restricts this to where it's semantically meaningful.

The tracking issue for this feature is: #59127

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #49726

The tracking issue for this feature is: #27732

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #58957

The tracking issue for this feature is: #40062

The tracking issue for this feature is: #41758

The tracking issue for this feature is: #47960

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #57977

The tracking issue for this feature is: #60728

The tracking issue for this feature is: #55981

This feature is internal to the Rust compiler and is not intended for general use.

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #32463

The Rust Unstable Book

The Unstable Book

Compiler flags

emit-stack-sizes

profile

Language features

aarch64_target_feature

abi_amdgpu_kernel

abi_msp430_interrupt

abi_ptx

abi_thiscall

abi_unadjusted

abi_vectorcall

abi_x86_interrupt

adx_target_feature

alloc_error_handler

allocator_internals

allow_fail

allow_internal_unsafe

allow_internal_unstable

arbitrary_self_types

arm_target_feature

asm

Assembly template

Operands

Clobbers

Options

More Information

associated_type_defaults

async_await

avx512_target_feature

await_macro

bind_by_move_pattern_guards

box_patterns

box_syntax

c_variadic

Examples

cfg_target_has_atomic

cfg_target_thread_local

cmpxchg16b_target_feature

compiler_builtins

concat_idents

Examples

const_compare_raw_pointers

const_fn

Examples

const_fn_union

const_generics

const_panic

const_raw_ptr_deref

const_raw_ptr_to_usize_cast

const_transmute

crate_visibility_modifier

custom_attribute

custom_inner_attributes

custom_test_frameworks

decl_macro

default_type_parameter_fallback

doc_alias

doc_cfg

doc_keyword

doc_masked

doc_spotlight

dropck_eyepatch

dropck_parametricity

exclusive_range_pattern

exhaustive_patterns

existential_type

extern_types

external_doc

f16c_target_feature

ffi_returns_twice

format_args_nl

fundamental

generators

The Generator trait

Closure-like semantics

Generators as state machines

generic_associated_types

global_asm

`emit-stack-sizes`

`profile`

`aarch64_target_feature`

`abi_amdgpu_kernel`

`abi_msp430_interrupt`

`abi_ptx`

`abi_thiscall`

`abi_unadjusted`

`abi_vectorcall`

`abi_x86_interrupt`

`adx_target_feature`

`alloc_error_handler`

`allocator_internals`

`allow_fail`

`allow_internal_unsafe`

`allow_internal_unstable`

`arbitrary_self_types`

`arm_target_feature`

`asm`

`associated_type_defaults`

`async_await`

`avx512_target_feature`

`await_macro`

`bind_by_move_pattern_guards`

`box_patterns`

`box_syntax`

`c_variadic`

`cfg_target_has_atomic`

`cfg_target_thread_local`

`cmpxchg16b_target_feature`

`compiler_builtins`

`concat_idents`

`const_compare_raw_pointers`

`const_fn`

`const_fn_union`

`const_generics`

`const_panic`

`const_raw_ptr_deref`

`const_raw_ptr_to_usize_cast`

`const_transmute`

`crate_visibility_modifier`

`custom_attribute`

`custom_inner_attributes`

`custom_test_frameworks`

`decl_macro`

`default_type_parameter_fallback`

`doc_alias`

`doc_cfg`

`doc_keyword`

`doc_masked`

`doc_spotlight`

`dropck_eyepatch`

`dropck_parametricity`

`exclusive_range_pattern`

`exhaustive_patterns`

`existential_type`

`extern_types`

`external_doc`

`f16c_target_feature`

`ffi_returns_twice`

`format_args_nl`

`fundamental`

`generators`

The `Generator` trait

`generic_associated_types`

`global_asm`

`hexagon_target_feature`

`impl_trait_in_bindings`

`in_band_lifetimes`

`infer_static_outlives_requirements`

`intrinsics`

`label_break_value`

`lang_items`

`link_args`

`link_cfg`

`link_llvm_intrinsics`

`linkage`

`lint_reasons`

`log_syntax`

`macros_in_extern`