Functions

^Syntax
Function :
   FunctionQualifiers fn IDENTIFIER GenericParams^?
      ( FunctionParameters^? )
      FunctionReturnType^? WhereClause^?
      ( BlockExpression | ; )

FunctionQualifiers :
   const^? async¹^? ItemSafety^? (extern Abi^?)^?

ItemSafety :
   safe² | unsafe

Abi :
   STRING_LITERAL | RAW_STRING_LITERAL

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

SelfParam :
   OuterAttribute^* ( ShorthandSelf | TypedSelf )

ShorthandSelf :
   (& | & Lifetime)^? mut^? self

TypedSelf :
   mut^? self : Type

FunctionParam :
   OuterAttribute^* ( FunctionParamPattern | ... | Type ³ )

FunctionParamPattern :
   PatternNoTopAlt : ( Type | ... )

FunctionReturnType :
   -> Type

¹
The async qualifier is not allowed in the 2015 edition.

²
The safe function qualifier is only allowed semantically within extern blocks.

³
Function parameters with only a type are only allowed in an associated function of a trait item in the 2015 edition.

A function consists of a block (that’s the body of the function), along with a name, a set of parameters, and an output type. Other than a name, all these are optional. Functions are declared with the keyword fn which defines the given name in the value namespace of the module or block where it is located. Functions may declare a set of input variables as parameters, through which the caller passes arguments into the function, and the output type of the value the function will return to its caller on completion. If the output type is not explicitly stated, it is the unit type.

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

For example, this is a simple function:

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

The safe function is semantically only allowed when used in an extern block.

Function parameters

Function parameters are irrefutable patterns, so any pattern that is valid in an else-less let binding is also valid as a parameter:

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

If the first parameter is a SelfParam, this indicates that the function is a method. Functions with a self parameter may only appear as an associated function in a trait or implementation.

A parameter with the ... token indicates a variadic function, and may only be used as the last parameter of an external block function. The variadic parameter may have an optional identifier, such as args: ....

Function body

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

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

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

Functions without a body block are terminated with a semicolon. This form may only appear in a trait or external block.

Generic functions

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

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

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

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

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

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

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

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

foo(&[1, 2]);
}

will instantiate type parameter T with i32.

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

Extern function qualifier

The extern function qualifier allows providing function definitions that can be called with a particular ABI:

extern "ABI" fn foo() { /* ... */ }

These are often used in combination with external block items which provide function declarations that can be used to call functions without providing their definition:

unsafe extern "ABI" {
  unsafe fn foo(); /* no body */
  safe fn bar(); /* no body */
}
unsafe { foo() };
bar();

When "extern" Abi?* is omitted from FunctionQualifiers in function items, the ABI "Rust" is assigned. For example:

#![allow(unused)]
fn main() {
fn foo() {}
}

is equivalent to:

#![allow(unused)]
fn main() {
extern "Rust" fn foo() {}
}

Functions can be called by foreign code, and using an ABI that differs from Rust allows, for example, to provide functions that can be called from other programming languages like C:

#![allow(unused)]
fn main() {
// Declares a function with the "C" ABI
extern "C" fn new_i32() -> i32 { 0 }

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

Just as with external block, when the extern keyword is used and the "ABI" is omitted, the ABI used defaults to "C". That is, this:

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

is equivalent to:

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

Functions with an ABI that differs from "Rust" do not support unwinding in the exact same way that Rust does. Therefore, unwinding past the end of functions with such ABIs causes the process to abort.

Note: The LLVM backend of the rustc implementation aborts the process by executing an illegal instruction.

Const functions

Functions qualified with the const keyword are const functions, as are tuple struct and tuple variant constructors. Const functions can be called from within const contexts.

Const functions may use the extern function qualifier, but only with the "Rust" and "C" ABIs.

Const functions are not allowed to be async.

Async functions

Functions may be qualified as async, and this can also be combined with the unsafe qualifier:

#![allow(unused)]
fn main() {
async fn regular_example() { }
async unsafe fn unsafe_example() { }
}

Async functions do no work when called: instead, they capture their arguments into a future. When polled, that future will execute the function’s body.

An async function is roughly equivalent to a function that returns impl Future and with an async move block as its body:

#![allow(unused)]
fn main() {
// Source
async fn example(x: &str) -> usize {
    x.len()
}
}

is roughly equivalent to:

#![allow(unused)]
fn main() {
use std::future::Future;
// Desugared
fn example<'a>(x: &'a str) -> impl Future<Output = usize> + 'a {
    async move { x.len() }
}
}

The actual desugaring is more complex:

The return type in the desugaring is assumed to capture all lifetime parameters from the async fn declaration. This can be seen in the desugared example above, which explicitly outlives, and hence captures, 'a.
The async move block in the body captures all function parameters, including those that are unused or bound to a _ pattern. This ensures that function parameters are dropped in the same order as they would be if the function were not async, except that the drop occurs when the returned future has been fully awaited.

For more information on the effect of async, see async blocks.

Edition differences: Async functions are only available beginning with Rust 2018.

Combining `async` and `unsafe`

It is legal to declare a function that is both async and unsafe. The resulting function is unsafe to call and (like any async function) returns a future. This future is just an ordinary future and thus an unsafe context is not required to “await” it:

#![allow(unused)]
fn main() {
// Returns a future that, when awaited, dereferences `x`.
//
// Soundness condition: `x` must be safe to dereference until
// the resulting future is complete.
async unsafe fn unsafe_example(x: *const i32) -> i32 {
  *x
}

async fn safe_example() {
    // An `unsafe` block is required to invoke the function initially:
    let p = 22;
    let future = unsafe { unsafe_example(&p) };

    // But no `unsafe` block required here. This will
    // read the value of `p`:
    let q = future.await;
}
}

Note that this behavior is a consequence of the desugaring to a function that returns an impl Future – in this case, the function we desugar to is an unsafe function, but the return value remains the same.

Unsafe is used on an async function in precisely the same way that it is used on other functions: it indicates that the function imposes some additional obligations on its caller to ensure soundness. As in any other unsafe function, these conditions may extend beyond the initial call itself – in the snippet above, for example, the unsafe_example function took a pointer x as argument, and then (when awaited) dereferenced that pointer. This implies that x would have to be valid until the future is finished executing, and it is the caller’s responsibility to ensure that.

Attributes on functions

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

This example shows an inner attribute on a function. The function is documented with just the word “Example”.

#![allow(unused)]
fn main() {
fn documented() {
    #![doc = "Example"]
}
}

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

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

Attributes on function parameters

Outer attributes are allowed on function parameters and the permitted built-in attributes are restricted to cfg, cfg_attr, allow, warn, deny, and forbid.

#![allow(unused)]
fn main() {
fn len(
    #[cfg(windows)] slice: &[u16],
    #[cfg(not(windows))] slice: &[u8],
) -> usize {
    slice.len()
}
}

Inert helper attributes used by procedural macro attributes applied to items are also allowed but be careful to not include these inert attributes in your final TokenStream.

For example, the following code defines an inert some_inert_attribute attribute that is not formally defined anywhere and the some_proc_macro_attribute procedural macro is responsible for detecting its presence and removing it from the output token stream.

#[some_proc_macro_attribute]
fn foo_oof(#[some_inert_attribute] arg: u8) {
}

The Rust Reference