Functions
Syntax
Function :
FunctionQualifiersfn
IDENTIFIER GenericParams?
(
FunctionParameters?)
FunctionReturnType? WhereClause?
( BlockExpression |;
)FunctionQualifiers :
const
?async
1?unsafe
? (extern
Abi?)?Abi :
STRING_LITERAL | RAW_STRING_LITERALFunctionParameters :
SelfParam,
?
| (SelfParam,
)? FunctionParam (,
FunctionParam)*,
?SelfParam :
OuterAttribute* ( ShorthandSelf | TypedSelf )ShorthandSelf :
(&
|&
Lifetime)?mut
?self
TypedSelf :
mut
?self
:
TypeFunctionParam :
OuterAttribute* ( FunctionParamPattern |...
| Type 2 )FunctionParamPattern :
PatternNoTopAlt:
( Type |...
)FunctionReturnType :
->
Type1The
async
qualifier is not allowed in the 2015 edition.2Function 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, 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
.
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; } }
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 block of a function is conceptually wrapped in a block that binds the
argument patterns and then return
s the value of the function's block. This
means that the tail expression of the block, if evaluated, ends up being
returned to the caller. As usual, an explicit return expression within
the body of the function will short-cut that implicit return, if reached.
For example, the function above behaves as if it was written as:
// argument_0 is the actual first argument passed from the caller
let (value, _) = argument_0;
return {
value
};
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:
extern "ABI" {
fn foo(); /* no body */
}
unsafe { foo() }
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 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) {
}