r[type.fn-pointer]

Function pointer types

r[type.fn-pointer.syntax]

BareFunctionType ->
    ForLifetimes? FunctionTypeQualifiers `fn`
       `(` FunctionParametersMaybeNamedVariadic? `)` BareFunctionReturnType?

FunctionTypeQualifiers -> `unsafe`? (`extern` Abi?)?

BareFunctionReturnType -> `->` TypeNoBounds

FunctionParametersMaybeNamedVariadic ->
    MaybeNamedFunctionParameters | MaybeNamedFunctionParametersVariadic

MaybeNamedFunctionParameters ->
    MaybeNamedParam ( `,` MaybeNamedParam )* `,`?

MaybeNamedParam ->
    OuterAttribute* ( ( IDENTIFIER | `_` ) `:` )? Type

MaybeNamedFunctionParametersVariadic ->
    ( MaybeNamedParam `,` )* MaybeNamedParam `,` OuterAttribute* `...`

r[type.fn-pointer.intro] Function pointer types, written using the fn keyword, refer to a function whose identity is not necessarily known at compile-time.

An example where Binop is defined as a function pointer type:

fn add(x: i32, y: i32) -> i32 {
    x + y
}

let mut x = add(5,7);

type Binop = fn(i32, i32) -> i32;
let bo: Binop = add;
x = bo(5,7);

r[type.fn-pointer.coercion] Function pointers can be created via a coercion from both function items and non-capturing, non-async closures.

r[type.fn-pointer.qualifiers] The unsafe qualifier indicates that the type's value is an unsafe function, and the extern qualifier indicates it is an extern function.

r[type.fn-pointer.constraint-variadic] Variadic parameters can only be specified with extern function types with the "C" or "cdecl" calling convention.

This also includes the corresponding [-unwind variants][items.fn.extern.unwind].

r[type.fn-pointer.attributes]

Attributes on function pointer parameters

Attributes on function pointer parameters follow the same rules and restrictions as regular function parameters.