r[type.fn-pointer]

Function pointer types

r[type.fn-pointer.syntax]

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 these calling conventions:

• C
• cdecl
• system
• aapcs
• sysv64
• win64
• efiapi

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.