Function pointer types

^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^* ...

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:

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

Function pointers can be created via a coercion from both function items and non-capturing, non-async closures.

The unsafe qualifier indicates that the type’s value is an unsafe function, and the extern qualifier indicates it is an extern function.

Variadic parameters can only be specified with extern function types with the "C" or "cdecl" calling convention.

This also includes the corresponding -unwind variants.

Attributes on function pointer parameters

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