function-pointer.md

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] A function pointer type, written using the fn keyword, refers to a function whose identity is not necessarily known at compile-time.

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

fnadd(x:i32,y:i32) -> i32{
x + y
}letmut x = add(5,7);typeBinop = 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] For the function to be variadic, its extern ABI must be one of those listed in [items.extern.variadic.conventions].

r[type.fn-pointer.extern-custom] An extern "custom" function pointer must follow the rules in [items.fn.extern.custom.signature].

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.