Attribute Macro substrate::type_dispatch::impl_dispatch

#[impl_dispatch]

Dispatches a trait implementation to a specified set of generic types.

Syntax

Sets are delimited with curly braces ({}) and have semicolon-separated elements. Sets are used to enumerate types to be assigned to a generic argument or set of generic arguments.

Comma-separated elements are combined using a cartesian product, making it easier to implement traits on any combination of the provided types.

Semantics

The supplied dispatch types are dispatched starting from the first generic type argument of the trait implementation.

Examples

struct GenericStruct<A, B>(A, B);

// Creates 4 trait implementations.
#[impl_dispatch({u64; u16}, {u32, usize; u8, u64})]
impl<A, B, C> Into<C> for GenericStruct<A, B> {
    fn into(self) -> C {
       self.0 as C + self.1 as C
   }
}

let x: usize = GenericStruct(1u64, 3u32).into();
assert_eq!(x, 4);
let x: u64 = GenericStruct(1u64, 3u8).into();
assert_eq!(x, 4);
let x: usize = GenericStruct(1u16, 3u32).into();
assert_eq!(x, 4);
let x: u64 = GenericStruct(1u16, 3u8).into();
assert_eq!(x, 4);

// The following two lines will not compile as `GenericStruct` does not implement
// these particular type combinations:
// ```
// let x: u64 = GenericStruct(1u64, 3u32).into();
// let x: usize = GenericStruct(1u64, 3u8).into();
// ```