Macro substrate::type_dispatch::dispatch_const

dispatch_const!() { /* proc-macro */ }

Dispatches a constant based on a given generic type.

Syntax

The syntax is effectively the same as a match statement, but the patterns are instead simply comma-separated lists of types.

Semantics

Unlike normal match statements, duplicate arms are not allowed. Only the constant corresponding to the unique matching arm will be dispatched.

dispatch_const! internally uses the DispatchConst trait to dispatch constant values, meaning that types do not have to match exactly (i.e. impl_dispatch might have std::vec::Vec while the dispatch_const! arm has Vec).

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 + dispatch_const!(
            match A, B {
                u64, u32 => 1: C,
                u64, u8 => 2: C,
                u16, u32 => 3: C,
                u16, u8 => 4: C,
            }           
        )
   }
}

let x: usize = GenericStruct(1u64, 3u32).into();
assert_eq!(x, 5);
let x: u64 = GenericStruct(1u64, 3u8).into();
assert_eq!(x, 6);
let x: usize = GenericStruct(1u16, 3u32).into();
assert_eq!(x, 7);
let x: u64 = GenericStruct(1u16, 3u8).into();
assert_eq!(x, 8);