Macros for dispatching based on generic types.
```rust struct GenericStruct(A, B);
// Creates 4 trait implementations.
impl Into
).len() as C
}
}
let x: usize = GenericStruct(1u64, 3u32).into(); asserteq!(x, 7); let x: u64 = GenericStruct(1u64, 3u8).into(); asserteq!(x, 5); let x: usize = GenericStruct(1u16, 3u32).into(); asserteq!(x, 7); let x: u64 = GenericStruct(1u16, 3u8).into(); asserteq!(x, 8); ```