Disjoint Impls

Crate will be maintained (at least) until this idiom is allowed by the Rust compiler directly

Description

Enables writing non-overlapping (disjoint) impls distinguished by a set of associated types.

Works for trait and inherent implementations alike (no special syntax).

Trait implementations

use disjoint_impls::disjoint_impls;

pub trait Dispatch {
    type Group;
}

disjoint_impls! {
    pub trait Kita {}

    impl<T: Dispatch<Group = u32>> Kita for T {}
    impl<T: Dispatch<Group = i32>> Kita for T {}
}

Inherent implementations

use disjoint_impls::disjoint_impls;

pub trait Dispatch {
    type Group;
}

struct Wrapper<T>(T);

disjoint_impls! {
    impl<T: Dispatch<Group = u32>> Wrapper<T> {}
    impl<T: Dispatch<Group = i32>> Wrapper<T> {}
}

Foreign(remote) traits

For traits defined outside the current crate (a.k.a. foreign or remote traits), duplicate the trait definition inside the macro and annotate it with #[disjoint_impls(remote)].

use disjoint_impls::disjoint_impls;
// A foreign trait must be brought into scope so
// the `disjoint_impls!` macro can refer to it.
use remote_trait::ForeignKita;

pub trait Dispatch {
    type Group;
}

// (orphan rule): You can define blanket impls only
// for types that are defined in the current crate
pub struct LocalType<T>(T);

disjoint_impls! {
    #[disjoint_impls(remote)]
    pub trait ForeignKita {
        fn kita() {
            unreachable!("method is accessed through the original trait");
        }
    }

    impl<T: Dispatch<Group = u32>> ForeignKita for LocalType<T> {}
    impl<T: Dispatch<Group = i32>> ForeignKita for LocalType<T> {}
}

Other, much more complex examples, can be found in tests.