Extending `impl Trait` to allow multiple return types

language design
37

The OP proposal is a bit too magical for me and I am not sure if we should do it (though it’s indeed a cool concept), so I would like to comment a bit on enum based approach.

Personally I don’t like enum impl Trait syntax and think that it will make function signatures over-cluttered. I would like to suggest an alternative aproach: why not introduce a macro which will construct anonymous enum and will wrap returns? Code could look like this:

fn foo(flag: bool) -> impl Display {
    if flag {
        enum_impl!(1)
    } else {
        enum_impl!("foo")
    }
}

It will be lowered into:

fn foo(flag: bool) -> Self::AnonEnum {
    enum AnonEnum {
        B1(u32),
        B2(&'static str),
    }
    impl std::fmt::Display for AnonEnum { .. }

    if flag {
        AnonEnum::B1(1)
    } else {
        AnonEnum::B2("foo")
    }
}

This approach is quite explicit and does not clutter signatures though enum_impl! is a bit magical and I am not sure if it will be possible to implement it as not-builtin, but then question is do we need enum_impl! or should it just work.

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