Document association of generic arguments in path segments

[ehuss]

There are some subtle behaviors around where generic arguments can appear in a path, particularly with enum variants. For example:

enum E<T> {
    V1 { x: T },
}

fn main() {
    // These seem to be the same.
    E::V1::<i32> { x: 1 };
    E::<i32>::V1 { x: 1 };
}

Are these exactly the same? I'm wondering if there are other oddities like this? This came up when looking at rust-lang/rust#154971.

#2165 is adding more documentation on generic arguments and how they are associated, but I don't think it covers this.

[fmease]

They're meant to be equivalent. Historically speaking, E::V1::<i32> was the original form and E::<i32>::V1 came second (but the details are slightly more messy). Years ago, some people advocated for deprecating one of the two forms but nothing ended up happening.

rust-lang/rust#154962 is only one example of a gross bug related to the "E::<…>::V form", rust-lang/rust#108224 is another one that I also fixed recently (in rust-lang/rust#154918).