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Irrefutable slice patterns should be inferred as arrays of that length #76342

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@scottmcm

Simple demo https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=5b0acd8b21c773c9c4a3ff73bc7a030e:

#![allow(unused_variables)]
struct Zeroes;
impl Into<[usize; 2]> for Zeroes {
    fn into(self) -> [usize; 2] { [0; 2] }
}
impl Into<[usize; 3]> for Zeroes {
    fn into(self) -> [usize; 3] { [0; 3] }
}
impl Into<[usize; 4]> for Zeroes {
    fn into(self) -> [usize; 4] { [0; 4] }
}
fn main() {
    let [a, b, c] = Zeroes.into(); // ERROR: type annotations needed
    let [d, e, f]: [_; 3] = Zeroes.into(); // Works great
}

This is presumably not happening because slice patterns in refutable patterns can be more than just an array of that exact size, but there's no other option for irrefutable patterns.

This would be particularly helpful as we're getting more and more [T; N]: TryFrom<_> implementations with const generics. See the example in #76310 for the place I first noticed this not working the way I'd hoped.

Zulip topic: https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Type.20inferrence.20.26.20irrefutable.20slice.20patterns/near/209052937

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A-arrayArea: `[T; N]`A-const-genericsArea: const generics (parameters and arguments)A-inferenceArea: Type inferenceC-enhancementCategory: An issue proposing an enhancement or a PR with one.E-mediumCall for participation: Medium difficulty. Experience needed to fix: Intermediate.

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