Extra `non_snake_case` warning when punning field name in pattern

[cole-miller]

Here's a minimized version of the code I'm working with:

#[derive(Clone, Copy)]
#[allow(non_snake_case)]
struct Entry {
    Δ: u16,
    _θ: u16,
    _μ: u8,
}

static TABLE: [Entry; 1] = [Entry { Δ: 0, _θ: 0, _μ: 0 }];

pub mod inner {
    pub fn f(k: usize) -> u16 {
        let super::Entry { Δ, _θ, _μ } = super::TABLE[k]; /* here */
        Δ
    }
}

Compiling this on stable or nightly gives a non_snake_case warning on the indicated line, despite the allow(non_snake_case) attribute on struct Entry. A similar issue was reported before in #66362, and addressed in a follow-up PR #66660, but it seems to be cropping up again here, maybe because a non-ASCII field name is involved. It was agreed in #66362 that punning of non-snake-case fields in patterns should not produce an additional warning, so I think this is a bona fide bug. (Incidentally, on stable the same warning appears twice in the compiler output, but this duplication seems to be fixed on nightly.)

[FabianWolff]

It's actually unrelated to the non-ASCII field names; reduced:

#[allow(non_snake_case)]
struct Entry {
    A: u16,
    a: u16
}

fn foo() -> Entry {todo!()}

pub fn f() {
    let Entry { A, a } = foo();
    let _ = (A, a);
}

@rustbot claim

[cole-miller]

Thanks @FabianWolff!