non_exhaustive_omitted_patterns should not warn matches! macro

[taiki-e]

I tried this code:

#![feature(non_exhaustive_omitted_patterns_lint)]
#![warn(non_exhaustive_omitted_patterns)]

use std::sync::atomic::Ordering; // #[non_exhaustive] enum

pub fn is_seqcst(order: Ordering) -> bool {
    matches!(order, Ordering::SeqCst)
}

I expected to see this happen: no warning

Instead, this happened:

warning: some variants are not matched explicitly
 --> src/lib.rs:7:14
  |
7 |     matches!(order, Ordering::SeqCst)
  |              ^^^^^ patterns `std::sync::atomic::Ordering::Relaxed`, `std::sync::atomic::Ordering::Release`, `std::sync::atomic::Ordering::Acquire` and 1 more not covered
  |
  = help: ensure that all variants are matched explicitly by adding the suggested match arms
  = note: the matched value is of type `std::sync::atomic::Ordering` and the `non_exhaustive_omitted_patterns` attribute was found

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rustc --version --verbose:

rustc 1.75.0-nightly (2f1bd0729 2023-10-27)
binary: rustc
commit-hash: 2f1bd0729b74787f55d4cbc7818cfd787cd43a99
commit-date: 2023-10-27
host: aarch64-apple-darwin
release: 1.75.0-nightly
LLVM version: 17.0.3

---

@rustbot label +F-non_exhaustive_omitted_patterns_lint

[clarfonthey]

This feels like something that could be easily solved by just adding an allow for the lint inside the macro. I can't think of a situation where you would want this lint to fire here.

[Jules-Bertholet]

I attempted to add the allow, but it turns out to not be so simple. Log of my struggles:

1. Add #[allow(non_exhaustive_omitted_patterns)] above the match, inside the macro: compiler complains that non_exhaustive_omitted_patterns is unstable
2. Add #[allow_internal_unstable(non_exhaustive_omitted_patterns_lint)]: doesn't change anything
3. Patched compiler to make allow_internal_unstable work with lints: doesn't help the bootstrap compiler
4. Use #[cfg_attr(not(bootstrap), allow(non_exhaustive_omitted_patterns)] instead: compiler complains that #![feature(stmt_expr_attributes)] is unstable
5. Add #[allow_internal_unstable(stmt_expr_attributes)]: but stmt_expr_attributes doesn't work with allow_internal_unstable either
6. Patch compiler again to make stmt_expr_attributes work with allow_internal_unstable: but that does nothing for the bootstrap compiler
7. A cfg_attred-out attribute still requires stmt_expr_attributes, so #[cfg_attr(not(bootstrap), ...] can't save us here. The entire macro definition would need to be duplicated and cfg-gated. (I gave up at this point)

@rustbot label -E-easy