Code

fn foo() -> Result<(), core::convert::Infallible> {
    todo!()
}

fn main() {
    match foo() {
        Ok(()) => {},
        Err(e) => match e {}
    }
}

Current output

warning: unreachable pattern
 --> src/lib.rs:8:9
  |
8 |         Err(e) => match e {}
  |         ^^^^^^
  |
  = note: this pattern matches no values because `Infallible` is uninhabited
  = note: `#[warn(unreachable_patterns)]` on by default

Desired output

Until the 2024 edition, or the code's MSRV is ≥ 1.82, no warning.

Rationale and extra context

Deleting the Err(e) => match e {} arm will produce code which does not warn on nightly/1.82, but which does not compile on 1.80. This means that users which wish to keep a warning-free-ish nightly (or beta, eventually) build will have to #[allow(unreachable_patterns)] from now until 1.82 is released, or perhaps even until their next MSRV increase.

Deferring the warning until an edition or explicit MSRV increase seems to me to be a good way to avoid churn in the form of #[allow]s. Such #[allow]s are particularly dangerous because unreachable_patterns is also the lint that detects binding patterns that were intended to be matching against constants instead, so declaring it on a wide scope might hide serious bugs, especially if (as is likely) they're not promptly removed upon the release of 1.82.

Rust Version

1.82.0-nightly (2024-08-11 41dd149fd6a6a06795fc)

Anything else?

I noticed this due to my CI against nightly, but comments on the stabilization PR #122792 (comment) mentioned it first. I'm filing this issue to make sure it is not overlooked by being only visible in that comment thread.