rust-lang / rust-lang/rust

`unreachable_patterns` emitted on `_` that matches a local of uninhabited type (that is uninitialized)

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A-lints C-bug L-unreachable_patterns T-compiler
Dominant language
Rust
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Description

I tried this code:

enum Void {}

fn main() {
    let void: Void;
    match void {
        _ => (),
    }
}

(playground)

I expected to see this happen: the code compiler without warnings and successfully runs doing nothing.

Instead, this happened: a warning is emitted (the code compiles and runs as expected):

warning: unreachable pattern
 --> src/main.rs:6:9
  |
6 |         _ => (),
  |         ^-------
  |         |
  |         matches no values because `Void` is uninhabited
  |         help: remove the match arm
  |
  = note: to learn more about uninhabited types, see https://doc.rust-lang.org/nomicon/exotic-sizes.html#empty-types
  = note: `#[warn(unreachable_patterns)]` on by default
Meta

rustc version: 1.85.0-nightly (2024-12-23 bdc6b3de48646345549f)

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First steps

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Research direction

Reproduce the warning from src/main.rs using the linked Rust Playground example and the reported nightly version. Compare the compiler's unreachable-pattern result for the uninitialized local with the expected behavior. Done means the example still compiles and runs without emitting the unreachable_patterns warning.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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