Non-exhaustive patterns lead to bogus "unused variable" / "dead code" warnings

[RalfJung]

Consider this code:

fn foo() -> Option<i32> { None }

pub fn bar(x: i32) {
    match foo() {
        Some(_) => return,
    }
    
    let _val = Box::new(x);
}

This generates the following diagnostics on current nightly:

warning: unreachable statement
 --> src/lib.rs:8:5
  |
8 |     let _val = Box::new(x);
  |     ^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unreachable_code)]` on by default

error[E0004]: non-exhaustive patterns: `None` not covered
 --> src/lib.rs:4:11
  |
4 |     match foo() {
  |           ^^^^^ pattern `None` not covered
  |
  = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms

warning: unused variable: `x`
 --> src/lib.rs:3:8
  |
3 | pub fn bar(x: i32) {
  |        ^ help: consider prefixing with an underscore: `_x`
  |
  = note: `#[warn(unused_variables)]` on by default

The "unused variable" and "dead code" lints are spurious, and not helpful. This only seems like dead code because there's a missing match arm. After adding _ => {} in the match, all diagnostics go away.

[shekhei]

I would like to try this.

What would be the preferred behavior? Should the "unused variable" / "dead code" warnings be suppressed for this case?

[Spoonbender]

Triage: no change