Incorrect "variant is never used" lint

[LukasKalbertodt]

A "variant is never used" lint is incorrectly triggered, presumably due to optimizations on MIR. I know my code sample looks pretty stupid as no human would write this, probably. But this kind of code might be generated. Anyway, the lint is incorrect I'd say.

I tried this code:

enum Baz {
    Foo,
    Bar,
}

fn main() {
    let x = Baz::Foo;
    match x {
        Baz::Foo => {},
        Baz::Bar if true => {},
        _ => {}
    }
}

I expected to see this happen: no warning

Instead, this happened:

warning: variant is never used: `Bar`
 --> src/main.rs:3:5
  |
3 |     Bar,
  |     ^^^
  |
  = note: #[warn(dead_code)] on by default

Meta

Happens with

• 1.19 stable,
• 1.20 beta,
• 1.21 nightly

[hanna-kruppe]

Just to be clear: Bar isn't considered unused because the Bar arm is main not reached. It's considered unused because the variant is never constructed. If the variant is never constructed, it doesn't make any difference whether matches are prepared to handle it (and besides, those matches will never be executed -- by analogy, we also consider functions unused if they're called from other dead code only).

[LukasKalbertodt]

@rkruppe Oh right. Then I guess this can be closed, since it's a dupe of #19140