dead code lint should say "never constructed" for variants that are matched on but never constructed

[pnkfelix]

mod a {
    #[deriving(Show)]
    pub enum E {
        Variant1,
        Variant2,
    }
}

fn main() {
    let x = a::E::Variant1;
    let y = match x {
        a::E::Variant1 => 1i,
        a::E::Variant2 => 2i,
    };
    println!("Hello world: {}", (x, y));
}

produces:

<anon>:5:9: 5:17 warning: variant is never used: `Variant2`, #[warn(dead_code)] on by default
<anon>:5         Variant2,
                 ^~~~~~~~

[pnkfelix]

(arguably this may be a feature, since the arm associated with the pattern here is nontheless never used.)

[ghost]

@pnkfelix Yeah, this was intentional although I'm going to reopen this as I think we can do better on the wording. Used is indeed misleading, constructed is what the lint is proving.

[steveklabnik]

Triage: still an issue today. Updated code:

mod a {
    #[derive(Debug)]
    pub enum E {
        Variant1,
        Variant2,
    }
}

fn main() {
    let x = a::E::Variant1;
    let y = match x {
        a::E::Variant1 => 1,
        a::E::Variant2 => 2,
    };
    println!("Hello world: {:?}", (x, y));
}

[nox]

Could we get some action on that? Seems important, with all the ergonomics stuff going on.

[JohnyL]

Any news on fixing this?

enum Mode {
    Open,
    Close
}

fn main() {
    let mode = Mode::Open;
    let num = match mode {
        Mode::Open => 1,
        Mode::Close => 2
    };
}

Warning:

warning: variant is never constructed: Close
--> src\main.rs:3:5
|
3 | Close
| ^^^^^
|
= note: #[warn(dead_code)] on by default

warning: 1 warning emitted