MIR enabled rustc nightly warns about error in unreachable code

[nwin]

Compiling the following code using rustc+mir

fn main() {
    let a = [1];
    if 1 == 1 {
        println!("{}", a[0])
    } else {
        println!("{}", a[1])
    }
}

emits the following warning

warning: this expression will panic at run-time
 --> <anon>:6:24
  |
6 |         println!("{}", a[1])
  |                        ^^^^ index out of bounds: the len is 1 but the index is 1
<std macros>:2:27: 2:58 note: in this expansion of format_args!
<std macros>:3:1: 3:54 note: in this expansion of print! (defined in <std macros>)
<anon>:6:9: 6:29 note: in this expansion of println! (defined in <std macros>)

although the else branch is unreachable. Switching off MIR by using -Z orbit=off causes the warning to disappear.

Edit: It seems that rustc lost the ability to detect this unreachable branch. Using if true { instead causes the warning to disappear as well.

[nagisa]

Constant propagation should help with this.

[eddyb]

@nwin rustc never actually checked, this warning is a new MIR feature.
We do need constant propagation to make it not trigger in cases like this.

[Mark-Simulacrum]

This is still the case today; however, I'm not sure that there's much purpose in keeping this issue around since it doesn't track anything specific other than "errors in unreachable code need to not happen," which as far as I can tell is really only fixable with constant propagation -- which this doesn't track.

[Mark-Simulacrum]

I'm going to close -- there's nothing inherently wrong with warning about unreachable code, and this issue isn't going to influence constant propagation being introduced into MIR/rustc.