Cannot assign twice to immutable variable with 'l: while break 'l {} as initializer

[Centril]

With:

fn _f() {
    let _x = 'label: while break 'label {};
}

we get:

warning[E0384]: cannot assign twice to immutable variable `_x`
 --> src/lib.rs:2:14
  |
2 |     let _x = 'label: while break 'label {};
  |         --   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot assign twice to immutable variable
  |         |
  |         first assignment to `_x`
  |         help: make this binding mutable: `mut _x`
  |
  = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
  = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future
  = note: for more information, try `rustc --explain E0729`

This seems wrong. At least from a source view, _x is assigned to once.

Current MIR dump:

fn  _f() -> () {
    let mut _0: ();                      // return place in scope 0 at src/lib.rs:1:9: 1:9
    let _1: ();                          // "_x" in scope 0 at src/lib.rs:2:9: 2:11
    scope 1 {
    }

    bb0: {
        StorageLive(_1);                 // bb0[0]: scope 0 at src/lib.rs:2:9: 2:11
        StorageDead(_1);                 // bb0[1]: scope 0 at src/lib.rs:3:1: 3:2
        return;                          // bb0[2]: scope 0 at src/lib.rs:3:2: 3:2
    }
}

This might possibly be fixed by #61988.

cc @eddyb @pnkfelix @matthewjasper

[pnkfelix]

@eddyb points out that the first bullet item of PR #61872 says that it fixes this.

[Centril]

Great. Then we'll just need to add a regression test to https://github.com/rust-lang/rust/blob/07e5297faf2966a77c466b0b5ad936c0deb828ac/src/test/ui/nll/assign-while-to-immutable.rs once the PR is merged.

[eddyb]

@Centril once it's merged? @matthewjasper could add this test and close this issue as part of the PR.