NLL: unused_mut lint triggered by for _ in <unreachable>

[pnkfelix]

Consider the following code (play):

#![feature(nll)]
#![allow(unreachable_code)]

fn main() {
    for _ in { return (); 0..3 } {
    }
}

It signals the following diagnostic:

warning: variable does not need to be mutable
 --> src/main.rs:5:14
  |
5 |     for _ in { return (); 0..3 } {
  |              --^^^^^^^^^^^^^^^^^
  |              |
  |              help: remove this `mut`
  |
  = note: #[warn(unused_mut)] on by default

Which is pretty nonsensical in appearance.

(It almost certainly arises due to a legitimate mut appeared in the expansion of for. We probably just need to check whether a mut that has been introduced is due to legitimate source code or if its due to an internal expansion, and use that knowledge when we decide whether to signal the unused_mut lint.)

[pnkfelix]

putting on the RC2 milestone under the assumption that this should be relatively straight-forward to resolve.

[varkor]

This should be fixed in #54125, when it's merged.

[pnkfelix]

I'm going to also assign @varkor since they have been participating in discussion about this. Even if @varkor makes the main PR, @blitzerr might be helpful in e.g. confirming the quality of the regression test, etc.