Incorrect "label name shadows a name already in scope" warning

[AaronC81]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=88ae8b7f1d4eb4b2fb318e957cd3695d

fn foo(x: u32) {
    match x {
        1 => {
            'outer: loop { break 'outer }
        }
        _ => {
            'outer: loop { break 'outer }
        }
    }
}

The current output is:

warning: label name `'outer` shadows a label name that is already in scope
 --> src/lib.rs:7:13
  |
4 |             'outer: loop { break 'outer }
  |             ------ first declared here
...
7 |             'outer: loop { break 'outer }
  |             ^^^^^^ label `'outer` already in scope

I don't think this warning should be emitted, since as far as I understand there's no valid way that one match arm's 'outer label could be used from the other match arm. So I don't think there is shadowing taking place here, but rather defining two entirely separate labels in two entirely separate scopes.

[ehuss]

This is currently intended behavior. See #21633 and https://internals.rust-lang.org/t/psa-rejecting-duplicate-loop-labels/1833 for more. There was an early intent that labels have function scope instead of block scope.

Closing as a duplicate of #31745, #81799, and #88225.