Incorrect label shadow warning

[WiSaGaN]

I tried this code:

fn main() {
    if true {
        'l : loop {
            break 'l;
        }
    } else {
        'l : loop {
            break 'l;
        }
    }
}

I expected to see this happen: no warning regarding to label

Instead, this happened: got warning about label shadowing

warning: label name `'l` shadows a label name that is already in scope
 --> src/main.rs:7:9
  |
3 |         'l : loop {
  |         -- first declared here
...
7 |         'l : loop {
  |         ^^ label `'l` already in scope

version

Build using the Nightly version: 1.56.0-nightly

(2021-08-21 d3e2578c31688619ddc0)

playground

[ehuss]

Thanks for the report! This is currently intentional, and a known issue. The warning could be clearer, though. Closing as a duplicate of #24278 and #31745.

[WiSaGaN]

Ok. I digged up an exact duplicate here: #81799

This warning is very unintuitive. I got that this is intended in the links provided, but could not find out why, i.e. what harm does it do if I write it this way. @ehuss can you provide a link to the reason why this is intended that this can be discussed?

[ehuss]

I don't think there is any information beyond what is linked in those issues. This was something added before 1.0, and it hasn't been revisited since. The why is primarily explained in https://internals.rust-lang.org/t/psa-rejecting-duplicate-loop-labels/1833, where there was an intent to add explicit syntax for labeling a region for a borrow, but that never got beyond the idea stage (AFAIK). #24278 also explains how there are macro hygiene problems.