Erroneous warning for loop name shadowing in separate match arms

[insanitybit]

http://is.gd/XAabFg

Despite the loop names not shadowing each other, as they exist within separate match arms, there's a warning for this code.

:13:13: 15:14 warning: label name 'l shadows a label name that is already in scope

[utkarshkukreti]

Shorter test case:

fn main() {
    match Some(0) {
        Some(_) => 'a: loop { break },
        None    => 'a: loop { break }
    }
}

Warnings:

<anon>:4:20: 4:38 warning: label name `'a` shadows a label name that is already in scope
<anon>:4         None    => 'a: loop { break }
                            ^~~~~~~~~~~~~~~~~~
<anon>:3:20: 3:38 note: shadowed label `'a` declared here
<anon>:3         Some(_) => 'a: loop { break },

[petrochenkov]

This is intentional (at least currently). Labels in a function are supposed to be uniquely named regardless of scoping. The warning message could be better though.

cc #24162

[GuillaumeGomez]

It seems very clear to me. In which way could it be improved?

[jonas-schievink]

Maybe "shadows another label already defined in this function"?

[GuillaumeGomez]

@jonas-schievink: That cannot be this because a scope is more precise than a function. Example:

fn labels() {
    {
        'a: for i in 0..10 {}
    }
    {
        'a: for i in 0..10 {}
    }
}

Same function, but not same scopes. Do you understand better?

[jonas-schievink]

The linked PR #24162 intentionally makes duplicated labels emit a warning (even when they mark different scopes)

[GuillaumeGomez]

Hum... Well then, if you want to avoid this in a function scope, the better would then be to add a help message after the warning: "To keep your code easy to read an maintain, try to avoid using the same label name in a function scope".

[petrochenkov]

Duplicate of #24278