false positive on unused_parens lint

[tomjakubowski]

fn main() {
    if (return) {}
}

sez:

<anon>:2:8: 2:16 warning: unnecessary parentheses around `if` condition, #[warn(unused_parens)] on by default
<anon>:2     if (return) {}
                ^~~~~~~~

However, removing the parentheses makes the file have illegal syntax, because {} is part of the return expression (so the if is missing its arm).

[tomjakubowski]

Also, by the way, it's very amusing to me that this is legal:

fn main() {
    if return { return } { return } else { return }
}

[samdoiron]

Triage: Warning still generated in nightly build. Seems to be an issue with contains_exterior_struct_lit not identifying return {} as an exterior expression.

[euclio]

Looks like this was fixed in #55166, and there's a test.