fix unused_braces suggestion for wildcard assignment

[pbkx]

Fixes #116536.

The unused_braces lint could suggest removing braces from wildcard assignments like:

_ = { e };

producing:

_ = e;

These are not equivalent: _ = { e }; moves/drops e, while _ = e; does not. This PR suppresses unused_braces for assignment RHS blocks when the assignment LHS contains _.

ui regression test covers both the simple wildcard assignment case and a tuple destructuring assignment case

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[TaKO8Ki]

Thank you. I left one comment.

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[TaKO8Ki]

I think this lint should not be triggered in some other cases like (..) = { e } [..] = { e }. Can we address them here as well?

#![deny(unused_braces)]

struct D(&'static str);

impl Drop for D {
    fn drop(&mut self) {
        println!("drop {}", self.0);
    }
}

fn main() {
    let e = (D("a"), D("b"));
    (..) = { e };
}

[rust-bors]

☔ The latest upstream changes (presumably #158720) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.