closure upvars disable `unused_assignments` lint

[lcnr]

fn main() {
    let mut overwritten = false;
    let _ = || overwritten = true;
}

does not trigger any lints while

fn main() {
    let mut overwritten = false;
    if false {
        overwritten = true;
    }
}

results in

warning: variable `overwritten` is assigned to, but never used
 --> src/main.rs:2:13
  |
2 |     let mut overwritten = false;
  |             ^^^^^^^^^^^
  |
  = note: consider using `_overwritten` instead
  = note: `#[warn(unused_variables)]` on by default

The unused_variables and lint should look into nested closures here.

[surechen]

@rustbot claim

[surechen]

@rustbot release-assignment