Weak keyword union oddity

[cyqsimon]

The "weak keyword" (see https://doc.rust-lang.org/reference/keywords.html#weak-keywords) union when used in the context of the function HashSet::union, sometimes receives erroneous highlighting.

Problematic code sample

use std::collections::HashSet;

fn main() {
    let set_a = HashSet::from([0, 1]);
    let set_b = HashSet::from([2, 3]);

    let set_c: HashSet<_> = set_a.union(&set_b).collect();
    dbg!(set_c);

    let set_d: HashSet<_> = [set_a, set_b]
        .into_iter()
        .reduce(|acc, set| acc.union(&set).cloned().collect())
        .unwrap();
    dbg!(set_d);
}

Observe that the two calls to HashSet::union are highlighted differently.

P.S. This issue was originally raised here: microsoft/vscode#141141