Incorrect unused_mut in let binding

[bartek358]

Summary

When destructuring a &mut reference to a Copy type in a let binding, clippy considers using a mut binding to be needless if the variable is not mutated later. However, the binding with mut causes dereference and shortens the borrow, so its removal might lead to borrow checking errors.

Lint Name

unused_mut

Reproducer

playground

#[allow(unused_assignments, unused_variables, clippy::disallowed_names)]
fn main() {
    let mut foo = (1, 2);
    let (bar, mut baz) = &mut foo;
    *bar = 3;
    foo = (4, 5);
    println!("{baz}");
}

Version

rustc 1.84.0-nightly (59cec72a5 2024-11-08)
binary: rustc
commit-hash: 59cec72a57af178767a7b8e7f624b06cc50f1087
commit-date: 2024-11-08
host: aarch64-apple-darwin
release: 1.84.0-nightly
LLVM version: 19.1.3

Additional Labels

No response

[y21]

unused_mut is a builtin rustc lint, so moving to the rustc issue tracker

[bartek358]

Actually, now that you pointed that out I see it is a duplicate of #113451