`needless_pass_by_ref_mut` should not trigger when values derived from the `&mut` are used in an unsafe block

Summary

Using values derived from a &mut inside of an unsafe block can still trigger the needless_pass_by_ref_mut lint.

It seems like it was partially addressed in #11586, which should already be present in 1.80, but that only seems to cover direct use of the &mut in an unsafe block, not values derived from the &mut.

Lint Name

needless_pass_by_ref_mut

Reproducer

I tried this code:

#![warn(clippy::needless_pass_by_ref_mut)]

use std::ptr::NonNull;

struct MyStruct {
    child: Option<NonNull<Self>>,
}

impl MyStruct {
    fn do_thing(&mut self) {
        if let Some(mut child) = self.child {
            unsafe { child.as_mut() }.do_thing()
        }
    }
}

I saw this happen:

this argument is a mutable reference, but not used mutably
for further information visit https://rust-lang.github.io/rust-clippy/master/
index.html#needless_pass_by_ref_mut
`-W clippy::needless-pass-by-ref-mut` implied by `-W clippy::nursery`
to override `-W clippy::nursery` add `#[allow(clippy::
needless_pass_by_ref_mut)]` (clippy needless_pass_by_ref_mut)

I expected to see this happen:

Nothing in this case. The compiler can't know if my unsafe usage of child is safe to call from a shared reference. Switching it to a shared reference could cause unsoundness.

Version

rustc 1.80.0 (051478957 2024-07-21)
binary: rustc
commit-hash: 051478957371ee0084a7c0913941d2a8c4757bb9
commit-date: 2024-07-21
host: x86_64-unknown-linux-gnu
release: 1.80.0
LLVM version: 18.1.7

Additional Labels

@rustbot label +I-suggestion-causes-bug

Start by reproducing the provided Rust example with the needless_pass_by_mut lint enabled, then trace the lint's handling of values derived from an &mut inside an unsafe block. Done means the reproducer no longer emits a warning while ordinary needless mutable references remain diagnosed, with a regression test covering the case.