Clippy reports a false error

[Makogan]

Summary

when running clippy it suggests to change a snippet which will result in non-compilation. It suggests that removing a clone is a better choice because the value is not consumed or mutated, and yet if the clone is removed the code won;t compile as the value is actually being mutated.

Reproducer

I tried this code:

pub fn conjugate_gradient(
    a: &CsrMatrix<f64>,
    threshold: f64,
    solution: &mut DVector<f64>,
) -> usize {
    let mut r = &*solution - a * &*solution;
    let mut iter_count = 0;
    let mut p = r.clone();

    loop {
        let alpha = r.dot(&r) / p.dot(&(a * &p));

        *solution = &*solution + alpha * &p;
        let r_next = &r - alpha * a * &p;

        let delta = r_next.norm_squared();
        debug_assert!(!delta.is_nan());

        if delta <= threshold {
            break;
        }

        let beta = r_next.dot(&r_next) / r.dot(&r);
        p = &r_next + beta * &p;
        r = r_next;

        iter_count += 1;

        // Arbitrary iteration count break.
        if iter_count > 1000 {
            break;
        }
    }

    iter_count
}

I expected to see this happen:
The clone is necessary, clippy should not suggest anything.

Instead, this happened:

warning: redundant clone
   --> crates/fse/src/classifier/classifier_rbf.rs:319:18
    |
319 |     let mut p = r.clone();
    |                  ^^^^^^^^ help: remove this
    |
note: cloned value is neither consumed nor mutated
   --> crates/fse/src/classifier/classifier_rbf.rs:319:17
    |
319 |     let mut p = r.clone();
    |                 ^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_clone
    = note: `#[warn(clippy::redundant_clone)]` on by default

You can clearly see both p and r mutate over the algorithm.

Version

rustc 1.70.0 (90c541806 2023-05-31)
binary: rustc
commit-hash: 90c541806f23a127002de5b4038be731ba1458ca
commit-date: 2023-05-31
host: x86_64-unknown-linux-gnu
release: 1.70.0
LLVM version: 16.0.2

Additional Labels

No response

[Jarcho]

Duplicate of #10577. redundant_clone is currently broken and in the nursery.