needless_range_loop makes suggestions that are impossible to implement

[koelzk]

Summary

The following code triggers needless_range_loop but it suggests code changes that do not work in this case.

Lint Name

needless_range_loop

Reproducer

I tried this code, which creates a symmetric nxn matrix:

pub fn create_distance_matrix(n: usize, distance: fn(usize, usize) -> f64) -> Vec<Vec<f64>> {
    let mut matrix: Vec<Vec<f64>> = vec![vec![0_f64; n]; n];

    for i in 0..n {
        for j in i+1..n {
            let cost = distance(i, j);

            matrix[i][j] = cost;
            matrix[j][i] = cost;
        }
    }

    matrix
}

I saw this happen:

warning: the loop variable `i` is used to index `matrix`
   --> src/tsp.rs:172:14
    |
172 |     for i in 0..n {
    |              ^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.92.0/index.html#needless_range_loop
help: consider using an iterator and enumerate()
    |
172 -     for i in 0..n {
172 +     for (i, <item>) in matrix.iter_mut().enumerate().take(n) {
    |

warning: the loop variable `j` is used to index `matrix`
   --> src/tsp.rs:173:18
    |
173 |         for j in i+1..n {
    |                  ^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.92.0/index.html#needless_range_loop
help: consider using an iterator and enumerate()
    |
173 -         for j in i+1..n {
173 +         for (j, <item>) in matrix.iter_mut().enumerate().take(n).skip(i+1) {

The lint suggests to work with iter_mut().enumerate(). If I try to do that, I run into borrow checker problems as the inner loop assigns values to 2 cells in different rows.

I expected to see this happen:

Rewriting the code without range loop does not seem to be straight forward. The lint should not suggest an approach that does not work.

Version

cargo 1.92.0 (344c4567c 2025-10-21)
release: 1.92.0
commit-hash: 344c4567c634a25837e3c3476aac08af84cf9203
commit-date: 2025-10-21
host: x86_64-unknown-linux-gnu

Additional Labels

No response

[mrchilliballs]

@rustbot claim