Suggestion from needless_range_loop removes too many array index operations

Summary

Clippy's suggestion for a needles_range_loop lint where the loop body indexes doubly-nested Vecs removes both sets of square brackets, instead of only the last one.

Reproducer

Playground link: https://play.rust-lang.org/?version=beta&mode=debug&edition=2024&gist=7b400efc4213e580381fa0fccf93459a

I tried this code:

use std::iter::repeat_n;

fn main() {
    let mut wire_polys = repeat_n(vec!['x'; 7], 10).collect::<Vec<_>>();
    let mut inp = repeat_n(String::new(), 10).collect::<Vec<_>>();
    for i in 0..10 {
        for j in 0..7 {
            wire_polys[i][j] = 'y';
        }
        inp[i] = format!("{i} {:?}", wire_polys[i]);
    }
}

This is minimized from https://github.com/divviup/libprio-rs/blob/574c75ecda60323e087f29e221862b1713459d6c/src/flp/gadgets.rs#L563-L568

I got the following suggestion:

warning: the loop variable `j` is only used to index `wire_polys`
 --> src/main.rs:7:18
  |
7 |         for j in 0..7 {
  |                  ^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/beta/index.html#needless_range_loop
  = note: `#[warn(clippy::needless_range_loop)]` on by default
help: consider using an iterator
  |
7 -         for j in 0..7 {
7 +         for <item> in wire_polys.iter_mut().take(7) {
  |

The for loop should instead iterate over wire_polys[i].iter_mut().take(7).

Version

rustc 1.91.0 (f8297e351 2025-10-28)
binary: rustc
commit-hash: f8297e351a40c1439a467bbbb6879088047f50b3
commit-date: 2025-10-28
host: x86_64-unknown-linux-gnu
release: 1.91.0
LLVM version: 21.1.2

Additional Labels

@rustbot label +I-suggestion-causes-error

Start with the needless_range_loop lint and reproduce the nested-Vec case using the linked Playground example. Compare the emitted suggestion with the requested wire_polys[i].iter_mut().take(7) form, then add coverage for the nested indexing case and verify that the suggestion no longer removes both index operations.