clippy::needless_range_loop: the loop variable `i` is only used to index `vec`

What you see

warning: the loop variable `i` is only used to index `vec`
 --> main.rs:4:10
  |
4 | for i in 0..vec.len() {
  |          ^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#needless_range_loop
  = note: `#[warn(clippy::needless_range_loop)]` on by default
help: consider using an iterator
  |
4 - for i in 0..vec.len() {
4 + for <item> in &vec {
  |

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

Just iterating the collection itself makes the intent more clear and is probably faster because it eliminates the bounds check that is done when indexing.

Code that works but is not idiomatic Rust. Warns by default.

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

let vec = vec!['a', 'b', 'c'];
for i in 0..vec.len() {
    println!("{}", vec[i]);
}

Write this instead

let vec = vec!['a', 'b', 'c'];
for i in vec {
    println!("{}", i);
}

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

#[allow(clippy::needless_range_loop)]
fn my_function() { /* ... */ }

[lints.clippy]
needless_range_loop = "allow"

cargo clippy -- -A clippy::needless-range-loop

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.