clone().iter_mut() is rarely what you want

[steventrouble]

What it does

This lint warns you when you've called .clone().iter_mut() without saving the result anywhere.

You rarely want to mutate a cloned value without first saving the data somewhere. You can't access the data if it's not stored in a variable, so mutating it would be a noop.

Advantage

• Avoids accidental noops
• Almost never necessary to use .clone().iter_mut()
• .clone().iter_mut() is relatively unused
  • On Github, only 290 Rust files use it, out of 590M.

Drawbacks

• Someone might use .clone().iter_mut() just for convenience when one of the functions in the for loop takes a mutable reference.
  • E.g. vec.append(&mut x)
  • Usually a non mutable version exists (e.g. vec.extend)

Example

fn add_one(a : &mut Vec<usize>) {
    for x in a.clone().iter_mut() {
        // oops, this does nothing
        *x += 2;
    }
}

Could be written as:

fn add_one(a : &mut Vec<usize>) {
    for x in a.iter_mut() {
        *x += 2;
    }
}

---

fn added_one(a : &Vec<usize>) -> Vec<usize> {
    // no need for it to be mutable
    a.clone().iter_mut().map(|&mut x| x + 1).collect()
}

Could be written as:

fn added_one(a : &Vec<usize>) -> Vec<usize> {
    a.iter().map(|&x| x + 1).collect()
}

[Centri3]

Could probably lint stuff like vec![1, 2].clone().get_mut() as well