A problem with needless_range_loop

[leonardo-m]

For code like:

fn main() {
    let mut v = [0; 10];
    for i in 0 .. v.len() {
        v[i] += i;
    }
}

Currently Clippy gives notes like:

warning: the loop variable `i` is used to index `v`
 --> .../main.rs:3:5
  |
3 | /     for i in 0 .. v.len() {
4 | |         v[i] += i;
5 | |     }
  | |_____^
  |
  = note: #[warn(needless_range_loop)] on by default
  = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.161/index.html#needless_range_loop
help: consider using an iterator
  |
3 |     for (i, <item>) in v.iter().enumerate() {
  |  

Replacing array-indexing Rust code with iterators sometimes helps the compiler remove some array bound tests (but in this case the compiler is able to remove then), but in general this code change increases the compilation time (and often the max amount of memory the compiler uses). This isn't a problem if the programmer replaces few loops-with-indexing with loops-with-iterators, but introducing hundreds or thousands of loops-with-iterators in a large Rust program could slow down the compilation a lot. So in my opinion Clippy needless_range_loop should be more careful and conservative in suggesting this kind of code change.

[ebroto]

Duplicate of #6075, leaving the new issue as it has more info.