False positive in clippy::while_let_on_iterator

[bertptrs]

I tried this code: (simplified, the original made more sense)

fn main() {
    let mut it = 1..40;

    while let Some(n) = it.next() {
        while let Some(m) = it.next() {
            if m % 10 == 0 {
                break;
            }

            println!("doing something with m: {}", m);
        }

        println!("n still is {}", n);
    }
}

I expected to see this happen: this passes cargo clippy just fine.

Instead, this happened: it suggests an invalid improvement:

warning: this loop could be written as a `for` loop
 --> src/main.rs:5:29
  |
5 |         while let Some(m) = it.next() {
  |                             ^^^^^^^^^ help: try: `for m in it { .. }`
  |
  = note: `#[warn(clippy::while_let_on_iterator)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#while_let_on_iterator

warning: 1 warning emitted

This moves out of it, which isn't allowed because it's needed in the next loop iteration.

Meta

• cargo clippy -V: clippy 0.0.212 (891e1a8 2020-04-18)
• rustc -Vv:

  rustc 1.48.0
  binary: rustc
  commit-hash: unknown
  commit-date: unknown
  host: x86_64-unknown-linux-gnu
  release: 1.48.0
  LLVM version: 11.0
  

• Installed from https://archlinux.org/packages/extra/x86_64/rust/

[giraffate]

This is similar to #5844.

[ThibsG]

@rustbot label +T-MIR