`while_let_on_iterator` does not consider the borrow checker

[jyn514]

This output is from running clippy on https://github.com/rust-lang/stdarch/blob/598e632e8f62ff5054dfe0d0fabb2a03ecee0a3d/crates/stdarch-test/src/disassembly.rs#L102-L116. The relevant snippet is

    while let Some(header) = cached_header.take().or_else(|| lines.next()) {
        while let Some(instruction) = lines.next() {
        }
    }

$ cargo clippy --fix -Z unstable-options
after fixes were automatically applied the compiler reported errors within these files:

  * crates/stdarch-test/src/disassembly.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see 
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/rust/issues
quoting the full output of this command we'd be very appreciative!
Note that you may be able to make some more progress in the near-term
fixing code with the `--broken-code` flag

The following errors were reported:
error[E0382]: borrow of moved value: `lines`
   --> crates/stdarch-test/src/disassembly.rs:117:59
    |
105 |     let mut lines = output.lines();
    |         --------- move occurs because `lines` has type `std::str::Lines<'_>`, which does not implement the `Copy` trait
...
117 |     while let Some(header) = cached_header.take().or_else(|| lines.next()) {
    |                                                           ^^ ----- borrow occurs due to use in closure
    |                                                           |
    |                                                           value borrowed here after move
...
125 |         for instruction in lines {
    |                            -----
    |                            |
    |                            value moved here, in previous iteration of loop
    |                            help: consider borrowing to avoid moving into the for loop: `&lines`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0382`.
Original diagnostics will follow.

warning: this loop could be written as a `for` loop
   --> crates/stdarch-test/src/disassembly.rs:125:9
    |
125 |         while let Some(instruction) = lines.next() {
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `for instruction in lines`
    |
    = 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: this loop could be written as a `for` loop
   --> crates/stdarch-test/src/disassembly.rs:125:9
    |
125 |         while let Some(instruction) = lines.next() {
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `for instruction in lines`
    |
    = 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

[ThibsG]

Related issue: #5844

@rustbot label +T-MIR

[jyn514]

I think this might be fixed by recommending for line in lines.by_ref() instead, but that's uglier in the common case where you don't need it.

[ThibsG]

Maybe for line in &mut lines could work also?

Playground using example given in #5844