ore: Add must_use to drain_filter_swapping

[jkosh44]

This commit adds the #[must_use] annotation to the drain_filter_swapping function. drain_filter_swapping is evaluated lazily, so if the result is not used then it has no effect, and it is almost certainly a mistake. The annotation helps avoid this mistake by emitting a compiler warning.

Motivation

This PR refactors existing code.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:
  • There are no user-facing behavior changes.

[teskje]

Seems like a good hotfix to prevent bugs, but the documentation of this method is misleading too:

    /// Using this method is equivalent to the following code:
    ///
    /// ```
    /// # let some_predicate = |x: &mut i32| { *x == 2 || *x == 3 || *x == 6 };
    /// # let mut vec = vec![1, 2, 3, 4, 5, 6];
    /// let mut i = 0;
    /// while i < vec.len() {
    ///     if some_predicate(&mut vec[i]) {
    ///         let val = vec.swap_remove(i);
    ///         // your code here
    ///     } else {
    ///         i += 1;
    ///     }
    /// }
    ///
    /// # assert_eq!(vec, vec![1, 5, 4]);
    /// ```

This should instead say something like "using this method and consuming the iterator is equivalent to the following code:". Also it might be nice to have a little warning in the # Note section as well.

[ParkMyCar]

LGTM

You can also add a msg to the must_use annotation to explain why it must be used