Handle Result<T, !> and ControlFlow<!, T> as T wrt #[must_use]

[samueltardieu]

There is a proposal to change the behaviour of rustc's must_use lint to consider Result<T, U> and ControlFlow<U, T> as T when U is uninhabited. See rust-lang/rust#148214.

This might make the user adding extra #[must_use] attributes to functions returning Result<T, !> or ControlFlow<!, T>, which would trigger the double_must_use lint in Clippy without the current change.

changelog: [double_muse_use, drop_non_drop, let_underscore_must_use]: consider Result<T, U> and ControlFlow<U, T> as T wrt the #[must_use] attribute if U is uninhabited.

[rustbot]

r? @dswij

rustbot has assigned @dswij.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[github-actions]

No changes for 3ed43d8

[samueltardieu]

The second push was made to prevent suggesting using #[must_use] on types that are already #[must_use] even though they will be treated otherwise by rustc in the future (this is the key part). This was detected by lint check on the first iteration.