Add #[must_use] to Option and Result overlap methods

[bobrippling]

This helps users avoid the dropping of errors/None without checking, for example Miserlou/Loop/pull/66.

[m-ou-se]

Unfortunately, it's quite common now to use .ok() to ignore a Result. Changing this would break start warning in quite a lot of existing code.

See #66116 for discussion.

(By the way, the #[must_use] attributes you added on the Option methods have no effect. They return a Result, which is already #[must_use].)

[bobrippling]

Ah, thanks for the explanation and link, I was wondering why I was finding so many discard usages!

[m-ou-se]

@bobrippling That doesn't mean that this is something we can never change, but it does require some more discussion first. Feel free to join the discussion about this on the issue and share your thoughts there. :)