Decide whether or not to have #[must_use] on PartialEq

[alexcrichton]

This is a "tracking issue" sort for #[must_use] annotations added in #43728. Mostly this is a placeholder to ensure this comes up at the next libs triage meeting! the libs team will want to make a decision here one way or the other during the next triage meeting.

[alexcrichton]

Discussed at libs triage conclusion was these are good to go!