New lint: swap_with_temporary

[samueltardieu]

This lint detects inefficient or useless {std,core}::mem::swap() calls such as:

    // Should be `a = temp();`
    swap(&mut a, &mut temp());
    // Should be `*b = temp();`
    swap(b, &mut temp());
    // Should be `temp1(); temp2();` if we want to keep the side effects
    swap(&mut temp1(), &mut temp2());

It also takes care of using a form appropriate for a () context if swap() is part of a larger expression (don't ask me why this wouldn't happen, I have no idea), by suggesting { x = y; } (statement in block) or {std,core}::mem::drop((temp1(), temp2()).

changelog: [swap_with_temporary]: new lint

Close #1968

[rustbot]

r? @y21

rustbot has assigned @y21.
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

[samueltardieu]

Rebased

[samueltardieu]

Rebased.

[samueltardieu]

Rebased

[samueltardieu]

Rebased

[y21]

Are there any cases where linting if the borrowed value is from an expansion would result in FPs? Not linting in this case prevents linting something like swap(&mut v, &mut vec![1]); which seems like it could be useful

[samueltardieu]

I was worried that some macros may sometimes return a temporary value and some other times return a non-temporary value. But indeed, if I don't check this, many more cases get linted.

I'll do a test run where I remove the expansion check in a second commit just to show how it behaves, and to get lintcheck results.

[samueltardieu]

Ok, that went well, I've squashed the commits. This lint now supports expanded arguments as well, thanks for the idea.

[y21]

This seems more like a complexity lint to me since it's just an unnecessarily complicated way to assign or drop. Does this actually make a difference in performance?

[samueltardieu]

Probably not, at least not a significant one. I'll switch it to complexity.

[samueltardieu]

Rebased

[samueltardieu]

@y21 Anything else needs to be done?

[samueltardieu]

Rebased

[samueltardieu]

@y21 should I reassign?

[y21]

Sorry, I've been meaning to get to this. I'll try to look into this again in the next few days!