Lint for let _ = with Drop types?

[Ekleog]

Hello,

I had an uncomfortable moment today, noticing my let _ = mutex.lock() didn't actually keep a lock on the mutex for the scope (it's just a Mutex<()>, given it's for some unsafe mmapped shared resource, design choice may be criticized but I think irrelevant to the issue). Changing it to let _a = mutex.lock() fixed it.

I raised this up on #rust-unternals, and @nox and Mutabah helped me understand (I think) why this was a sensible design choice (that is, _ not moving the value but leaving it where it was, in this case the temporary).

Now, I think this is treacherous, and could lead to security issues when used unknowingly from unsafe code (or deadlocks or other things from normal code), as I did before knowing (thankfully discovered thanks to unit tests).

So I think maybe a warning lint could be added when assigning a Drop type to _, as this 1/ would maybe not be often legitimately done (I think?), and 2/ may have unexpected consequences on the drop order.

What do you think about this?

[F001]

For reference, this issue is in large part a duplication of #40096. And there is an issue in clippy https://github.com/Manishearth/rust-clippy/issues/1574 tracking this.

[nagisa]

Closing as a duplicate.