mem::forget on !Drop

[Manishearth]

We should warn if you call mem::forget on:

• a generic type known to be Copy
• a non generic type known to not be Drop

since it's a no op, and likely means you mem::forgot the wrong thing.

(I hit this issue today)

[oli-obk]

https://github.com/rust-lang-nursery/rust-clippy/wiki#drop_copy ;)

There's a use case for types that are not Drop: if the type is !Copy, then you just got rid of it. But we might prefer to suggest let _ = foo; instead, not sure?

[Manishearth]

Yeah, that makes sense