New lint for statement with no effect

[sanxiyn]

This is an implementation of rust-lang/rust#16170.

Many extensions are possible (tuples, for example), but this implements just struct/enum constructors.

[Ryman]

Theoretically, if any of these unit structs implemented Drop it could have an effect?

[sanxiyn]

Yes, Drop should be considered. On the other hand, HasDrop; is still worth warning, because the destructor will run immediately, which is usually not the intention. You need to use let to run the destructor at the end of the block.

[llogiq]

I too think that at least the lint message should be different when the constructed type implements Drop. Perhaps even give it another lint specifier – what about naked_drop?

[Manishearth]

Personal preference is that we should not have an extra lint -- I'm okay with both warning and not warning on Drop.

[Manishearth]

As far as the rest of it goes, we should add a macro check (in_macro), otherwise LGTM

[sanxiyn]

Added the macro check. Leaving warning on Drop as-is, we can open a follwup issue for that. See also discussion on Rust's built-in lint re Drop at rust-lang/rust#21775.

[llogiq]

LGTM.