Lint on useless transmutes

[Manishearth]

rust-lang/rust#11400

We should lint on transmutes between the same type, e.g. transmute::<u8,u8>(...)

The easiest thing to do here is to find ExprCalls which resolve to transmute, and checking if the expr_ty of the argument matches that of the whole call expression.

[llogiq]

This absolutely needs a macro check. And it should be mem::transmute, right? Just to make sure we don't catch anyone's own transmute(_) function, say, in a medieval fantasy game...

[Manishearth]

Well, yeah, match_def_path takes care of that

[apasel422]

I'm trying to implement this, but I'm wondering how to get a DefId from the Path in the ExprCall so that I can call match_def_path.

[Manishearth]

use the def_map in tcx on the NodeId of the call?

[apasel422]

Thanks! And to check if two types are equal?

[Manishearth]

ty::Ty has an Eq impl.

[Manishearth]

I can't recall if that actually works, or if it computes some arcane form of equality 😝. Worth a try though. If it works, see if it can also catch:

fn foo<T>(t: T) {
transmute::<T,T>(t);
}

or something.

[Manishearth]

It looks like it does a pointer comparison, which sound scary but since it's in an explicit impl I'll believe it. Probably because all the TySs are borrowed from the same cache/arena.

[llogiq]

Otherwise eq_op.rs should have a comparison function. It's not public yet, though.