vec: avoid some unsafe code in MoveIterator's dtor

[thestinger]

No description provided.

[erickt]

This for loop is what was causing my 30% hit I mentioned in #10988. Would it be possible to wrap the for loop in a needs_drop::<T>() if-block (assuming that's the right intrinsic)? That appears to allow my micro-benchmark to generate the same code as push_all_move.

edit: I'm guessing this is needed so the values are appropriately dropped, so it is needed :)

[alexcrichton]

I am in favor of this change, although I find @erickt's comment a little disturbing. I would have hoped that LLVM would realize for values without a destructor that it can optimize away the value inside the loop, and then it'd optimize the whole loop away.

@thestinger, could you look into why LLVM isn't optimizing it away? I would like to avoid the needs_drop conditional if possible, and if we can invert some condition or something like that to get LLVM to optimize it away that would be nice.

[thestinger]

@alexcrichton: The substantial change already landed for this and this is just some cleanup not really changing much.

[thestinger]

@erickt: I'm okay with doing that, but would rather fix whatever is wrong with our code generation. It could be caused by the stupid way we handle self combined with the lack of any aliasing information.