[pcwalton]

The main thing I can think of that would help here is the fact that Rust has stricter alignment requirements than C++ does. Any live reference can more or less be assumed to point to validly-aligned memory at all times, which isn't true in C++.

As to whether LLVM actually takes advantage of this effectively, I don't know. I know that we do supply the necessary attributes to LLVM in most cases, but I haven't looked at the individual transform and optimization passes to see whether they take advantage of this (e.g. emitting movdqa vs. falling back to movdqu).