Why LLVM needs this explaining? Probably somewhere an optimiser could inspect something and it doesn't.
Rust cuts the slice in half and swaps between the halves, so probably LLVM doesn't convince itself that if you did those swaps directly they aren't ever aliased, but once there are two slices which can't overlap it can see it's fine.
Anyone want to explain why Rust needs to explicitly de-alias this yet? What a useless digression...