This is really a minor quibble, if you think it's uninteresting, let's just keep it short.
Range<u64> implements the trait
ExactSizeIterator when compiled on 64-bit platforms and not on 32-bit platforms. That means it's reasonably easy to write code that fails to compile on 32-bit.
This was added in PR 19858.
My opinion is that it's wrong to be implemented at all.