Other than what other people said, the question is to why that desugaring is chosen, not that it's because of a specific one. That is, why

  let mut iter = IntoIterator::into_iter(y);

instead of

  let mut iter = y.into_iter();

That could have been done, but it wasn't.

> Otherwise you'd be writing methods with absolutely no idea how the language would interact with it.

Right, this is what's guessed, because it makes the most sense.