Why is Iterator::rev defined on Iterator and not DoubleEndedIterator

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Rust has two main traits for iteration: the standard Iterator for things that can be traversed in order, and DoubleEndedIterator for things that can additionally be iterated from the back.

This makes sense to me. A lot of useful methods are defined on Iterator, like map, filter, and whatnot. And then things that operate from the "back", so to speak, like rfind and rfold are defined on DoubleEndedIterator since such operations only make sense on such iterators.

But then we've got Iterator::rev

fn rev(self) -> Rev<Self>
where
    Self: Sized + DoubleEndedIterator,
{
    Rev::new(self)
}

rev is a function defined on Iterator but with an additional constraint on Self that it be DoubleEndedIterator as well.

What is the practical benefit to having this function defined on Iterator? We can only call it on DoubleEndedIterator implementors, and DoubleEndedIterator extends Iterator so we'll never end up in a situation where something implements the former and not the latter. So why is rev not defined on DoubleEndedIterator like rfind and rfold are?

rust bidirectional iterator
1 Answers

This allow to:

  • see all features (doc) of Iterator in one place.
  • use rev() on an Iterator without need to use DoubleEndedIterator, the prelude include it so it's not a problem actually. I would so add this reduce cognitive load by letting the user ignore DoubleEndedIterator exist.
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