Alternative to `drain`

Right now, a lot of types implement drain without any parameters. Personally, I think that this method should be made part of IntoIterator for these types. Here's an example of what the new trait would look like:

pub trait IntoIterator where Self::IntoIter::Item == Self::Item {
    type Item;
    type IntoIter: Iterator;
    fn into_iter(self) -> Self::IntoIter;

    // additions
    type Drain: Iterator = Self::IntoIter;
    fn drain(&mut self) -> Self::Drain where Self: Default {
        ::std::mem::replace(self, Self::default()).into_iter()
    }
}

Really, drain(..) is just a form of into_iter which reuses resources. Once associated type defaults are no longer gated, we may be able to make this change to the standard library without breaking user code, assuming that we delete the native impl or provide a different name. (although, we'd have to check that)

What do you think?

Start with the proposed IntoIterator trait and the discussion of associated type defaults, native implementation removal, and compatibility. Review the 12-comment thread for unresolved objections; done would require a settled RFC direction that addresses these compatibility concerns.