Potential specialisation: DoubleEndedIterator::last

[clarfonthey]

Right now, Iterator::last defaults to:

let mut last = None;
for x in self { last = Some(x); }
last

If the iterator implements DoubleEndedIterator, it should default to:

self.next_back()

Many standard library types manually implement this specialisation, but a blanket implementation would be nice too!

Another potential specialisation for nth with DoubleEndedIterator and ExactSizeIterator:

if self.len() - n < n {
    for x in self.rev() {
        if n == 0 { return Some(x) }
        n -= 1;
    }
} else {
    for x in self {
        if n == 0 { return Some(x) }
        n -= 1;
    }
}
None

There's a bunch of stuff here and it might be worth looking into what extra implementations the standard library has, and how we could potentially make this default using specialisation.

[Mark-Simulacrum]

Closing as per #42584 (comment).