a boxed iterator can't use methods in the IteratorExt trait

[Kroisse]

Example

let v = vec![1, 2, 3];
let boxed = Box::new(v.iter()) as Box<Iterator<Item=i32>>;
boxed.max()  // IteratorExt::max(*boxed) also fails

This code will be failed to compile because boxed holds a trait object that is unsized, and IteratorExt requires the Sized kind so IteratorExt can only be implemented for sized types. Like IteratorExt::map(), many methods in IteratorExt consume self, so requiring a sized type is necessary.

Adding impl Iterator for Box<Iterator> (like Reader for Box<Reader>) might solve this problem, but I'm worrying about the confliction with autoderef.

[japaric]

I think a possible solution would be impl Iterator for &mut Iterator, that way boxed.max() would autoderef boxed from Box<Iterator> into &mut Iterator (because Box implements DerefMut<Target=T>) and then since &mut Iterator implements the Iterator trait, it would also implement IteratorExt. Let me see if that works.

[japaric]

The idea works with the old Iterator<T> trait:

#![no_implicit_prelude]

use std::boxed::Box;
use std::marker::Sized;
use std::option::Option;

trait Iterator<T> {
    fn next(&mut self) -> Option<T>;
}

impl<'a, T> Iterator<T> for &'a mut (Iterator<T> + 'a) {
    fn next(&mut self) -> Option<T> {
        loop {}
    }
}

trait IteratorExt<T>: Iterator<T> + Sized {
    fn last(self) -> Option<T> {
        loop {}
    }
}

impl<I, T> IteratorExt<T> for I where I: Iterator<T> {}

fn test(mut it: Box<Iterator<u8> + 'static>) {
    it.last();  // <-- it works
}

fn main() {}

But I'm not quite sure how to impl<'a> Iterator for &'a mut (Iterator + 'a) with associated types. This gets rejected:

impl<'a, T> Iterator for &'a mut (Iterator<Item=T> + 'a) {
//~^ error: the type parameter `T` is not constrained by the impl trait, self type, or predicates
    type Item = T;
}

[japaric]

I think the last impl should be accepted, but it's not because of a bug, so I have filed #21363.