Replace the current `for` protocol with external iterators

[thestinger]

The std::iterator::Iterator trait will need a lang item and Iterator implementations on functions will likely have to be forbidden to avoid ambiguity with internal iterators.

The body of the loop won't have to be a closure and return/break will never fail to work as expected.

See: https://mail.mozilla.org/pipermail/rust-dev/2013-June/004364.html

[thestinger]

Nominating for the backwards compatible milestone. This will change the Iterator API by removing the need for the advance method.

[bblum]

Is there some reason you can't add an each method to the Iterator typeclass, and just write for iterator_value.each ... like today's syntax supports? Much less magic required.

[thestinger]

@bblum: they do have that with an advance method, but beyond being ugly it doesn't take advantage of the fact that they don't need a closure and can always successfully break and return. The borrow/move checker is painful with our current loop.

[thestinger]

Here's what it would do:

// for loop sugar
for xs.take_while(|x| *x < 5).zip(ys) |(x, y)| { return x + y; }

// equivalent low-calorie code, already compiling today
let mut it = xs.take_while(|x| *x < 5).zip(ys);
loop {
    match it.next() {
        Some((x, y)) => return x + y, // note that this will always return successfully
        None => break
    }
}

No more errors like this from return and break being lies:

foo.rs:17:24: 17:35 error: cannot borrow `(*foo)[]` as mutable more than once at a time
foo.rs:17             return Some(&mut foo[idx])
                                  ^~~~~~~~~~~
foo.rs:17:24: 17:35 note: second borrow of `(*foo)[]` as mutable occurs here
foo.rs:17             return Some(&mut foo[idx])
                                  ^~~~~~~~~~~
error: aborting due to previous error

They'll also compile much faster than closures and avoid the noisy .advance tacked on to every external iterator.

[graydon]

This was accepted for milestone 2. We're mid way through doing it now in eg. #8141 and #8184 .

[bblum]

👍 👍 👍 👍 👍