Pre-RFC: Save and load state for `Iterator`

[tuxzz]

On my project, I recently found a problem that I can't implement a comfortable interface that do operation on Iterator directly using existing API.

/* not work because iterator state is changed after each operation */
pub fn do_operateion<'a, T, U>(input_iter: &mut T, output_iter: &mut U)
where
    T: Iterator<Item = &'a f32>,
    U: Iterator<Item = &'a mut f32>,
{
    let temp = input_iter.skip(2).map(...).sum();
    output_iter.step_by(2).zip(input_iter).for_each(|a, b| *a * temp);
    output_iter.skip(1).step_by(2).zip(input_iter).for_each(|a, b| *a * temp * 123.0);
}

Hence the idea of adding StateSavableIterator trait:

pub trait StateSavableIterator: Iterator {
    type IteratorState;

    fn save(&self) -> IteratorState;
    fn load(&mut self, state: &IteratorState) -> Self;
}

So that I can:

pub fn do_operateion<'a, T, U>(input_iter: &mut T, output_iter: &mut U)
where
    T: StateSavableIterator<Item = &'a f32>,
    U: StateSavableIterator<Item = &'a mut f32>,
{
    let input_init = input_iter.save();
    let out_init = output_iter.save();
    let temp = input_iter.load(input_init).skip(2).map(...).sum();
    output_iter.load(out_init).step_by(2).zip(input_iter.load(input_init)).for_each(|a, b| *a * temp);
    output_iter.load(out_init).skip(1).step_by(2).zip(input_iter.load(input_init)).for_each(|a, b| *a * temp * 123.0);
}

We don't have to pass &slice anymore! This trait benefits to writing more generic and readable API.

[SimonSapin]

You can use the Clone trait for this. Require a T: Iterator<Item = …> + Clone bound, then iter.save() is iter.clone() and iter.load(x) is iter = x.

[tuxzz]

You can use the Clone trait for this. Require a T: Iterator<Item = …> + Clone bound, then iter.save() is iter.clone() and iter.load(x) is iter = x.

Clone does not work on IterMut because of noalias rule.
But an output_iter have to be an iterator that has &mut Item

[SimonSapin]

slice::IterMut<T> could not implement StateSavableIterator for exactly the same reason. Save, call next() to get a &mut T, load, call next again and get a second &mut T aliasing the same item.

[SimonSapin]

To give a more helpful answer, if output_iter in your code is indeed slice::IterMut, consider passing the actual slice instead and calling .iter_mut() on it twice. This will let the borrow checker make sure that the lifetimes of the two iterators do not overlap. (Until NLL lands you’ll also likely need to add a {…} anonymous block to limit the scope of the first iterator.)

[burdges]

Iterator::by_ref almost sounds like what you want here.

If you actually want to abstract slice::IterMut to restart iteration, then an for<'a> &'a mut T: IntoIterator bound should theoretically work, except right now a compiler bug makes this mostly useless.

In my opinion, the best solution would be FnBorrow* closure traits that supports reborrowing in closures by providing access to the lifetime of the closure's self reference. It'd make tricks like this work.

fn foo<F>(f: F) where F: FnBorrowMut() -> impl Iterator<Item: 'borrow> { ... }

foo(|| my_collection.iter_mut().take(5).filter(bar))

In the short term, foo<B: BorrowMut<T>>(&mut [B]) is more flexible than foo(&mut [T]).

[clarfonthey]

I don't see any case where Clone isn't viable but a load-and-save state is both viable and sound.

[jonas-schievink]

Closing as per the above comments, this sort of interface does not seem to solve the problem.