windows_mut()!

use ::lending_iterator::prelude::*;

let mut array = [0; 15];
array[1] = 1;
// Cumulative sums are trivial with a `mut` sliding window,
// so let's showcase that by generating a Fibonacci sequence.
let mut iter = array.windows_mut::<3>();
while let Some(&mut [a, b, ref mut next]) = iter.next() {
    *next = a + b;
}
assert_eq!(
    array,
    [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377],
);

Rolling your own version of it using the handy from_fn constructor

• (Or even the FromFn flavor of it to enjoy "named arguments")

use ::lending_iterator::prelude::*;

let mut array = [0; 15];
array[1] = 1;
// Let's hand-roll our iterator lending `&mut` sliding windows:
let mut iter = {
    let mut start = 0;
    lending_iterator::FromFn::<HKT!(&mut [u16; 3]), _, _> {
        state: &mut array,
        next: move |array| {
            let to_yield =
                array
                    .get_mut(start..)?
                    .get_mut(..3)?
                    .try_into() // `&mut [u8] -> &mut [u8; 3]`
                    .unwrap()
            ;
            start += 1;
            Some(to_yield)
        },
        _phantom: <_>::default(),
    }
};
while let Some(&mut [a, b, ref mut next]) = iter.next() {
    *next = a + b;
}
assert_eq!(
    array,
    [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377],
);

• where that HKT!(&mut [u16; 3]) is a higher-kinded type parameter that has to be turbofished to let the generic context properly figure out the return type of the next closure.

  Indeed, if we were to let type inference, alone, figure it out, it wouldn't be able to know which lifetimes would be fixed/tied to call-site captures, and which would be tied to the "lending-ness" of the iterator (higher-order return type). See ::higher-order-closure for more info about this.

LendingIterator adapters

See lending_iterator::adapters.

use ::lending_iterator::higher_kinded_types::{*, Apply as A};

fn slice_sort_by_key<Key, Item, KeyGetter> (
    slice: &'_ mut [Item],
    mut get_key: KeyGetter,
)
where
    Key : HKT, // "Key : <'_>"
    KeyGetter : for<'item> FnMut(&'item Item) -> A!(Key<'item>),
    for<'item>
        A!(Key<'item>) : Ord
    ,
{
    slice.sort_by(|a, b| Ord::cmp(
        &get_key(a),
        &get_key(b),
    ))
}

// ---- Demo ----

struct Client { key: String, version: u8 }

fn main ()
{
    let clients: &mut [Client] = &mut [];

    // Error: cannot infer an appropriate lifetime for autoref due to conflicting requirements
    // clients.sort_by_key(|c| &c.key);

    // OK
    slice_sort_by_key::<HKT!(&str), _, _>(clients, |c| &c.key);

    // Important: owned case works too!
    slice_sort_by_key::<HKT!(u8), _, _>(clients, |c| c.version);
}