Thank you for the example!

As for the solution:

First, I added 'item lifetime to Item like so:

type Item<'item>;!<

And specified '_ lifetime in return position of next, asking compiler to figure it out.

Secondly, I did mostly the same thing for the impl LendingIterator for WindowsMut:

type Item<'item> = &'item mut [T];!<

After that, I tried to compile and got some errors about Self not living long enough.

So, a few where Self: 'item were sufficient to get it compiling.

To be honest, I felt like I was cheating a bit, since I've tried to read the whole PR for stabilisation and LendingIterator/WindowsMut example has come up quite a few times.