Spitballing: This is a bit of an abuse of notation, but maybe
-> IterMut<'_, K, V, Item = (&K, &mut V)>
-> Read<'a, Self, Output = Result<usize, Error>>
Alternatively, Pre-RFC: Absolute minimum for in-impl imports has me pondering inherent trait impls as a potential definition for "notable", though of course that wouldn't be helpful to anyone for a while.