i would say the biggest factor for nonreadability is not being able to chase function calls using simple text search, with as few indirections as possible. proc macros and to a lesser degree traits make that difficult.
That's just because you got used to it ;) (same as with modern C++ really, if you've used C++ long enough you become blind to its problems)
"Regular" Rust and C++ is fairly readable, but a quick Google for "Higher Kinded Types in Rust" ends up with [0]:
fn ap<A, B, F: Fn(&A) -> B>(x: &<Self as Plug<F>>::R, arg: &<Self as Plug<A>>::R) -> <Self as Plug<B>>::R
where Self: Plug<A> + Plug<B> + Plug<F>;