<zeenix[m]> "I'll try and help but I'll..." <- The docs explaining why this is an issue in the first place are a bit hidden.
I expected to find an explanation at https://doc.rust-lang.org/reference/subtyping.html#variance or perhaps at https://doc.rust-lang.org/nomicon/subtyping.html, but while they explain the general concept of variance nicely, they don't cover the case from https://github.com/rust-embedded/heapless/issues/501.
The important information can be found at https://rustc-dev-guide.rust-lang.org/variance.html#variance-and-associated-types:
"Traits with associated types – or at minimum projection expressions – must be invariant with respect to all of their inputs."