InteriorAssign trait { a := b }

We don’t even have traits for customizing “assignment” operations for the non-interior-mutable case; we also don’t have special syntax for borrow_mut()-style projections in the non-interior-mutable case.

A main design choice of Rust is to keep interior mutability limited / the exceptional case, as far as resonably possible; the syntax overhead is not a limitation, it’s intentional.