InteriorAssign trait { a := b }

It's pretty much baked into the foundational philosophy of the language, as evidenced by the fact that regular mutability has syntax support (and has had it since before 1.0) where interior mutability does not. That syntactic divide was no accident. It follows from a recognition that interior mutability occupies a point on a spectrum between "useful" (eg perf optimization) and "necessary" (in the occasional case where regular CREW semantics are too restrictive).