InteriorAssign trait { a := b }

To be pedantic, := always denotes defining some new binding, not mutation or interior mutation. Rust has let and let mut instead of :=, but this does leave := free for other uses. Instead Rust should never use := for anything.

Now one could consider safer partial initialisation than MaybeUninit, but recent conversations suggest other ideas, probably adding some new assignment syntax. And really tricky stuff like partial initialisation should probably not benefit from really short syntax.