> I'm still waiting for a language feature that lets you say "This field of this struct is only mutable while this module is running, and once it completes it will never be changed", though. A lot of data structures are initialized in passes and then passed along as constant data: you construct the basic object structure with nothing but a few IDs, then you compute extra fields as necessary, then you're done and the object is never written to again. If you could keep the fields mutable during initialization (which may be a longer process than just constructor calls) and then seal them off once that's complete, you eliminate a whole class of bugs where a read-only client decides to mutate a field that should only be mutated from designated initializer.

Why do you want to think of it as "the same" struct? It seems to me that the way to achieve this is having the partially initialised thing be a different type than the fully initialised thing, along with a linear type system that makes sure the partially-initialised one is consumed in the process of making the fully-initialised one.