Generic mutability parameters

[tobz1000]

@vitalyd

They don’t cause a conflict. It would limit the type of code you can write in the fn body. In the immutable case the code can stick a 'static; in the mutable they may not. It’s just an example of an extra restriction on the code.

True, I hadn't considered this. A mutably-generic collection would also be unable to interact with other always-immutable arguments to the function. Previously, I was thinking any immutable-only function body would work, but now I can see that's not that case.

Still, I think it could be worthwhile, obviously just for the cases which still work given this restriction.

@vitalyd

But maybe that’s fine as what you’re suggesting is mutability forwarding or “passthrough” mutability. I’m not sure I’d consider it abstraction, at least in the same sense as AsRef/AsMut/Borrow/BorrowMut but perhaps exact nomenclature is unimportant.

Maybe that's not the right word then. Certainly, being able to pass-through mutability is the goal here, whatever that's called.

@RalfJung

I wonder how much of this can be implemented with associated type constructors and the family pattern :

This is interesting. Not as neat as syntax-level support, but would be even more versatile. It's kind of a shame we already have so many FooRef/FooMut-type trait pairs... although maybe you could unify such pairs, as needed, using a similar family pattern?