Fair, but feels like that significantly reduces the utility of pinned types then if for certain values you lose the ability to mutate values when the compiler overwise knows that there is a single mutable reference.
pub fn get_mut_pinned(this: &mut Pin<Arc<T>>) -> Option<Pin<&mut T>>
, which for pinning-aware values should give you enough mutability and for `T: Unpin` can give you `Option<&mut T>` through `Option::as_deref_mut`. Unfortunately, I haven't seen any `Arc` implementations that actually provide it, since almost none of the third-party ones are pinning-aware. (My `tiptoe::Arc`'s `get_mut` has this signature, but that's a specialty container with additional requirements for the contained value.)
The same goes for `make_mut_pinned`, that's missing too (but to be fair would be much less useful, since pinning and `Clone` don't often mix that well).
These functions are indeed generally sound (also for `Rc`), as any value that cares would be `!Unpin`, which would still bar access to `&mut T`. `make_mut_pinned` also shouldn't cause too much confusion, as availability of `Clone` would have to be declared explicitly just about everywhere that's relevant.
They can be implemented as very thin wrappers around their non-pinning equivalents, with only a few `unsafe` operations to make the types fit.
The internals forum is good for informal discussion, so start there. Posting a RFC proposal is of course the formal step for actually getting the feature included in Rust proper.