Proposal: implement StableDeref

[vigna]

The StableDeref marker trait (https://crates.io/crates/stable_deref_trait) has been designed to mark structures whose deref is stable, exactly like the case of memory mapping. Yoke (https://crates.io/crates/yoke) uses the trait to make it possible to attach in a safe way anything implementing the trait and providing a reference. It would be very useful (for us, at least) if Mmap would implement StableDeref as we could yoke the reference of an Mmap to the backing Mmap. We are using a newtype for that presently, but it would be nice if Mmap would provide the feature directly. I can create a pull request if there's interest.

[StephanvanSchaik]

Having StableDeref implemented for Mmap definitely sounds interesting. A PR would definitely be welcome.

It does seem to impose some limitations, namely that Deref (and DerefMut) must always return the same pointer, and that this pointer must remain valid even when calling methods on &self until the object is dropped, but this restriction does not apply to methods on &mut self (other than DerefMut), if I understand this correctly at least. Assuming that my understanding is correct, I think it should be sound to implement StableDeref for both Mmap and MmapMut.

Mmap::split_to() introduced by PR #18 as part of the API to split Mmap objects, does change the underlying pointer, but should meet those conditions, as it requires &mut self.

Similarly, it is also possible to use Mmap::make_none() to restrict the access, such that the pointer returned by Deref can no longer be dereferenced. However, since Mmap::make_none() takes ownership of Mmap to return MmapNone, that should also meet those conditions (and MmapNone doesn't implement Deref anyway).

Assuming that this is correct, there shouldn't be any issues with soundness as far as I can tell, but I am just mentioning this to be sure.