fn DisjointImmutGuard::get_unchecked: add unsafe fn to bypass DisjointMut

[kkysen]

For use in #1439 (comment).

[randomPoison]

I'm not sure I understand what the point of this change is. DisjointImmutGuard already implements Deref, which gives us a safe way to get the full slice out, and I can't tell what is different about this new unsafe function. Why does the existing Deref impl not work for our purposes here?

[kkysen]

I'm not sure I understand what the point of this change is. DisjointImmutGuard already implements Deref, which gives us a safe way to get the full slice out, and I can't tell what is different about this new unsafe function. Why does the existing Deref impl not work for our purposes here?

The existing deref impl returns a slice that can't outlive the guard. This method does.

[randomPoison]

Ah, I see now, the returned reference isn't tied to the guard, it's tied to the borrow the guard represents, i.e. the original DisjointMut. In that case, I think that should be called out in the documentation for the function and spelled out as part of the safety invariants.