Add comment on why RefCell::unwrap is safe.

[tbu-]

No description provided.

[huonw]

I think it's actually safe: moving the RefCell (i.e taking self by-value) requires that there are no existing borrows, and both .borrow() and .borrow_mut() store borrows, so it's impossible to call unwrap while either of those exist. The debug_assert! is just a final check to ensure that RefCell is maintaining its own invariants.

If you adjust this patch to leave the debug_assert! as is, but add a comment describing the above for future readers, I'll r+ in a flash since it's definitely subtle.

[nikomatsakis]

Actually, I think we intentionally used debug_assert there to avoid undue performance impact for something we believe to be statically verified.