Just a clarification:
The borrow checker is working correctly. This signature:
fn<'this>(&'this mut self)->&'this T
Says that so long as the return type is used ('this is active), *self remains exclusively borrowed (the &mut self also has lifetime 'this).
That relationship can be relied on for soundness.
So the desired get_or_insert semantics will indeed require a new type of API, be it super borrow or something else; the existing API semantic is not a bug and can't be "fixed".