Upgrading mutability based on exclusivity inference

language design
4

The recent proposal to allow downgradable mut references seems like it would extend naturally to this:

    pub fn get_some_ref<'a, 'm>( &'a mut<'m> self, key: i32 ) -> &'a mut<'m> i32 {
        if key == 42 {
            &mut self.data1
        } else {
            &mut self.data2
        }
    }

…if we can say that &'a T is equivalent to &'a mut<'!> T or something.