&mut inputs don't "downgrade" to &

Still talking about this signature:

fn main() {
fn quz(&mut self) -> &str { todo!() }

Newcomers often expect self to only be shared-borrowed after quz returns, because the return is a shared reference. But that's not how things work; self remains exclusively borrowed for as long as the returned &str is valid.

I find looking at the exact return type a trap when trying to build a mental model for this pattern. The fact that the lifetimes are connected is crucial, but beyond that, instead focus on the input parameter: You cannot call the method until you have created a &mut self with a lifetime as long as the return type has. Once that exclusive borrow (or reborrow) is created, the exclusiveness lasts for the entirety of the lifetime. Moreover, you give the &mut self away by calling the method (it is not Copy), so you can't create any other reborrows to self other than through whatever the method returns to you (in this case, the &str).