When providing a blanket implmentation of T also optionally provide an appropriate as_boxed method

[nwalfield]

Let's say I have a Box<T> that implements T, and some function fn f(...) -> impl T. f can return a Box<T>, because Box<T> implements T. If the caller needs to put the return t in a box, then the caller has to do: Box::new(t). That's unfortunate, because t was actually already a Box<T>, we just didn't know (or rather, couldn't exploit the fact).

A solution is to add a method to T: fn as_boxed(self) -> Box<Self>. The implementation on T would do: return Box::new(self), and the implementation on Box<T> would do: return self. No double boxing. This is what we use in Sequoia 1, 2 where we ended up with an object being boxed millions of time.

As far as I can tell, blanket wraps all functions. So, we can't use blanket in this case. I'd like an option to either not wrap particular functions, or a mechanism to add an as_boxed method. As I can't imagine other functions that shouldn't be wrapped, this point solution is probably sufficient.