I've seen uses of &*s where s is a String; this can be reduced to &s when calling a function that accepts &str.

Similarly, in the AST, I've seen (in clippy and racer) &**x or &*x where just x or &x is fine; call syntax will autoderef a &P<T> to a &T.

However, I don't really know yet how to do this in a general way.