Theoretically, if the standard library only optimizes about interfaces whose semantics are in the standard library (i.e., standard library interfaces) that's pe

Theoretically, if the standard library only optimizes about interfaces whose semantics are in the standard library (i.e., standard library interfaces) that's perfectly fine. What worries me about this approach is that Go is essentially duck-typed -- implementing an interface isn't necessary for something to appear like it implements an interface, and a method signature doesn't implicitly carry its semantic information alongside it.