That's a helpful explanation, thank you. I do generally agree that overuse of generics can produce bad interfaces. I also agree that generics tempt people to assume that the type fully determines the desired behavior, rather than giving any other means of selecting it.

(It's worth noting, for instance, that HashMap lets you replace the hasher, if you have some strings with a property that make them more hashable. But if we didn't have that, then the generic would force a given type to have only one hasher, and force a newtype wrapper to change the hasher, which would make for suboptimal designs.)