I guess I had mistakenly thought that run time dispatch and a single binary implementation was somehow required for parametric polymorphism since it requires treating all types identically. Upon reflection I can see that it's not actually exclusive of compile time dispatch though.

From a pragmatic perspective it seems unfortunate that such generics can't be used in an API without corresponding access to the source code. I saw the ability to work with arbitrary future types at runtime as largely being the point of type erasure.

From a theoretical perspective I'm having some trouble accepting the idea that Rust generics are an example of true parametric polymorphism. In the earlier example a function could behave differently based on the size of the type. That's not uniformly dispatching to a behavior implemented by the type though, which seems like the core idea behind parametric polymorphism to me. (I suppose my concerns are purely academic though. AFAIK runtime reflection in Java allows utterly breaking type erasure but in practice Java generics still seem to be referred to as an example of parametric polymorphism.)

The effect without concepts is that you can (in principle) get things that "accidentally" match, like applying "+" to strings; and that the error messages when they don't match are unpleasant. Concepts resolve both.