> That's not the expression problem.

To me it looks like this is exactly the expression problem. The expression problem is not about adding methods to a trait, it's about adding an operation to a set of types. In this case every operation is defined by a single trait.

The idea behind the expression problem is to be able to define either a new operation or a new type in such a way that the code is nicely together. Rust trait system accomplish this beautifully.

> That's not unique to Rust, you can add new interfaces in any language...

Many languages have interfaces, but most of them don't allow you to implement them for an arbitrary type that you have not defined. For example, in Java, if you create an interface called `PrettyPrintable`, but you can't implement it for the `ArrayList` type from the standard library. In Rust you can do this kind of things.