In general I find that people restrict the idea of "interface" too much. If it has a downstream user / client, it is an interface despite not being `interface X

In general I find that people restrict the idea of "interface" too much. If it has a downstream user / client, it is an interface despite not being `interface X`. This becomes very obvious in a language like Rust where you are encouraged to use concrete types as your 'interface' to downstream clients / consumers / library users.