I'm aware that my example isn't possible, which is why I said, "if they wanted to, they (presumably) could allow for..."
My point being that Rust restricts what one can do with higher-order types, but those types still exist and are expressible in the language. From a theoretical point of view, there doesn't seem to be any reason that they couldn't allow for traits on higher-kinded types -- they simply choose not to. Perhaps for performance reasons, or for simplicity, or for syntax or whatever, but I haven't seen any reason why they couldn't. That was the point that I was making.
My point being that Rust restricts what one can do with higher-order types, but those types still exist and are expressible in the language. From a theoretical point of view, there doesn't seem to be any reason that they couldn't allow for traits on higher-kinded types -- they simply choose not to. Perhaps for performance reasons, or for simplicity, or for syntax or whatever, but I haven't seen any reason why they couldn't. That was the point that I was making.