compiler/rustc_error_codes/src/error_codes/E0253.md

Attempt was made to import an unimportable type. This can happen when trying
to import a type from a trait.

Erroneous code example:

```compile_fail,E0253
mod foo {
    pub trait MyTrait {
        type SomeType;
    }
}

use foo::MyTrait::SomeType;
// error: `SomeType` is not directly importable

fn main() {}
```

It's invalid to directly import types belonging to a trait.