What is the difference between using a type as a different name and a type alias?

Question

What is the difference between

use hyper::status::StatusCode as Error;

and

type Error = hyper::status::StatusCode;

Are the any more differences between them except that type can be also pub type? What are the benefits between using one or another?

Answer 1

In case of simple types, like in your example, there doesn't seem to be any semantic difference. Moreover, there is a direct analogue with use to pub type, it's pub use:

// will be available to other modules
pub use hyper::status::StatusCode as Error;

However, there are differences in more complex cases. For example, you can define generic type aliases or aliases for specialized generic types:

type Result<T> = ::std::result::Result<T, MyError>;

type OptionI32 = Option<i32>;

The general idea is that you usually use type aliases because they are more powerful and suggest the intent more clearly, like with Result, and you use use .. as .. when you only want to import that specific name but it conflicts with something which is already in the current namespace:

use std::io::Read as StdRead;

trait Read: StdRead { ... }

Note that using path-qualified identifiers should be preferred to use renaming. The above is better written as

use std::io;

trait Read: io::Read { ... }

(unless Read methods are used for some concrete type in the same file, of course).

Using use .. as .. as a substitute for type (in case where it is possible) is uncommon and I think it should be avoided.