rust - std::ops::Add or core::ops::Add?

Question

These two traits (std::ops::Add, core::ops::Add) provide the same functionality, and they both use the same example (both utilize std::ops::Add). Their set of implementors differ somewhat.

Should one default to using std::ops::Add? Why do both, as opposed to one, of them exist?

Answer 1

There aren't two traits. There is one trait which is exported under several interchangeable names. This is far from unique. Virtually everything in core is also exported from std, and virtually always under exactly the same path (i.e., you can just replace the "core" prefix with "std").

As for which one you should use: If you have a reason to not link to the standard library (#![no_std]), then the std::* one isn't available so obviously you use core::*. If on the other hand you do use the standard library, you should use the std::* re-export. It is more customary and requires less typing.