How to use one module from another module in a Rust cargo project?

You'll have to include b.rs somewhere, typically with mod b;. If b is a child of a (instead of being a sibling of a), there are two ways to do this:

• Recommended: rename a.rs into a/mod.rs and b.rs into a/b.rs. Then you can mod b; in a/mod.rs.
• Instead, you can just #[path = "b.rs"] mod b; in a.rs without renaming sources.

If b is intended to be a sibling of a (instead of being a child of a), you can just mod b; in main.rs and then use crate::b; in a.rs.

The method from the accepted answer doesn't work for me in Rust 1.33. Instead, I use the sibling module like this:

use crate::b;