rust - How to include a module from another file from the same project?

Question

By following this guide I created a Cargo project.

src/main.rs

fn main() {
    hello::print_hello();
}

mod hello {
    pub fn print_hello() {
        println!("Hello, world!");
    }
}

which I run using

cargo build && cargo run

and it compiles without errors. Now I'm trying to split the main module in two but cannot figure out how to include a module from another file.

My project tree looks like this

├── src
 ?? ├── hello.rs
 ?? └── main.rs

and the content of the files:

src/main.rs

use hello;

fn main() {
    hello::print_hello();
}

src/hello.rs

mod hello {
    pub fn print_hello() {
        println!("Hello, world!");
    }
}

When I compile it with cargo build I get

error[E0432]: unresolved import `hello`
 --> src/main.rs:1:5
  |
1 | use hello;
  |     ^^^^^ no `hello` external crate

I tried to follow the compiler's suggestions and modified main.rs to:

#![feature(globs)]

extern crate hello;

use hello::*;

fn main() {
    hello::print_hello();
}

But this still doesn't help much, now I get this:

error[E0463]: can't find crate for `hello`
 --> src/main.rs:3:1
  |
3 | extern crate hello;
  | ^^^^^^^^^^^^^^^^^^^ can't find crate

Is there a trivial example of how to include one module from the current project into the project's main file?

Answer 1

You don't need the mod hello in your hello.rs file. Code in any file but the crate root (main.rs for executables, lib.rs for libraries) is automatically namespaced in a module.

To include the code from hello.rs in your main.rs, use mod hello;. It gets expanded to the code that is in hello.rs (exactly as you had before). Your file structure continues the same, and your code needs to be slightly changed:

main.rs:

mod hello;

fn main() {
    hello::print_hello();
}

hello.rs:

pub fn print_hello() {
    println!("Hello, world!");
}