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

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!");
}

If you wish to have nested modules...

Rust 2018

It's no longer required to have the file mod.rs (although it is still supported). The idiomatic alternative is to name the file the name of the module:

$ tree src
src
├── main.rs
├── my
│   ├── inaccessible.rs
│   └── nested.rs
└── my.rs

main.rs

mod my;

fn main() {
    my::function();
}

my.rs

pub mod nested; // if you need to include other modules

pub fn function() {
    println!("called `my::function()`");
}

Rust 2015

You need to put a mod.rs file inside your folder of the same name as your module. Rust by Example explains it better.

$ tree src
src
├── main.rs
└── my
    ├── inaccessible.rs
    ├── mod.rs
    └── nested.rs

main.rs

mod my;

fn main() {
    my::function();
}

mod.rs

pub mod nested; // if you need to include other modules

pub fn function() {
    println!("called `my::function()`");
}

I really like Gardener's response. I've been using the suggestion for my module declarations. Someone please chime in if there is a technical issue with this.

./src
├── main.rs
├── other_utils
│   └── other_thing.rs
└── utils
    └── thing.rs

main.rs

#[path = "utils/thing.rs"] mod thing;
#[path = "other_utils/other_thing.rs"] mod other_thing;

fn main() {
  thing::foo();
  other_thing::bar();
}

utils/thing.rs

pub fn foo() {
  println!("foo");
}

other_utils/other_thing.rs

#[path = "../utils/thing.rs"] mod thing;

pub fn bar() {
  println!("bar");
  thing::foo();
}