How do you use parent module imports in Rust?

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I’m going to answer this question too, for anyone else who finds this and is (like me) totally confused by the difficult-to-comprehend answers.

It boils down to two things I feel are poorly explained in the tutorial:

  • The mod blah; syntax imports a file for the compiler. You must use this on all the files you want to compile.

  • As well as shared libraries, any local module that is defined can be imported into the current scope using use blah::blah;.

A typical example would be:

src/main.rs
src/one/one.rs
src/two/two.rs

In this case, you can have code in one.rs from two.rs by using use:

use two::two;  // <-- Imports two::two into the local scope as 'two::'

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

However, main.rs will have to be something like:

use one::one::bar;        // <-- Use one::one::bar 
mod one { pub mod one; }  // <-- Awkwardly import one.rs as a file to compile.

// Notice how we have to awkwardly import two/two.rs even though we don't
// actually use it in this file; if we don't, then the compiler will never
// load it, and one/one.rs will be unable to resolve two::two.
mod two { pub mod two; }  

fn main() {
    bar();
}

Notice that you can use the blah/mod.rs file to somewhat alleviate the awkwardness, by placing a file like one/mod.rs, because mod x; attempts x.rs and x/mod.rs as loads.

// one/mod.rs
pub mod one.rs

You can reduce the awkward file imports at the top of main.rs to:

use one::one::bar;       
mod one; // <-- Loads one/mod.rs, which loads one/one.rs.
mod two; // <-- This is still awkward since we don't two, but unavoidable.    

fn main() {
    bar();
}

There’s an example project doing this on Github.

It’s worth noting that modules are independent of the files the code blocks are contained in; although it would appear the only way to load a file blah.rs is to create a module called blah, you can use the #[path] to get around this, if you need to for some reason. Unfortunately, it doesn’t appear to support wildcards, aggregating functions from multiple files into a top-level module is rather tedious.

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