How do I do a basic import/include of a function from one module to another in Rust 2015?

I can't find how to include (or import, inject, or some other word) a function from one file (module) to another.

I start a new project with

$ cd ~/projects
$ cargo new proj --bin
$ cd proj
$ tree
.
|
-- Cargo.toml
-- src
|
-- main.rs


I modify main.rs and create a new file a.rs (inside the src dir) with the following code:

main.rs

fn main() {
    println!("{}", a::foo());
}

a.rs

pub fn foo() -> i32 { 42 }

I run the project with cargo run and get the error:

error[E0433]: failed to resolve: use of undeclared type or module a
--> src/main.rs:2:20
|
2 | println!("{}", a::foo());
| ^ use of undeclared type or module a


It seems that I need to import the a somehow. I tried to add following things as a first line to main.rs

-
use a;

error[E0432]: unresolved import a
--> src/main.rs:1:5
|
1 | use a;
| ^ no a in the root


-
use a::*;

error[E0432]: unresolved import a
--> src/main.rs:1:5
|
1 | use a::*;
| ^ maybe a missing extern crate a;?

error[E0433]: failed to resolve: use of undeclared type or module a
--> src/main.rs:4:20
|
4 | println!("{}", a::foo());
| ^ use of undeclared type or module a


-
use a::foo;

error[E0432]: unresolved import a
--> src/main.rs:1:5
|
1 | use a::foo;
| ^ maybe a missing extern crate a;?

error[E0433]: failed to resolve: use of undeclared type or module a
--> src/main.rs:4:20
|
4 | println!("{}", a::foo());
| ^ use of undeclared type or module a


-
extern crate a; use a::foo;

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


-
extern crate proj; use proj::a::foo;

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


I have read the guide but still cannot figure out how to do imp

In a mainish module (main.rs, lib.rs, or subdir/mod.rs), you need to write mod a; for all other modules that you want to use in your whole project (or in the subdir).

In any other module, you need to write use a; or use a::foo;

You're far from the only person to be confused by this, and it's certainly possible to do better, but any changes to the module system will get rejected as "too confusing".

Edit: this answer was written for the "Rust 2015" language standard. Changes were made for the "Rust 2018" standard, see this blog post and the edition guide

Stack Overflow Q#26224947, score: 58