🧀 mod usage between modules

Here we create a project named modtest

My goal is that functions in test2 can refer to the functions defined in test1

test1.rs:

pub fn test1_foo() {
    println!("test1 foo");
}

test2.rs:

mod test1;
use crate::test1::test1_foo;
pub fn test2_foo() {
    test1_foo();
}

main.rs:

mod test2;

fn main() {
    test2::test2_foo();
}

But it fails in the reason

-*- mode: compilation; default-directory: "~/Develop/Rust/modtest/" -*-
Compilation started at Fri Apr 12 20:25:06

cargo run --package modtest --bin modtest --
   Compiling modtest v0.1.0 (/Users/mmmmmcclxxvii/Develop/Rust/modtest)
error[E0583]: file not found for module `test1`
 --> src/test2.rs:1:1
  |
1 | mod test1;
  | ^^^^^^^^^^
  |
  = help: to create the module `test1`, create file "src/test2/test1.rs" or "src/test2/test1/mod.rs"
  = note: if there is a `mod test1` elsewhere in the crate already, import it with `use crate::...` instead

error[E0432]: unresolved import `test1::test1_foo`
 --> src/test2.rs:2:5
  |
2 | use test1::test1_foo;
  |     ^^^^^^^^^^^^^^^^ no `test1_foo` in `test2::test1`

Some errors have detailed explanations: E0432, E0583.
For more information about an error, try `rustc --explain E0432`.
error: could not compile `modtest` (bin "modtest") due to 2 previous errors

Compilation exited abnormally with code 101 at Fri Apr 12 20:25:07

defining modules

The doc above tells me that mod test1 declared in test2.rs is nothing more than declaring a submodule

So I move mod test1 to main.rs

mod test1; // added, and removed in test2.rs
mod test2;

fn main() {
    test2::test2_foo();
}

It works.

Submodules must always be declared in the module they reside in !!!

We have to organize our project into a reasonable hierarchy in such a way that every module have a parent module.

So that we can just declare all the son modules of crate root.