Rust: no `module` in the root

Question

When I run use crate::feed; in src/cmdline.rs I expect that to import src/feed.rs, but it doesn't. Instead I get,

error[E0432]: unresolved import `crate::feed`
 --> src/cmdline.rs:2:5
  |
2 | use crate::feed;
  |     ^^^^^^^^^^^ no `feed` in the root

Despite the fact that src/feed.rs exists. However, if I panic and change it to mod feed; then I get

error[E0583]: file not found for module `feed`
 --> src/cmdline.rs:2:1
  |
2 | mod feed;
  | ^^^^^^^^^
  |
  = help: to create the module `feed`, create file "src/cmdline/feed.rs"

Using mod super::

error: expected identifier, found keyword `super`
 --> src/cmdline.rs:2:5
  |
2 | mod super::feed;
  |     ^^^^^ expected identifier, found keyword

Or with use super::

error[E0432]: unresolved import `super::feed`
 --> src/cmdline.rs:2:5
  |
2 | use super::feed;
  |     ^^^^^^^^^^^ no `feed` in the root

File structure for files in question looks like this,

src/feed.rs
src/cmdline.rs
src/main.rs

Answer 1

I was able to make my imports work the way you describe by doing the following.

First in main.rs I import the module cmdline and each module I want to be able to use via crate::.

// file src/main.rs
mod cmdline;
mod feed; // <== import module to be able to use via `crate::feed`

fn main() {
  cmdline::do_something();
}

Then in cmdline.rs I use create::feed.

// file src/cmdline.rs

use crate::feed; // <== import sibling module

pub fn do_something() {
  feed::do_something_else();
}

And my feed.rs looks something like this.

// file src/feed.rs
pub fn d_something_else() {
  // ...
}

From what I understand from experimenting is that you need to first use mod in your main.rs to define which modules are included in your crate.

Answer 2

I figured it out. The rust module system doesn't permit importing sibling files,

• src/a.rs
• src/b.rs

Full stop: a.rs can not import b.rs. What it will do is try to source it from

• src/a/b.rs

If you're on this answer, none of this probably makes sense to you and you've wasted hours on this. This was a source of my confusion:

• src/main.rs

Is actually special. Inside src/main.rs a mod will import a sibling file, (and also with the now deprecated mod.rs; or, with lib.rs). But the point is that your own rust files can't make use of rust code in sibling files.