Rust: no `module` in the root - import

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 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.

Answer 2

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.