Rust

Best to not think of files as modules. Instead a rust crate is just a tree of modules with the src/main.rs or src/lib.rs being the main entry point.

Inside these files you define the module structure you want like:

mod foo {
    mod bar {}
}

This creates two modules, crate::foo and crate::foo::bar. Now, because you don't want to have all your code in main.rs/lib.rs rust lets you move the module contents to separate files. But the location of the file needs to match its location in the module structure - from the crates root (not the file it was declared in).

So when you call mod foo; from src/main.rs it will look for src/foo.rs or src/foo/mod.rs (and you can only have one of these). And the foo::bar - no matter if that is declared in src/main.rs (as above) or inside src/foo.rs or src/foo/mod.rs, it will always look for the bar module contents inside src/foo/bar.rs or src/foo/bar/mod.rs as it is nested inside the foo module - not because the file is next to the current one.

This means if you had this inside main.rs/lib.rs:

mod foo {
    mod bar {
        mod baz;
    }
}

Then it will look for the baz module contents inside src/foo/bar/baz.rs or src/foo/bar/baz/mod.rs - even though those might be the only two files you have.