Why rust can't find a function in a submodule? [duplicate] - module

Question

This question already has answers here:
How can I include a module from another file from the same project?
(6 answers)
Closed 5 years ago.
I'm trying to call a function from a module located in a separate file, the function is public and I am calling it using a full path, but rustc still complains about "unresolved name".
a.rs
pub mod b;
fn main() {
b::f()
}
b.rs
pub mod b {
pub fn f(){
println!("Hello World!");
}
}
compilation
$ rustc a.rs
a.rs:3:5: 3:9 error: unresolved name `b::f`.
When I move the module to the crate's main file, everything works fine.
one_file.rs
pub mod b {
pub fn f(){
println!("Hello World!");
}
}
fn main() {
b::f()
}
Aren't these two ways supposed to be equivalent?
Am I doing something wrong, or it's a bug in rustc?

Answer 1

When you have separate files, they are automatically considered as separate modules.
Thus, you can do :
othermod.rs
pub fn foo() {
}
pub mod submod {
pub fn subfoo() {
}
}
main.rs
mod othermod;
fn main () {
othermod::foo();
othermod::submod::subfoo();
}
Note that if you use sub-directories, the file mod.rs is special and will be considered as the root of your module :
directory/file.rs
pub fn filebar() {
}
directory/mod.rs
pub mod file;
pub fn bar() {
}
main.rs
mod directory;
fn main() {
directory::bar();
directory::file::filebar();
}