src/a.rs, src/b.rs, src/lib.rs

a.rs

pub fn fn_a() -> u32 { 1 }

b.rs

use a;

// error[E0463]: can't find crate for `a`
//   --> src/b.rs:1:5
//   |
// 1 | use a::fn_a;
//   |     ^ can't find crate

pub fn fn_b() -> u32 {
    a::fn_a()
}

lib.rs

mod a;
mod b;