//something.rs
use other::do_stuff;
pub fn start() {
    do_stuff(/*What do i put here*/);
}

//main.rs
mod something;
mod other;
mod third;
use third::Third;
fn main() {
    let th = Third::new();
    something::start();
}

//other.rs
use third::Third;
pub fn do_stuff(thrd: Third) {
    /* do something with it */
}

//third.rs
pub struct Third {
    /* Omitted */
}