t1.rs

use t2::t3::fun;
mod t2;

fn main() { 
    t2::fun();
    fun(); // calling t3::fun
}

t2.rs

pub mod t3; // `pub` so that main can import it in `t2::t3::fun`.

pub fn fun() {
    t3::fun();
}

t3.rs

pub fn fun() {}