You can also make a whole module public, to allow access

src/lib.rs

@@ -1,4 +1,2 @@
 mod geometry;
-mod math;
-
-pub use math::Point3;
+pub mod math;

src/main.rs

@@ -1,4 +1,4 @@
-use hello::Point3;
+use hello::math::Point3;
 
 fn main() {
     let p = Point3::new(0.0, 0.0, 0.0);