Use MeshRayCast to render debug ray

src/main.rs

@@ -3,11 +3,20 @@ use bevy::prelude::*;
 mod free_camera;
 use free_camera::*;
 
+mod placement;
+use placement::*;
+
 fn main() {
     App::new()
         .add_plugins(DefaultPlugins)
         .add_systems(Startup, setup)
         .add_systems(Update, (camera_free_look, noclip_controller).chain())
+        // This system requires `GlobalTransform`, so for a most
+        // up-to-date value, run it right after it's been updated.
+        .add_systems(
+            PostUpdate,
+            debug_ray_cast.after(TransformSystems::Propagate),
+        )
         .run();
 }
 

src/placement.rs

@@ -0,0 +1,27 @@
+use bevy::prelude::*;
+use bevy::window::{CursorGrabMode, CursorOptions};
+
+pub fn debug_ray_cast(
+    mut gizmos: Gizmos,
+    mut ray_cast: MeshRayCast,
+    window: Single<(&Window, &CursorOptions)>,
+    camera: Single<(&GlobalTransform, &Camera)>,
+) {
+    let (window, cursor) = window.into_inner();
+    let (transform, camera) = camera.into_inner();
+
+    let ray = if cursor.grab_mode == CursorGrabMode::Locked {
+        Ray3d::new(transform.translation(), transform.forward())
+    } else if let Some(cursor_pos) = window.cursor_position() {
+        camera.viewport_to_world(transform, cursor_pos).unwrap()
+    } else {
+        return; // cursor outside window area
+    };
+
+    let settings = &MeshRayCastSettings::default();
+    let Some((_entity, hit)) = ray_cast.cast_ray(ray, settings).first() else {
+        return; // ray didn't hit anything
+    };
+
+    gizmos.arrow(hit.point, hit.point + hit.normal / 2., Color::WHITE);
+}