Fix stale CCD target cache after idle scene changes

crates/rapier2d/tests/issue_985_ccd_stale_fixed_target.rs

@@ -0,0 +1,66 @@
+//! Regression test for https://github.com/dimforge/rapier/issues/985.
+//!
+//! A fixed-target cache built by one CCD pass must not retain a collider that is
+//! removed during a later step where no body is fast enough to run CCD.
+
+use rapier2d::prelude::*;
+
+#[test]
+fn fixed_target_removed_without_active_ccd_is_not_reused_by_later_sweep() {
+    let mut world = PhysicsWorld::default();
+    world.gravity = Vector::new(0.0, -20.0);
+
+    world.insert(
+        RigidBodyBuilder::fixed().translation(Vector::new(0.0, -0.5)),
+        ColliderBuilder::cuboid(50.0, 0.5),
+    );
+
+    let (obstacle, _) = world.insert(
+        RigidBodyBuilder::fixed().translation(Vector::new(10.0, 5.0)),
+        ColliderBuilder::cuboid(1.0, 1.0),
+    );
+
+    world.insert(
+        RigidBodyBuilder::dynamic()
+            .translation(Vector::new(-10.0, 10.0))
+            .linvel(Vector::new(0.0, -30.0)),
+        ColliderBuilder::ball(0.25).density(8.0),
+    );
+
+    let mut falling_ball = None;
+    for tick in 0..=202 {
+        if tick == 120 {
+            assert!(
+                world
+                    .bodies
+                    .iter()
+                    .filter(|(_, body)| body.is_dynamic())
+                    .all(|(_, body)| !body.is_ccd_active()),
+                "the fixed target must be removed on a step with no active CCD body"
+            );
+            assert!(world.remove_body(obstacle).is_some());
+        }
+        if tick == 121 {
+            let ball = world
+                .insert(
+                    RigidBodyBuilder::dynamic().translation(Vector::new(10.0, 25.0)),
+                    ColliderBuilder::ball(0.25).density(8.0),
+                )
+                .0;
+            assert!(!world.bodies[ball].is_ccd_active());
+            falling_ball = Some(ball);
+        }
+
+        world.step();
+    }
+
+    let falling_ball = falling_ball.unwrap();
+    assert!(
+        world.bodies[falling_ball].is_ccd_active(),
+        "the later body must become fast enough to exercise the fixed-target sweep"
+    );
+    assert!(
+        world.bodies[falling_ball].translation().y.is_finite(),
+        "the later CCD sweep should complete with only live fixed-target handles"
+    );
+}

src/dynamics/ccd/ccd_solver.rs

@@ -43,6 +43,11 @@ impl CCDSolver {
         Self::default()
     }
 
+    /// Invalidates the fixed-target list after user changes to the scene.
+    pub(crate) fn invalidate_fixed_targets_cache(&mut self) {
+        self.fixed_targets_cache = None;
+    }
+
     /// Updates the set of bodies that needs CCD to be resolved.
     ///
     /// Returns `true` if any rigid-body must have CCD resolved.

src/pipeline/physics_pipeline/substep.rs

@@ -339,6 +339,11 @@ impl PhysicsPipeline {
         let ccd_scene_changed = !modified_colliders.is_empty()
             || !removed_colliders.is_empty()
             || !modified_bodies.is_empty();
+        if ccd_scene_changed {
+            // This step may have no fast body, so invalidate now instead of relying on
+            // the next motion-clamping pass to observe this per-step signal.
+            ccd_solver.invalidate_fixed_targets_cache();
+        }
 
         // Join islands based on new joints.
         #[cfg(feature = "enhanced-determinism")]