Preserve warm assembly caches across Save As renames (#2120)

Thanks @yutyan0119.

crates/engine/src/cmd/file.rs

@@ -294,12 +294,23 @@ fn saved_name(name: &str, old_path: Option<&str>, path: &str) -> Option<String>
 /// Rename the design, and the undo and redo states that still carry the old name, so undo
 /// does not bring the default name back and leave the design looking changed.
 fn rename_design(s: &mut Session, from: &str, to: &str) {
+    // A prepared world is keyed by the document's Arc. Changing only its name
+    // must not make an opted-in save lose warm geometry and display caches.
+    // Retag only a cache that already matches: cold or stale placements stay so.
+    let model = s.model.state();
+    let keep_world = s.world_cache.lock().ok().is_some_and(|cache| cache.as_ref().is_some_and(|cache| cache.matches(s, &model)));
     s.doc_mut().name = to.to_string();
     for snap in s.undo.iter_mut().chain(s.redo.iter_mut()) {
         if snap.doc.name == from {
             std::sync::Arc::make_mut(&mut snap.doc).name = to.to_string();
         }
     }
+    if keep_world
+        && let Ok(mut cache) = s.world_cache.lock()
+        && let Some(cache) = cache.as_mut()
+    {
+        cache.doc = std::sync::Arc::clone(&s.doc);
+    }
 }
 
 fn write(s: &mut Session, path: &str, format: &str, compression: &str, preview: bool, cache: bool) -> Result<Value> {

crates/engine/tests/save_as_cache.rs

@@ -0,0 +1,58 @@
+//! Optional save caches survive a filename-driven rename, without warming or
+//! certifying placements that do not match the document being saved.
+use serde_json::json;
+use solidcraft_doc::Document;
+use solidcraft_engine::Session;
+
+fn assembly() -> Session {
+    let mut session = Session::new(Document::new("Untitled"));
+    session.execute("component.create", &json!({"name":"PlacedPart"})).unwrap();
+    session.execute("solid.box", &json!({"length":2,"width":3,"height":4})).unwrap();
+    session
+}
+
+fn warm(session: &Session) {
+    for body in &session.world_state().bodies {
+        body.mesh();
+    }
+}
+
+fn save(session: &mut Session, label: &str) -> Vec<u8> {
+    let path = std::env::temp_dir().join(format!("solidcraft-save-as-{label}-{}.solidcraft", std::process::id()));
+    session.execute("file.save_as", &json!({"path":path,"cache":true,"preview":true})).unwrap();
+    let bytes = std::fs::read(&path).unwrap();
+    assert_eq!(solidcraft_io::read_design(&bytes).unwrap().name, path.file_stem().unwrap().to_string_lossy());
+    std::fs::remove_file(&path).unwrap();
+    let _ = std::fs::remove_file(format!("{}.bak", path.display()));
+    bytes
+}
+
+#[test]
+fn a_filename_rename_preserves_opted_in_warm_assembly_caches() {
+    let mut session = assembly();
+    warm(&session);
+    for label in ["first-warm", "renamed-warm"] {
+        let bytes = save(&mut session, label);
+        let geometry = solidcraft_io::read_design_geometry(&bytes).expect("warm geometry survives the rename");
+        assert_eq!(geometry.len(), 1);
+        assert!((solidcraft_kernel::measure(&geometry[0].body).unwrap().volume - 24.0).abs() < 1e-9);
+        assert!(solidcraft_io::read_design_preview(&bytes).is_some(), "warm display meshes survive the rename");
+        warm(&session);
+    }
+}
+
+#[test]
+fn naming_a_design_does_not_warm_cold_or_revalidate_stale_placements() {
+    let original = assembly();
+    let mut cold = Session::new((*original.doc).clone());
+    let bytes = save(&mut cold, "cold");
+    assert!(solidcraft_io::read_design_geometry(&bytes).is_none());
+    assert!(solidcraft_io::read_design_preview(&bytes).is_none());
+
+    let mut stale = assembly();
+    warm(&stale);
+    stale.doc_mut().occurrences.first_mut().unwrap().transform[3][0] = 25.0;
+    let bytes = save(&mut stale, "stale");
+    assert!(solidcraft_io::read_design_geometry(&bytes).is_none(), "old placements must not be certified for a changed document");
+    assert!(solidcraft_io::read_design_preview(&bytes).is_none());
+}