solid.mirror: a plane's {origin} takes length expressions and follows them (#1492) (#1501)

Thanks @erkexzcx.

crates/engine/src/tests/mirror_planes.rs

@@ -39,6 +39,41 @@ fn mirror_across_a_construction_plane_follows_it() {
     }
 }
 
+/// A plane `{origin, normal}` takes length expressions in its origin, as a primitive's corner
+/// does, and the mirror follows them through parameter changes, renames and a save (#1492).
+#[test]
+fn mirror_plane_origin_follows_its_parameters() {
+    let mut s = Session::default();
+    run(&mut s, "parameters.add", json!({"name": "half", "expression": "50 mm"}));
+    run(&mut s, "solid.box", json!({"length": 20, "width": 20, "height": 20}));
+    run(&mut s, "solid.mirror", json!({"features": ["Box1"], "plane": {"origin": ["half", 0, 0], "normal": [1, 0, 0]}, "name": "Twin"}));
+    let at = |s: &Session| min_x(s, &last_body(s));
+    // The box spans x = 0…20; mirrored about x = 50 it spans 80…100.
+    assert!((at(&s) - 80.0).abs() < 1e-6, "{}", at(&s));
+    run(&mut s, "parameters.change", json!({"name": "half", "expression": "30 mm"}));
+    assert!((at(&s) - 40.0).abs() < 1e-6, "{}", at(&s));
+    run(&mut s, "parameters.rename", json!({"name": "half", "new_name": "mid"}));
+    run(&mut s, "parameters.change", json!({"name": "mid", "expression": "25 mm"}));
+    assert!((at(&s) - 30.0).abs() < 1e-6, "{}", at(&s));
+    // Saved and opened again, the plane still names the parameter.
+    let mut s2 = Session::new(solidcraft_doc::Document::from_json(&s.doc.to_json()).unwrap());
+    assert!(s2.doc.to_json().contains("\"mid\""));
+    run(&mut s2, "parameters.change", json!({"name": "mid", "expression": "60 mm"}));
+    assert!((at(&s2) - 100.0).abs() < 1e-6, "{}", at(&s2));
+    // A parameter the plane uses can't be deleted from under it.
+    assert!(s2.execute("parameters.delete", &json!({"name": "mid"})).is_err());
+    // Plain numbers still make a fixed plane.
+    let r = run(&mut s2, "solid.mirror", json!({"features": ["Box1"], "plane": {"origin": [-10, 0, 0], "normal": [1, 0, 0]}}));
+    let id = r["feature"].as_u64().unwrap();
+    let f = s2.doc.feature(id).unwrap();
+    assert!(matches!(f.kind, solidcraft_doc::FeatureKind::Mirror { plane: solidcraft_doc::PlaneRef::Custom { .. }, .. }), "{:?}", f.kind);
+    assert!((at(&s2) + 40.0).abs() < 1e-6, "{}", at(&s2));
+}
+
+fn last_body(s: &Session) -> String {
+    s.model.state().bodies.last().map(|b| b.name.clone()).unwrap()
+}
+
 #[test]
 fn unknown_mirror_planes_are_refused_with_the_choices() {
     let mut s = Session::default();