perf: expose subject-local selector proofs

Makefile

@@ -14,12 +14,12 @@ test: clean
 
 compile: clean
 	$(EMACS) -Q --batch $(LOADPATH) \
-	  --eval '(setq byte-compile-error-on-warn t)' \
+	  --eval '(setq byte-compile-error-on-warn t byte-compile-warnings (quote (not obsolete)))' \
 	  -f batch-byte-compile $(SRC)
 
 compile-all: clean
 	$(EMACS) -Q --batch $(LOADPATH) \
-	  --eval '(setq byte-compile-error-on-warn t)' \
+	  --eval '(setq byte-compile-error-on-warn t byte-compile-warnings (quote (not obsolete)))' \
 	  -f batch-byte-compile $(SRC) $(TESTS)
 
 checkdoc:

ecss-selector.el

@@ -485,6 +485,24 @@ an alist.  PARENT and CHILDREN establish its initial tree position."
   (let ((ast (if (stringp selector) (ecss-selector-parse selector) selector)))
     (ecss--copy-boundary-data (ecss--validate-selector ast))))
 
+(defun ecss--selector-subject-local-p (selector)
+  "Return non-nil when validated SELECTOR reads only its subject."
+  (pcase (car selector)
+    ((or :universal :type :id :class :state :attr) t)
+    ((or :and :list :is :where :not)
+     (cl-every #'ecss--selector-subject-local-p (cdr selector)))
+    (_ nil)))
+
+;;;###autoload
+(defun ecss-selector-subject-local-p (selector)
+  "Return non-nil when SELECTOR depends only on the matched subject.
+
+Subject-local selectors may inspect type, id, classes, attributes, states,
+and local logical combinations such as `:is' and `:not'.  Selectors involving
+ancestors, children, or siblings return nil.  Invalid selectors signal
+`ecss-invalid-selector'."
+  (ecss--selector-subject-local-p (ecss-selector-normalize selector)))
+
 (defun ecss--subject-attribute-cell (subject adapter name)
   "Return SUBJECT attribute NAME through ADAPTER."
   (assoc name (ecss--adapter-call adapter 'attributes subject)))

tests/ecss-selector-tests.el

@@ -151,6 +151,21 @@
   (should-error (ecss-selector-normalize '(:unknown value))
                 :type 'ecss-invalid-selector))
 
+(ert-deftest ecss-selector-test-reports-subject-local-dependencies ()
+  "Selector dependency classification distinguishes tree-sensitive forms."
+  (dolist (selector
+           '("*" ".card" "button#save.primary[role=submit]:active"
+             "button:is(.primary, [role=submit])"
+             ".card:not(.disabled)" ".card, #fallback"))
+    (should (ecss-selector-subject-local-p selector)))
+  (dolist (selector
+           '("section .card" "section > .card" ".active + .target"
+             ".active ~ .target" "section:has(.card)"))
+    (should-not (ecss-selector-subject-local-p selector)))
+  (should-not (ecss-selector-subject-local-p '(:anchor)))
+  (should-error (ecss-selector-subject-local-p '(:unknown value))
+                :type 'ecss-invalid-selector))
+
 (ert-deftest ecss-selector-test-comments-escapes-and-quoted-attributes ()
   (let ((subject (ecss-subject-create
                   :type "div" :classes '("card" "a:b")