lua: proper early-return via guard+raise sentinel; fixes if-then-return-end-rest +3 tests

lib/lua/runtime.sx

@@ -1503,9 +1503,15 @@
     (guard
       (e (true (list (quote lua-multi) nil (str e))))
       (let ((compiled (lua-transpile src)))
-        (let ((wrapped (list (make-symbol "fn") (list (make-symbol "&rest") (make-symbol "__args"))
-                             (list (make-symbol "let") (list) compiled))))
-          (eval-expr wrapped))))))
+        (let ((guarded (list
+                         (make-symbol "guard")
+                         (list (make-symbol "e")
+                           (list
+                             (list (make-symbol "lua-return-sentinel?") (make-symbol "e"))
+                             (list (make-symbol "lua-return-value") (make-symbol "e"))))
+                         (list (make-symbol "let") (list) compiled))))
+          (let ((wrapped (list (make-symbol "fn") (list (make-symbol "&rest") (make-symbol "__args")) guarded)))
+            (eval-expr wrapped)))))))
 
 (define loadstring lua-loadstring)
 (define load lua-loadstring)
@@ -1630,3 +1636,13 @@
         (va-build skip)
         (dict-set! t "n" n)
         t))))
+
+;; Return-sentinel: wrap function bodies so mid-block `return` can escape.
+(define
+  lua-return-sentinel?
+  (fn (e)
+    (and (= (type-of e) "list") (> (len e) 0) (= (first e) (quote lua-ret)))))
+
+(define
+  lua-return-value
+  (fn (e) (if (> (len e) 1) (nth e 1) nil)))