Pin the builtin registry shape and grow as a lower bound, and correct stale comments

builtins_test.go

@@ -181,3 +181,20 @@ func ibanValid(s string) bool {
 	}
 	return rem == 1
 }
+
+// TestRegistryShapes pins the builtin contract bind relies on: exactly one of prep
+// or call, and args parsed at compile only behind a check. A registry entry with
+// neither would compile and then panic inside a render.
+func TestRegistryShapes(t *testing.T) {
+	for name, b := range builtins {
+		switch {
+		case b.prep == nil && b.call == nil:
+			t.Errorf("builtin %q has neither prep nor call: bind would return a nil-func closure", name)
+		case b.prep != nil && b.call != nil:
+			t.Errorf("builtin %q has both prep and call; call is dead", name)
+		}
+		if b.prep != nil && b.arity != 0 && b.check == nil {
+			t.Errorf("builtin %q parses args in prep with no check", name)
+		}
+	}
+}