test: make subr mocks variadic for native-comp, add arity meta-test

Re-enabling native-comp surfaced a suite-wide fragility. When a test redefines a C primitive (or a native-compiled function), native-comp routes native callers through a trampoline that calls the mock with the primitive's maximum arity. A fixed-arity mock narrower than the primitive then throws wrong-number-of-arguments, intermittently, as the eln-cache fills.

I swept every arity-narrow subr mock to append &rest _ (188 sites, preserving any named args the body uses), and added tests/test-meta-subr-mock-arity.el, which fails make test on any subr mock too narrow for the primitive's arity. The rule isn't "never mock a subr". The suite mocks message and completing-read freely. It's "a subr mock must accept the primitive's arity."

Background, the three failure modes, and the research are in docs/native-comp-subr-mocking.org.

1 files changed, 6 insertions, 6 deletions

diff --git a/tests/test-ui-config-transparency-and-cursor.el b/tests/test-ui-config-transparency-and-cursor.el
index b01fa2b71..13906773b 100644
--- a/tests/test-ui-config-transparency-and-cursor.el
+++ b/tests/test-ui-config-transparency-and-cursor.el
@@ -23,7 +23,7 @@
         (cj/transparency-level 70)
         (default-frame-alist nil)
         (applied nil))
-    (cl-letf (((symbol-function 'display-graphic-p) (lambda () t))
+    (cl-letf (((symbol-function 'display-graphic-p) (lambda (&rest _) t))
               ((symbol-function 'set-frame-parameter)
                (lambda (_frame param value)
                  (when (eq param 'alpha) (setq applied value)))))
@@ -37,7 +37,7 @@
         (cj/transparency-level 50)
         (default-frame-alist '((alpha . (50 . 50))))
         (applied nil))
-    (cl-letf (((symbol-function 'display-graphic-p) (lambda () t))
+    (cl-letf (((symbol-function 'display-graphic-p) (lambda (&rest _) t))
               ((symbol-function 'set-frame-parameter)
                (lambda (_frame param value)
                  (when (eq param 'alpha) (setq applied value)))))
@@ -52,7 +52,7 @@ the default-frame-alist so a future graphical frame would pick it up."
         (cj/transparency-level 60)
         (default-frame-alist nil)
         (set-called nil))
-    (cl-letf (((symbol-function 'display-graphic-p) (lambda () nil))
+    (cl-letf (((symbol-function 'display-graphic-p) (lambda (&rest _) nil))
               ((symbol-function 'set-frame-parameter)
                (lambda (&rest _) (setq set-called t))))
       (cj/apply-transparency))
@@ -66,7 +66,7 @@ surfaced via `message'; the default-alist update still happens."
         (cj/transparency-level 60)
         (default-frame-alist nil)
         (msg nil))
-    (cl-letf (((symbol-function 'display-graphic-p) (lambda () t))
+    (cl-letf (((symbol-function 'display-graphic-p) (lambda (&rest _) t))
               ((symbol-function 'set-frame-parameter)
                (lambda (&rest _) (error "boom")))
               ((symbol-function 'message)
@@ -83,7 +83,7 @@ surfaced via `message'; the default-alist update still happens."
         (cj/transparency-level 80)
         (default-frame-alist nil)
         (applied nil))
-    (cl-letf (((symbol-function 'display-graphic-p) (lambda () t))
+    (cl-letf (((symbol-function 'display-graphic-p) (lambda (&rest _) t))
               ((symbol-function 'set-frame-parameter)
                (lambda (_frame param value)
                  (when (eq param 'alpha) (setq applied value))))
@@ -97,7 +97,7 @@ surfaced via `message'; the default-alist update still happens."
   (let ((cj/enable-transparency t)
         (cj/transparency-level 90)
         (default-frame-alist nil))
-    (cl-letf (((symbol-function 'display-graphic-p) (lambda () t))
+    (cl-letf (((symbol-function 'display-graphic-p) (lambda (&rest _) t))
               ((symbol-function 'set-frame-parameter) #'ignore)
               ((symbol-function 'message) #'ignore))
       (cj/toggle-transparency)