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, 4 insertions, 4 deletions

diff --git a/tests/test-hugo-config-commands.el b/tests/test-hugo-config-commands.el
index 01df5fc18..07bc27ca3 100644
--- a/tests/test-hugo-config-commands.el
+++ b/tests/test-hugo-config-commands.el
@@ -134,7 +134,7 @@ stubbed before the org-mode-derived guard runs."
               ((symbol-function 'completing-read)
                (lambda (&rest _) "Foo Post"))
               ((symbol-function 'find-file)
-               (lambda (f) (setq opened f))))
+               (lambda (f &rest _) (setq opened f))))
       (cj/hugo-open-draft))
     (should (equal opened "/tmp/foo.org"))))
 
@@ -196,7 +196,7 @@ stubbed before the org-mode-derived guard runs."
         (msg nil))
     (cl-letf (((symbol-function 'process-live-p) (lambda (_) t))
               ((symbol-function 'kill-process)
-               (lambda (p) (setq killed p)))
+               (lambda (p &rest _) (setq killed p)))
               ((symbol-function 'message)
                (lambda (fmt &rest args)
                  (setq msg (apply #'format fmt args)))))
@@ -210,7 +210,7 @@ stubbed before the org-mode-derived guard runs."
   (let ((cj/hugo--preview-process nil)
         (start-args nil))
     (cl-letf (((symbol-function 'process-live-p) (lambda (_) nil))
-              ((symbol-function 'executable-find) (lambda (_) "/usr/bin/hugo"))
+              ((symbol-function 'executable-find) (lambda (_ &rest _) "/usr/bin/hugo"))
               ((symbol-function 'start-process)
                (lambda (&rest args)
                  (setq start-args args)
@@ -226,7 +226,7 @@ stubbed before the org-mode-derived guard runs."
   "Error: a missing hugo binary signals user-error before start-process."
   (let ((cj/hugo--preview-process nil))
     (cl-letf (((symbol-function 'process-live-p) (lambda (_) nil))
-              ((symbol-function 'executable-find) (lambda (_) nil))
+              ((symbol-function 'executable-find) (lambda (_ &rest _) nil))
               ((symbol-function 'start-process)
                (lambda (&rest _) (error "start-process should not run")))
               ((symbol-function 'message) #'ignore))