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, 2 insertions, 2 deletions

diff --git a/tests/test-flyspell-and-abbrev.el b/tests/test-flyspell-and-abbrev.el
index 793fdc0f4..ef8cc6375 100644
--- a/tests/test-flyspell-and-abbrev.el
+++ b/tests/test-flyspell-and-abbrev.el
@@ -32,12 +32,12 @@
 (ert-deftest test-flyspell-require-spell-checker-present ()
   "Normal: a checker on PATH means no error."
   (cl-letf (((symbol-function 'executable-find)
-             (lambda (cmd) (equal cmd (car cj/--spell-checker-executables)))))
+             (lambda (cmd &rest _) (equal cmd (car cj/--spell-checker-executables)))))
     (should-not (cj/--require-spell-checker))))
 
 (ert-deftest test-flyspell-require-spell-checker-missing ()
   "Error: no checker on PATH signals user-error."
-  (cl-letf (((symbol-function 'executable-find) (lambda (_) nil)))
+  (cl-letf (((symbol-function 'executable-find) (lambda (_ &rest _) nil)))
     (should-error (cj/--require-spell-checker) :type 'user-error)))
 
 ;; --------------------- cj/find-previous-flyspell-overlay ---------------------