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, 3 insertions, 3 deletions

diff --git a/tests/test-gptel-tools-web-fetch.el b/tests/test-gptel-tools-web-fetch.el
index b6dbefccb..10abe6eba 100644
--- a/tests/test-gptel-tools-web-fetch.el
+++ b/tests/test-gptel-tools-web-fetch.el
@@ -106,13 +106,13 @@
 
 (ert-deftest test-gptel-tools-web-fetch-html-to-text-error-when-neither-on-path ()
   "Error: when neither pandoc nor w3m is 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/gptel-web-fetch--html-to-text "<p>x</p>"))))
 
 (ert-deftest test-gptel-tools-web-fetch-html-to-text-error-on-tool-failure ()
   "Error: a failing HTML stripping command is reported."
   (cl-letf (((symbol-function 'executable-find)
-             (lambda (program) (and (equal program "pandoc") "/bin/pandoc")))
+             (lambda (program &rest _) (and (equal program "pandoc") "/bin/pandoc")))
             ((symbol-function 'call-process-region)
              (lambda (&rest _args) 9)))
     (should-error (cj/gptel-web-fetch--html-to-text "<p>x</p>"))))
@@ -121,7 +121,7 @@
   "Boundary: w3m is used when pandoc is unavailable."
   (let (called-program)
     (cl-letf (((symbol-function 'executable-find)
-               (lambda (program) (and (equal program "w3m") "/bin/w3m")))
+               (lambda (program &rest _) (and (equal program "w3m") "/bin/w3m")))
               ((symbol-function 'call-process-region)
                (lambda (start end program delete output display &rest _args)
                  (setq called-program program)