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-prog-json--json-format-buffer.el b/tests/test-prog-json--json-format-buffer.el
index 70d7e98bb..c6297a404 100644
--- a/tests/test-prog-json--json-format-buffer.el
+++ b/tests/test-prog-json--json-format-buffer.el
@@ -16,7 +16,7 @@
 (ert-deftest test-prog-json--json-format-buffer-invokes-jq-argv ()
   "Normal: with jq present, the formatter calls jq via argv, no shell."
   (let (program args)
-    (cl-letf (((symbol-function 'executable-find) (lambda (_p) "/usr/bin/jq"))
+    (cl-letf (((symbol-function 'executable-find) (lambda (_p &rest _) "/usr/bin/jq"))
               ((symbol-function 'call-process-region)
                (lambda (_start _end prog &rest rest)
                  (setq program prog
@@ -31,7 +31,7 @@
 
 (ert-deftest test-prog-json--json-format-buffer-no-clobber-on-failure ()
   "Error: a non-zero jq exit leaves the buffer untouched and signals an error."
-  (cl-letf (((symbol-function 'executable-find) (lambda (_p) "/usr/bin/jq"))
+  (cl-letf (((symbol-function 'executable-find) (lambda (_p &rest _) "/usr/bin/jq"))
             ((symbol-function 'call-process-region)
              (lambda (_start _end _prog _delete buffer &rest _)
                (with-current-buffer buffer (insert "jq: parse error"))
@@ -112,7 +112,7 @@
 
 (ert-deftest test-prog-json--json-format-buffer-fallback-formats-without-jq ()
   "Falls back to built-in formatter when jq is not found."
-  (cl-letf (((symbol-function 'executable-find) (lambda (_) nil)))
+  (cl-letf (((symbol-function 'executable-find) (lambda (_ &rest _) nil)))
     (with-temp-buffer
       (insert "{\"b\":1,\"a\":2}")
       (cj/json-format-buffer)