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-slack-config-commands.el b/tests/test-slack-config-commands.el
index 8944662ef..21cbb3e5a 100644
--- a/tests/test-slack-config-commands.el
+++ b/tests/test-slack-config-commands.el
@@ -194,7 +194,7 @@
               ((symbol-function 'slack-buffer-update-mark-request)
                (lambda (_buf ts) (setq marked ts)))
               ((symbol-function 'bury-buffer)
-               (lambda () (setq buried t))))
+               (lambda (&rest _) (setq buried t))))
       (cj/slack-mark-read-and-bury))
     (should (equal marked "1234.5678"))
     (should buried)))
@@ -207,7 +207,7 @@
     (cl-letf (((symbol-function 'slack-buffer-update-mark-request)
                (lambda (&rest _) (setq marked t)))
               ((symbol-function 'bury-buffer)
-               (lambda () (setq buried t))))
+               (lambda (&rest _) (setq buried t))))
       (cj/slack-mark-read-and-bury))
     (should-not marked)
     (should buried)))