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-dev-fkeys--f6-test-runner-cmd-for.el b/tests/test-dev-fkeys--f6-test-runner-cmd-for.el
index 9a5526125..d7b6a0597 100644
--- a/tests/test-dev-fkeys--f6-test-runner-cmd-for.el
+++ b/tests/test-dev-fkeys--f6-test-runner-cmd-for.el
@@ -126,13 +126,13 @@ neither tool is present, the user gets a clear runner-not-found error
 rather than a silent nil that F6's outer wrapper interprets as
 \"language unsupported.\""
   (cl-letf (((symbol-function 'executable-find)
-             (lambda (_) nil)))
+             (lambda (_ &rest _) nil)))
     (should (equal
              (cj/--f6-test-runner-cmd-for
               'typescript t "src/foo.test.ts" "foo" "src")
              "npx --no-install jest src/foo.test.ts")))
   (cl-letf (((symbol-function 'executable-find)
-             (lambda (p) (when (equal p "vitest") "/usr/bin/vitest"))))
+             (lambda (p &rest _) (when (equal p "vitest") "/usr/bin/vitest"))))
     (should (equal
              (cj/--f6-test-runner-cmd-for
               'typescript t "src/foo.test.ts" "foo" "src")