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, 1 insertions, 1 deletions

diff --git a/tests/test-dev-fkeys--f6-current-file-tests-impl.el b/tests/test-dev-fkeys--f6-current-file-tests-impl.el
index 1cf222305..2d8e43858 100644
--- a/tests/test-dev-fkeys--f6-current-file-tests-impl.el
+++ b/tests/test-dev-fkeys--f6-current-file-tests-impl.el
@@ -111,7 +111,7 @@ runner instead of erroring as unsupported."
   (let ((compile-called nil))
     (cl-letf (((symbol-function 'compile)
                (lambda (cmd) (setq compile-called cmd)))
-              ((symbol-function 'executable-find) (lambda (_) nil)))
+              ((symbol-function 'executable-find) (lambda (_ &rest _) nil)))
       (cj/--f6-current-file-tests-impl
        "/home/u/proj/src/foo.test.ts" "/home/u/proj/")
       (should (stringp compile-called))