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-test-runner.el b/tests/test-dev-fkeys--f6-test-runner.el
index eb9cec5ef..d5f58a66d 100644
--- a/tests/test-dev-fkeys--f6-test-runner.el
+++ b/tests/test-dev-fkeys--f6-test-runner.el
@@ -79,7 +79,7 @@ Components integrated:
                (lambda (&rest _) "Current file's tests"))
               ((symbol-function 'projectile-test-project) (lambda (_arg) nil))
               ((symbol-function 'cj/--f4-project-root) (lambda () "/p/"))
-              ((symbol-function 'buffer-file-name) (lambda () "/p/foo.el"))
+              ((symbol-function 'buffer-file-name) (lambda (&rest _) "/p/foo.el"))
               ((symbol-function 'cj/--f6-current-file-tests-impl)
                (lambda (file root)
                  (setq seen-file file seen-root root))))