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-keybindings--jump-open-var.el b/tests/test-keybindings--jump-open-var.el
index bd04f4cf1..041f4a7d3 100644
--- a/tests/test-keybindings--jump-open-var.el
+++ b/tests/test-keybindings--jump-open-var.el
@@ -25,7 +25,7 @@ CAPTURE-VAR is set to the path passed to `find-file', or stays nil if
 the mock is never called."
   (declare (indent 1) (debug t))
   `(cl-letf (((symbol-function 'find-file)
-              (lambda (path) (setq ,capture-var path))))
+              (lambda (path &rest _) (setq ,capture-var path))))
      ,@body))
 
 (defmacro test-keybindings--with-fixture (value &rest body)