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, 4 insertions, 4 deletions

diff --git a/tests/test-config-utilities--compile-this-elisp-buffer.el b/tests/test-config-utilities--compile-this-elisp-buffer.el
index fb5e288a1..a06440abb 100644
--- a/tests/test-config-utilities--compile-this-elisp-buffer.el
+++ b/tests/test-config-utilities--compile-this-elisp-buffer.el
@@ -21,7 +21,7 @@ effects."
   (declare (indent 1) (debug t))
   `(with-temp-buffer
      (setq buffer-file-name ,path)
-     (cl-letf (((symbol-function 'save-buffer) (lambda () nil)))
+     (cl-letf (((symbol-function 'save-buffer) (lambda (&rest _) nil)))
        ,@body)))
 
 (ert-deftest test-config-utilities-compile-buffer-not-elisp-raises ()
@@ -47,7 +47,7 @@ effects."
                 ((symbol-function 'native-compile)
                  (lambda (_) (error "should not call sync native-compile")))
                 ((symbol-function 'byte-compile-file)
-                 (lambda (_) (error "should not call byte-compile-file"))))
+                 (lambda (&rest _) (error "should not call byte-compile-file"))))
         (cj/compile-this-elisp-buffer)
         (should (equal called-with "/tmp/some.el"))))))
 
@@ -60,7 +60,7 @@ effects."
                 ((symbol-function 'native-compile)
                  (lambda (file) (setq called-with file)))
                 ((symbol-function 'byte-compile-file)
-                 (lambda (_) (error "should not call byte-compile-file"))))
+                 (lambda (&rest _) (error "should not call byte-compile-file"))))
         (cj/compile-this-elisp-buffer)
         (should (equal called-with "/tmp/some.el"))))))
 
@@ -71,7 +71,7 @@ effects."
       (cl-letf (((symbol-function 'fboundp)
                  (lambda (sym) (eq sym 'byte-compile-file)))
                 ((symbol-function 'byte-compile-file)
-                 (lambda (file) (setq called-with file) "/tmp/some.elc")))
+                 (lambda (file &rest _) (setq called-with file) "/tmp/some.elc")))
         (cj/compile-this-elisp-buffer)
         (should (equal called-with "/tmp/some.el"))))))