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, 3 insertions, 3 deletions

diff --git a/tests/test-prog-c-mode-settings.el b/tests/test-prog-c-mode-settings.el
index 37a77a213..33c503377 100644
--- a/tests/test-prog-c-mode-settings.el
+++ b/tests/test-prog-c-mode-settings.el
@@ -18,7 +18,7 @@
     (cl-letf (((symbol-function 'auto-fill-mode) (lambda (&rest _) nil))
               ((symbol-function 'electric-pair-local-mode) (lambda (&rest _) nil))
               ((symbol-function 'lsp-deferred) (lambda (&rest _) nil))
-              ((symbol-function 'executable-find) (lambda (_) nil)))
+              ((symbol-function 'executable-find) (lambda (_ &rest _) nil)))
       (cj/c-mode-settings))
     (should (eq indent-tabs-mode nil))
     (should (= c-basic-offset 4))
@@ -33,7 +33,7 @@
       (cl-letf (((symbol-function 'auto-fill-mode) (lambda (&rest _) nil))
                 ((symbol-function 'electric-pair-local-mode) (lambda (&rest _) nil))
                 ((symbol-function 'lsp-deferred) (lambda () (cl-incf lsp-calls)))
-                ((symbol-function 'executable-find) (lambda (_) "/usr/bin/clangd")))
+                ((symbol-function 'executable-find) (lambda (_ &rest _) "/usr/bin/clangd")))
         (cj/c-mode-settings)))
     (should (= lsp-calls 1))))
 
@@ -44,7 +44,7 @@
       (cl-letf (((symbol-function 'auto-fill-mode) (lambda (&rest _) nil))
                 ((symbol-function 'electric-pair-local-mode) (lambda (&rest _) nil))
                 ((symbol-function 'lsp-deferred) (lambda () (cl-incf lsp-calls)))
-                ((symbol-function 'executable-find) (lambda (_) nil)))
+                ((symbol-function 'executable-find) (lambda (_ &rest _) nil)))
         (cj/c-mode-settings)))
     (should (zerop lsp-calls))))