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-hugo-config-open-blog-dir-external.el b/tests/test-hugo-config-open-blog-dir-external.el
index 0bf68982..05f116e6 100644
--- a/tests/test-hugo-config-open-blog-dir-external.el
+++ b/tests/test-hugo-config-open-blog-dir-external.el
@@ -44,7 +44,7 @@ filesystem checks."
      (cl-letf (((symbol-function 'env-macos-p) (lambda () ,macos-p))
                ((symbol-function 'env-windows-p) (lambda () ,windows-p))
                ((symbol-function 'file-directory-p) (lambda (_d) t))
-               ((symbol-function 'executable-find) (lambda (cmd) cmd))
+               ((symbol-function 'executable-find) (lambda (cmd &rest _) cmd))
                ((symbol-function 'start-process)
                 (lambda (_name _buf cmd &rest _args)
                   (setq test-hugo--captured-process-cmd cmd))))
@@ -86,7 +86,7 @@ filesystem checks."
               ((symbol-function 'file-directory-p) (lambda (_d) nil))
               ((symbol-function 'make-directory)
                (lambda (_dir &rest _args) (setq mkdir-called t)))
-              ((symbol-function 'executable-find) (lambda (cmd) cmd))
+              ((symbol-function 'executable-find) (lambda (cmd &rest _) cmd))
               ((symbol-function 'start-process) #'ignore))
       (cj/hugo-open-blog-dir-external)
       (should mkdir-called))))
@@ -99,7 +99,7 @@ filesystem checks."
               ((symbol-function 'file-directory-p) (lambda (_d) t))
               ((symbol-function 'make-directory)
                (lambda (_dir &rest _args) (setq mkdir-called t)))
-              ((symbol-function 'executable-find) (lambda (cmd) cmd))
+              ((symbol-function 'executable-find) (lambda (cmd &rest _) cmd))
               ((symbol-function 'start-process) #'ignore))
       (cj/hugo-open-blog-dir-external)
       (should-not mkdir-called))))
@@ -111,7 +111,7 @@ filesystem checks."
   (cl-letf (((symbol-function 'env-macos-p) (lambda () nil))
             ((symbol-function 'env-windows-p) (lambda () nil))
             ((symbol-function 'file-directory-p) (lambda (_d) t))
-            ((symbol-function 'executable-find) (lambda (_) nil))
+            ((symbol-function 'executable-find) (lambda (_ &rest _) nil))
             ((symbol-function 'start-process)
              (lambda (&rest _) (error "start-process should not run"))))
     (should-error (cj/hugo-open-blog-dir-external) :type 'user-error)))