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, 6 insertions, 6 deletions

diff --git a/tests/test-prog-general--find-file-respecting-split.el b/tests/test-prog-general--find-file-respecting-split.el
index 6d45c51c0..821cc79d6 100644
--- a/tests/test-prog-general--find-file-respecting-split.el
+++ b/tests/test-prog-general--find-file-respecting-split.el
@@ -23,9 +23,9 @@
     (delete-other-windows)
     (let (current-arg other-called)
       (cl-letf (((symbol-function 'find-file)
-                 (lambda (f) (setq current-arg f)))
+                 (lambda (f &rest _) (setq current-arg f)))
                 ((symbol-function 'find-file-other-window)
-                 (lambda (_f) (setq other-called t))))
+                 (lambda (_f &rest _) (setq other-called t))))
         (cj/--find-file-respecting-split "/tmp/proj/todo.org"))
       (should (equal current-arg "/tmp/proj/todo.org"))
       (should-not other-called))))
@@ -37,9 +37,9 @@
     (split-window-right)
     (let (other-arg current-called)
       (cl-letf (((symbol-function 'find-file-other-window)
-                 (lambda (f) (setq other-arg f)))
+                 (lambda (f &rest _) (setq other-arg f)))
                 ((symbol-function 'find-file)
-                 (lambda (_f) (setq current-called t))))
+                 (lambda (_f &rest _) (setq current-called t))))
         (cj/--find-file-respecting-split "/tmp/proj/todo.org"))
       (should (equal other-arg "/tmp/proj/todo.org"))
       (should-not current-called))))
@@ -52,9 +52,9 @@
     (split-window-below)
     (let (other-called current-called)
       (cl-letf (((symbol-function 'find-file-other-window)
-                 (lambda (_f) (setq other-called t)))
+                 (lambda (_f &rest _) (setq other-called t)))
                 ((symbol-function 'find-file)
-                 (lambda (_f) (setq current-called t))))
+                 (lambda (_f &rest _) (setq current-called t))))
         (cj/--find-file-respecting-split "/tmp/proj/todo.org"))
       (should other-called)
       (should-not current-called))))