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-host-environment--display-predicates.el b/tests/test-host-environment--display-predicates.el
index 15dff2ef8..5a87b5009 100644
--- a/tests/test-host-environment--display-predicates.el
+++ b/tests/test-host-environment--display-predicates.el
@@ -26,7 +26,7 @@ GRAPHIC-P becomes the return of `(display-graphic-p)'."
   `(cl-letf (((symbol-function 'window-system)
               (lambda (&optional _) ,window-system-value))
              ((symbol-function 'getenv)
-              (lambda (name)
+              (lambda (name &rest _)
                 (when (string= name "WAYLAND_DISPLAY") ,wayland-display)))
              ((symbol-function 'display-graphic-p)
               (lambda (&optional _) ,graphic-p)))