gh-95991: Add some infrastructure for testing Limited API in _testcapi (GH-95992)

- Limited API needs to be enabled per source file
- Some builds don't support Limited API, so Limited API tests must be skipped on those builds
  (currently this is `Py_TRACE_REFS`, but that may change.)
- `Py_LIMITED_API` must be defined before `<Python.h>` is included.

This puts the hoop-jumping in `testcapi/parts.h`, so individual
test files can be relatively simple. (Currently that's only
`vectorcall_limited.c`, imagine more.)

1 files changed, 9 insertions, 3 deletions

diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 8d9a0c1..2d4c73c 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -6544,9 +6544,6 @@ PyInit__testcapi(void)
     if (_PyTestCapi_Init_Vectorcall(m) < 0) {
         return NULL;
     }
-    if (_PyTestCapi_Init_VectorcallLimited(m) < 0) {
-        return NULL;
-    }
     if (_PyTestCapi_Init_Heaptype(m) < 0) {
         return NULL;
     }
@@ -6554,6 +6551,15 @@ PyInit__testcapi(void)
         return NULL;
     }
 
+#ifndef LIMITED_API_AVAILABLE
+    PyModule_AddObjectRef(m, "LIMITED_API_AVAILABLE", Py_False);
+#else
+    PyModule_AddObjectRef(m, "LIMITED_API_AVAILABLE", Py_True);
+    if (_PyTestCapi_Init_VectorcallLimited(m) < 0) {
+        return NULL;
+    }
+#endif
+
     PyState_AddModule(m, &_testcapimodule);
     return m;
 }