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, 30 insertions, 3 deletions

diff --git a/Modules/_testcapi/parts.h b/Modules/_testcapi/parts.h
index a76ddd9..304e592 100644
--- a/Modules/_testcapi/parts.h
+++ b/Modules/_testcapi/parts.h
@@ -1,9 +1,36 @@
-#include "Python.h"
+#ifndef Py_TESTCAPI_PARTS_H
+#define Py_TESTCAPI_PARTS_H
+
+#include "pyconfig.h"  // for Py_TRACE_REFS
+
+// Figure out if Limited API is available for this build. If it isn't we won't
+// build tests for it.
+// Currently, only Py_TRACE_REFS disables Limited API.
+#ifdef Py_TRACE_REFS
+#undef LIMITED_API_AVAILABLE
+#else
+#define LIMITED_API_AVAILABLE 1
+#endif
 
-/* Always enable assertions */
+// Always enable assertions
 #undef NDEBUG
 
+#if !defined(LIMITED_API_AVAILABLE) && defined(Py_LIMITED_API)
+// Limited API being unavailable means that with Py_LIMITED_API defined
+// we can't even include Python.h.
+// Do nothing; the .c file that defined Py_LIMITED_API should also do nothing.
+
+#else
+
+#include "Python.h"
+
 int _PyTestCapi_Init_Vectorcall(PyObject *module);
-int _PyTestCapi_Init_VectorcallLimited(PyObject *module);
 int _PyTestCapi_Init_Heaptype(PyObject *module);
 int _PyTestCapi_Init_Unicode(PyObject *module);
+
+#ifdef LIMITED_API_AVAILABLE
+int _PyTestCapi_Init_VectorcallLimited(PyObject *module);
+#endif // LIMITED_API_AVAILABLE
+
+#endif
+#endif // Py_TESTCAPI_PARTS_H