gh-143667: Specialize FOR_ITER for dict.items(), and scalar replace the pair in the JIT

Include/internal/pycore_dict.h

@@ -344,6 +344,19 @@ extern void _PyDict_EnablePerThreadRefcounting(PyObject *op);
 
 PyDictObject *_PyObject_MaterializeManagedDict_LockHeld(PyObject *);
 
+PyAPI_FUNC(PyObject *) _PyDictIter_IterNextItem(PyObject *self);
+
+PyAPI_FUNC(int) _PyDictIter_IterNextItemPair(PyObject *self, PyObject **key, PyObject **value);
+
+typedef struct {
+    PyObject_HEAD
+    PyDictObject *di_dict; /* Set to NULL when iterator is exhausted */
+    Py_ssize_t di_used;
+    Py_ssize_t di_pos;
+    PyObject* di_result; /* reusable result tuple for iteritems */
+    Py_ssize_t len;
+} _PyDictIterObject;
+
 // See `_Py_INCREF_TYPE()` in pycore_object.h
 #ifndef Py_GIL_DISABLED
 #  define _Py_INCREF_DICT Py_INCREF

Include/internal/pycore_magic_number.h

@@ -290,6 +290,7 @@ Known values:
     Python 3.15a4 3657 (Add BINARY_OP_SUBSCR_USTR_INT)
     Python 3.15a4 3658 (Optimize bytecode for list/set called on genexp)
     Python 3.15a4 3659 (Add CALL_FUNCTION_EX specialization)
+    Python 3.15a4 3660 (Add FOR_ITER_DICT_ITEMS specialization)
 
 
     Python 3.16 will start with 3700
@@ -303,7 +304,7 @@ PC/launcher.c must also be updated.
 
 */
 
-#define PYC_MAGIC_NUMBER 3659
+#define PYC_MAGIC_NUMBER 3660
 /* This is equivalent to converting PYC_MAGIC_NUMBER to 2 bytes
    (little-endian) and then appending b'\r\n'. */
 #define PYC_MAGIC_NUMBER_TOKEN \