bpo-23867: Argument Clinic: inline parsing code for a single positional parameter. (GH-9689)

diff --git a/Modules/clinic/_struct.c.h b/Modules/clinic/_struct.c.h
index b1fa067..3a186e4 100644
--- a/Modules/clinic/_struct.c.h
+++ b/Modules/clinic/_struct.c.h
@@ -57,7 +57,11 @@
     PyObject *return_value = NULL;
     Py_buffer buffer = {NULL, NULL};
 
-    if (!PyArg_Parse(arg, "y*:unpack", &buffer)) {
+    if (PyObject_GetBuffer(arg, &buffer, PyBUF_SIMPLE) != 0) {
+        goto exit;
+    }
+    if (!PyBuffer_IsContiguous(&buffer, 'C')) {
+        _PyArg_BadArgument("unpack", "contiguous buffer", arg);
         goto exit;
     }
     return_value = Struct_unpack_impl(self, &buffer);
@@ -166,7 +170,7 @@
     PyStructObject *s_object = NULL;
     Py_ssize_t _return_value;
 
-    if (!PyArg_Parse(arg, "O&:calcsize", cache_struct_converter, &s_object)) {
+    if (!cache_struct_converter(arg, &s_object)) {
         goto exit;
     }
     _return_value = calcsize_impl(module, s_object);
@@ -303,4 +307,4 @@
 
     return return_value;
 }
-/*[clinic end generated code: output=a73b0453174e4b51 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=01516bea2641fe01 input=a9049054013a1b77]*/