gh-117549: Don't use designated initializers in headers (#118580)

The designated initializer syntax in static inline functions in pycore_backoff.h
causes problems for C++ or MSVC users who aren't yet using C++20.
While internal, pycore_backoff.h is included (indirectly, via pycore_code.h)
by some key 3rd party software that does so for speed.

Author: gvanrossum

Include/internal/pycore_backoff.h

@@ -44,13 +44,18 @@ make_backoff_counter(uint16_t value, uint16_t backoff)
 {
     assert(backoff <= 15);
     assert(value <= 0xFFF);
-    return (_Py_BackoffCounter){.backoff = backoff, .value = value};
+    _Py_BackoffCounter result;
+    result.value = value;
+    result.backoff = backoff;
+    return result;
 }
 
 static inline _Py_BackoffCounter
 forge_backoff_counter(uint16_t counter)
 {
-    return (_Py_BackoffCounter){.as_counter = counter};
+    _Py_BackoffCounter result;
+    result.as_counter = counter;
+    return result;
 }
 
 static inline _Py_BackoffCounter

Misc/NEWS.d/next/Core and Builtins/2024-05-05-12-04-02.gh-issue-117549.kITawD.rst

@@ -0,0 +1,5 @@
+Don't use designated initializer syntax in inline functions in internal
+headers. They cause problems for C++ or MSVC users who aren't yet using the
+latest C++ standard (C++20). While internal, pycore_backoff.h, is included
+(indirectly, via pycore_code.h) by some key 3rd party software that does so
+for speed.