[Bug 54226] widl: implement `cstruct_out` option to generate C bindings compatible with small struct return values

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=54226

Zeb Figura  changed:

What    |Removed                     |Added
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                 CC|                            |[e-mail]

--- Comment #1 from Zeb Figura  ---
(In reply to Alvin Wong from comment #0)
> (I have not checked what `/cstruct_out` on MIDL actually does to the C
> bindings.)

This, basically:

@@ -129,8 +129,9 @@ EXTERN_C const IID IID_ID2D1Bitmap;
             ID2D1Bitmap * This);

DECLSPEC_XFGVIRT(ID2D1Bitmap, b)
-        struct apple ( STDMETHODCALLTYPE *b )( 
-            ID2D1Bitmap * This);
+        struct apple *( STDMETHODCALLTYPE *b )( 
+            ID2D1Bitmap * This,
+            apple * RetVal);

END_INTERFACE
     } ID2D1BitmapVtbl;
@@ -158,8 +159,8 @@ EXTERN_C const IID IID_ID2D1Bitmap;
 #define ID2D1Bitmap_a(This)    \
     ( (This)->lpVtbl -> a(This) )

-#define ID2D1Bitmap_b(This)    \
-    ( (This)->lpVtbl -> b(This) ) 
+#define ID2D1Bitmap_b(This,RetVal)     \
+    ( (This)->lpVtbl -> b(This,RetVal) )

#endif /* COBJMACROS */

But we already emit the C type definition (the first hunk) with ABI
compatibility in all cases. For the macro we currently have e.g.

#define ID2D1Bitmap_a(This) (This)->lpVtbl->a(This)
#define ID2D1Bitmap_b(This)
ID2D1Bitmap_b_define_WIDL_C_INLINE_WRAPPERS_for_aggregate_return_support

but is there really any point to doing this? Do we care about API compatibility
if the API is just broken? Should we just implement the ABI-compatible
behaviour for the macros and not bother checking cstruct_out?