[clang] [clang] Handle trivial_abi attribute for Microsoft ABI. (PR #88857)

[Reid Kleckner via cfe-commits]

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@@ -1105,6 +1105,11 @@ bool MicrosoftCXXABI::hasMostDerivedReturn(GlobalDecl GD) const {
 
 static bool isTrivialForMSVC(const CXXRecordDecl *RD, QualType Ty,
                              CodeGenModule &CGM) {
+  // If the record is marked with the trivial_abi attribute, we don't
+  // have to conform to the standard MSVC ABI.
+  if (RD->hasAttr<TrivialABIAttr>())
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rnk wrote:

Yes, I think I do want to block this on the recursive search. Even if it's ugly and we ultimately replace it with an explicitly tracked and type trait (`containsTrivialAbiSubobject`?), it reduces the number of externally visible ABI changes. I know we're saying this is ABI unstable already, but I still think fewer breaks is better.

https://github.com/llvm/llvm-project/pull/88857