[clang] [C23] Implement WG14 N3037 (PR #132939)

[Aaron Ballman via cfe-commits]

================
@@ -450,6 +453,116 @@ class StmtComparer {
 };
 } // namespace
 
+static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
+                                     const Attr *Attr1, const Attr *Attr2) {
+  // Two attributes are structurally equivalent if they are the same kind
+  // of attribute, spelled with the same spelling kind, and have the same
+  // arguments. This means that [[noreturn]] and __attribute__((noreturn)) are
+  // not structurally equivalent, nor are [[nodiscard("foo")]] and
+  // [[nodiscard("bar")]].
+  if (Attr1->getKind() != Attr2->getKind())
+    return false;
+
+  if (Attr1->getSyntax() != Attr2->getSyntax())
+    return false;
+
+  if (Attr1->getSpellingListIndex() != Attr2->getSpellingListIndex())
+    return false;
+
+  auto GetAttrName = [](const Attr *A) {
+    if (const IdentifierInfo *II = A->getAttrName())
+      return II->getName();
+    return StringRef{};
+  };
+
+  if (GetAttrName(Attr1) != GetAttrName(Attr2))
+    return false;
+
+  // FIXME: check the attribute arguments. Attr does not track the arguments on
+  // the base class, which makes this awkward. We may want to tablegen a
+  // comparison function for attributes? In the meantime, we're doing this the
+  // cheap way by pretty printing the attributes and checking they produce
----------------
AaronBallman wrote:

> I don't think this applies here.

I think it does, but I think WG14 has some serious problems around their interpretation of attributes as the feature was intended when I standardized it, at least as they relate to types. I'm not surprised we come down differently here. :-)

> Sounds reasonable, but I would exempt standard attributes, because I believe this is what the standard requires.

We make no distinction between standard and vendor attributes in our AST, they're both represented by an `Attr` subclass. And given that we have to handle things like differences between `[[noreturn]]` and `__attribute__((noreturn))`, I don't think we should exempt standard attributes.

https://github.com/llvm/llvm-project/pull/132939