[clang] [BoundsSafety] WIP: Make 'counted_by' work for pointer fields; late parsing for 'counted_by' on decl attr position (PR #87596)

[Dan Liew via cfe-commits]

================
@@ -4872,6 +4886,79 @@ void Parser::ParseStructDeclaration(
   }
 }
 
+// Parse all attributes in LA, and attach them to Decl D.
+void Parser::ParseLexedCAttributeList(LateParsedAttrList &LA, bool EnterScope, ParsedAttributes *OutAttrs) {
+  assert(LA.parseSoon() &&
+         "Attribute list should be marked for immediate parsing.");
+  for (unsigned i = 0, ni = LA.size(); i < ni; ++i) {
+    ParseLexedCAttribute(*LA[i], EnterScope, OutAttrs);
+    delete LA[i];
+  }
+  LA.clear();
+}
+
+/// Finish parsing an attribute for which parsing was delayed.
+/// This will be called at the end of parsing a class declaration
+/// for each LateParsedAttribute. We consume the saved tokens and
+/// create an attribute with the arguments filled in. We add this
+/// to the Attribute list for the decl.
+void Parser::ParseLexedCAttribute(LateParsedAttribute &LA, bool EnterScope, ParsedAttributes *OutAttrs) {
+  // Create a fake EOF so that attribute parsing won't go off the end of the
+  // attribute.
+  Token AttrEnd;
+  AttrEnd.startToken();
+  AttrEnd.setKind(tok::eof);
+  AttrEnd.setLocation(Tok.getLocation());
+  AttrEnd.setEofData(LA.Toks.data());
+  LA.Toks.push_back(AttrEnd);
+
+  // Append the current token at the end of the new token stream so that it
+  // doesn't get lost.
+  LA.Toks.push_back(Tok);
+  PP.EnterTokenStream(LA.Toks, true, /*IsReinject=*/true);
+  // Consume the previously pushed token.
+  ConsumeAnyToken(/*ConsumeCodeCompletionTok=*/true);
+
+  ParsedAttributes Attrs(AttrFactory);
+
+  assert(
+      LA.Decls.size() < 2 &&
+      "late field attribute expects to have at most a single declaration.");
+
+  Decl *D = LA.Decls.empty() ? nullptr : LA.Decls[0];
+
+  // If the Decl is on a function, add function parameters to the scope.
+  {
+    std::unique_ptr<ParseScope> Scope;
----------------
delcypher wrote:

Actually I guess this isn't testable because of the `  let Subjects = SubjectList<[Field], ErrorDiag>;`?

https://github.com/llvm/llvm-project/pull/87596