Temporarily allow @actorIndependent as an alias for nonisolated.

Addresses rdar://78830857.

Author: DougGregor

include/swift/AST/DiagnosticsParse.def

@@ -1428,6 +1428,8 @@ ERROR(attr_renamed, none,
       "'@%0' has been renamed to '@%1'", (StringRef, StringRef))
 WARNING(attr_renamed_warning, none,
         "'@%0' has been renamed to '@%1'", (StringRef, StringRef))
+WARNING(attr_renamed_to_modifier_warning, none,
+        "'@%0' has been renamed to '%1'", (StringRef, StringRef))
 ERROR(attr_name_close_match, none,
       "no attribute named '@%0'; did you mean '@%1'?", (StringRef, StringRef))
 ERROR(attr_unsupported_on_target, none,

lib/Parse/ParseDecl.cpp

@@ -2706,7 +2706,7 @@ bool Parser::parseNewDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc,
       .highlight(DuplicateAttribute->getRange());
   }
 
-    // If this is a decl modifier spelled with an @, emit an error and remove it
+  // If this is a decl modifier spelled with an @, emit an error and remove it
   // with a fixit.
   if (AtLoc.isValid() && DeclAttribute::isDeclModifier(DK))
     diagnose(AtLoc, diag::cskeyword_not_attribute, AttrName).fixItRemove(AtLoc);
@@ -2989,6 +2989,16 @@ ParserStatus Parser::parseDeclAttribute(
   checkInvalidAttrName(
       "concurrent", "Sendable", DAK_Sendable, diag::attr_renamed_warning);
 
+  // Historical name for 'nonisolated'.
+  if (DK == DAK_Count && Tok.getText() == "actorIndependent") {
+    diagnose(
+        Tok, diag::attr_renamed_to_modifier_warning, "actorIndependent", 
+        "nonisolated")
+      .fixItReplace(SourceRange(AtLoc, Tok.getLoc()), "nonisolated");
+    DK = DAK_Nonisolated;
+    AtLoc = SourceLoc();
+  }
+
   if (DK == DAK_Count && Tok.getText() == "warn_unused_result") {
     // The behavior created by @warn_unused_result is now the default. Emit a
     // Fix-It to remove.