[NFC] Extract a utility routine to parse a UUID string literal.

Author: rjmccall

include/swift/Parse/Parser.h

@@ -52,6 +52,7 @@ namespace swift {
   class SourceManager;
   class TupleType;
   class TypeLoc;
+  class UUID;
 
   struct EnumElementInfo;
 
@@ -1018,6 +1019,11 @@ class Parser {
   parseExtendedAvailabilitySpecList(SourceLoc AtLoc, SourceLoc AttrLoc,
                                     StringRef AttrName);
 
+  /// Parse a string literal whose contents can be interpreted as a UUID.
+  ///
+  /// \returns false on success, true on error.
+  bool parseUUIDString(UUID &uuid, Diag<> diag);
+
   /// Parse the Objective-C selector inside @objc
   void parseObjCSelector(SmallVector<Identifier, 4> &Names,
                          SmallVector<SourceLoc, 4> &NameLocs,

lib/Parse/ParseDecl.cpp

@@ -3651,6 +3651,25 @@ bool Parser::parseConventionAttributeInternal(
   return false;
 }
 
+bool Parser::parseUUIDString(UUID &uuid, Diag<> diagnostic) {
+  if (!Tok.is(tok::string_literal)) {
+    diagnose(Tok, diagnostic);
+    return true;
+  }
+
+  bool failed = true;
+  auto literalText = Tok.getText().slice(1, Tok.getText().size() - 1);
+  llvm::SmallString<UUID::StringBufferSize> text(literalText);
+  if (auto id = UUID::fromString(text.c_str())) {
+    uuid = *id;
+    failed = false;
+  } else {
+    diagnose(Tok, diagnostic);
+  }
+  consumeToken(tok::string_literal);
+  return failed;
+}
+
 /// \verbatim
 ///   attribute-type:
 ///     'noreturn'
@@ -3840,24 +3859,12 @@ ParserStatus Parser::parseTypeAttribute(TypeAttributes &Attributes,
     // Parse the opened existential ID string in parens
     SourceLoc beginLoc = Tok.getLoc(), idLoc, endLoc;
     if (consumeIfNotAtStartOfLine(tok::l_paren)) {
-      if (Tok.is(tok::string_literal)) {
-        UUID openedID;
-        idLoc = Tok.getLoc();
-        auto literalText = Tok.getText().slice(1, Tok.getText().size() - 1);
-        llvm::SmallString<UUID::StringBufferSize> text(literalText);
-        if (auto openedID = UUID::fromString(text.c_str())) {
-          Attributes.OpenedID = openedID;
-        } else {
-          diagnose(Tok, diag::opened_attribute_id_value);
-        }
-        consumeToken();
-      } else {
-        diagnose(Tok, diag::opened_attribute_id_value);
-      }
-
-      if (Tok.is(tok::comma)) {
-        consumeToken(tok::comma);
+      idLoc = Tok.getLoc();
+      UUID id;
+      if (!parseUUIDString(id, diag::opened_attribute_id_value))
+        Attributes.OpenedID = id;
 
+      if (consumeIf(tok::comma)) {
         auto constraintType = parseType(diag::expected_type);
         if (constraintType.isNonNull())
           Attributes.ConstraintType = constraintType.getPtrOrNull();