diff --git a/include/swift/AST/DiagnosticsParse.def b/include/swift/AST/DiagnosticsParse.def
index 55819a27def21..3f087fe6988f2 100644
--- a/include/swift/AST/DiagnosticsParse.def
+++ b/include/swift/AST/DiagnosticsParse.def
@@ -1296,8 +1296,6 @@ ERROR(swift_native_objc_runtime_base_must_be_identifier,none,
 
 ERROR(attr_interpolated_string,none,
 "'%0' cannot be an interpolated string literal", (StringRef))
-ERROR(attr_multiline_string,none,
-"'%0' cannot be a multiline string literal", (StringRef))
 ERROR(attr_extended_escaping_string,none,
 "'%0' cannot be an extended escaping string literal", (StringRef))
 
diff --git a/include/swift/Parse/Lexer.h b/include/swift/Parse/Lexer.h
index 0b388ae4b09c6..d71c0d6e8c9a2 100644
--- a/include/swift/Parse/Lexer.h
+++ b/include/swift/Parse/Lexer.h
@@ -426,15 +426,19 @@ class Lexer {
   /// If a copy needs to be made, it will be allocated out of the provided
   /// \p Buffer.
   static StringRef getEncodedStringSegment(StringRef Str,
-                                           SmallVectorImpl<char> &Buffer) {
+                                           SmallVectorImpl<char> &Buffer,
+                                           bool IsFirstSegment = false,
+                                           bool IsLastSegment = false,
+                                           unsigned IndentToStrip = 0,
+                                           unsigned CustomDelimiterLen = 0) {
     SmallString<128> TerminatedStrBuf(Str);
     TerminatedStrBuf.push_back('\0');
     StringRef TerminatedStr = StringRef(TerminatedStrBuf).drop_back();
     StringRef Result = getEncodedStringSegmentImpl(TerminatedStr, Buffer,
-                                                   /*IsFirstSegment*/false,
-                                                   /*IsLastSegment*/false,
-                                                   /*IndentToStrip*/0,
-                                                   /*CustomDelimiterLen*/0);
+                                                   IsFirstSegment,
+                                                   IsLastSegment,
+                                                   IndentToStrip,
+                                                   CustomDelimiterLen);
     if (Result == TerminatedStr)
       return Str;
     assert(Result.data() == Buffer.data());
diff --git a/lib/Parse/Lexer.cpp b/lib/Parse/Lexer.cpp
index eb9bc8aed933d..347c0a4282b3b 100644
--- a/lib/Parse/Lexer.cpp
+++ b/lib/Parse/Lexer.cpp
@@ -1560,9 +1560,9 @@ static size_t commonPrefixLength(StringRef shorter, StringRef longer) {
 
 /// getMultilineTrailingIndent:
 /// Determine trailing indent to be used for multiline literal indent stripping.
-static std::tuple<StringRef, SourceLoc>
-getMultilineTrailingIndent(const Token &Str, DiagnosticEngine *Diags) {
-  StringRef Bytes = getStringLiteralContent(Str);
+StringRef
+getMultilineTrailingIndent(StringRef Bytes, DiagnosticEngine *Diags = nullptr,
+                           unsigned CustomDelimiterLen = 0) {
   const char *begin = Bytes.begin(), *end = Bytes.end(), *start = end;
   bool sawNonWhitespace = false;
 
@@ -1575,11 +1575,9 @@ getMultilineTrailingIndent(const Token &Str, DiagnosticEngine *Diags) {
     case '\n':
     case '\r': {
       ++start;
-      auto startLoc = Lexer::getSourceLoc(start);
-      auto string = StringRef(start, end - start);
 
       // Disallow escaped newline in the last line.
-      if (Diags && Str.getCustomDelimiterLen() == 0) {
+      if (Diags && !CustomDelimiterLen) {
         auto *Ptr = start - 1;
         if (*Ptr == '\n') --Ptr;
         if (*Ptr == '\r') --Ptr;
@@ -1595,7 +1593,7 @@ getMultilineTrailingIndent(const Token &Str, DiagnosticEngine *Diags) {
         }
       }
 
-      return std::make_tuple(string, startLoc);
+      return StringRef(start, end - start);
     }
     default:
       sawNonWhitespace = true;
@@ -1609,7 +1607,7 @@ getMultilineTrailingIndent(const Token &Str, DiagnosticEngine *Diags) {
       .fixItInsert(loc, "\n");
   }
 
-  return std::make_tuple("", Lexer::getSourceLoc(end - 1));
+  return "";
 }
 
 /// diagnoseInvalidMultilineIndents:
@@ -1673,12 +1671,13 @@ static void diagnoseInvalidMultilineIndents(
 /// Diagnose contents of string literal that have inconsistent indentation.
 static void validateMultilineIndents(const Token &Str,
                                      DiagnosticEngine *Diags) {
-  StringRef Indent;
-  SourceLoc IndentStartLoc;
-  std::tie(Indent, IndentStartLoc) = getMultilineTrailingIndent(Str, Diags);
+  StringRef Bytes = getStringLiteralContent(Str);
+  StringRef Indent =
+    getMultilineTrailingIndent(Bytes, Diags, Str.getCustomDelimiterLen());
   if (Indent.empty())
     return;
-  
+  SourceLoc IndentStartLoc = Lexer::getSourceLoc(Indent.data());
+
   // The offset into the previous line where it experienced its first indentation 
   // error, or Indent.size() if every character matched.
   size_t lastMistakeOffset = std::numeric_limits<size_t>::max();
@@ -1688,7 +1687,6 @@ static void validateMultilineIndents(const Token &Str,
   // Prefix of indentation that's present on all lines in linesWithLastMatchLength.
   StringRef commonIndentation = "";
   
-  StringRef Bytes = getStringLiteralContent(Str);
   for (size_t pos = Bytes.find('\n'); pos != StringRef::npos; pos = Bytes.find('\n', pos + 1)) {
     size_t nextpos = pos + 1;
     auto restOfBytes = Bytes.substr(nextpos);
@@ -2109,6 +2107,11 @@ StringRef Lexer::getEncodedStringSegmentImpl(StringRef Bytes,
   // BytesPtr to avoid a range check subscripting on the StringRef.
   const char *BytesPtr = Bytes.begin();
 
+  // Special case when being called from EncodedDiagnosticMessage(...)
+  // This should allow multiline strings to work as attribute messages.
+  if (IndentToStrip == ~0U)
+    IndentToStrip = getMultilineTrailingIndent(Bytes).size();
+
   bool IsEscapedNewline = false;
   while (BytesPtr < Bytes.end()) {
     char CurChar = *BytesPtr++;
@@ -2203,8 +2206,7 @@ void Lexer::getStringLiteralSegments(
   bool MultilineString = Str.isMultilineString(), IsFirstSegment = true;
   unsigned IndentToStrip = 0, CustomDelimiterLen = Str.getCustomDelimiterLen();
   if (MultilineString)
-    IndentToStrip = 
-      std::get<0>(getMultilineTrailingIndent(Str, /*Diags=*/nullptr)).size();
+    IndentToStrip = getMultilineTrailingIndent(Bytes).size();
 
   // Note that it is always safe to read one over the end of "Bytes" because
   // we know that there is a terminating " character.  Use BytesPtr to avoid a
diff --git a/lib/Parse/ParseDecl.cpp b/lib/Parse/ParseDecl.cpp
index 3c904dcf4efcf..759b95c91db67 100644
--- a/lib/Parse/ParseDecl.cpp
+++ b/lib/Parse/ParseDecl.cpp
@@ -307,15 +307,11 @@ bool Parser::parseTopLevel() {
 static Optional<StringRef>
 getStringLiteralIfNotInterpolated(Parser &P, SourceLoc Loc, const Token &Tok,
                                   StringRef DiagText) {
-  // FIXME: Support extended escaping / multiline string literal.
+  // FIXME: Support extended escaping string literal.
   if (Tok.getCustomDelimiterLen()) {
     P.diagnose(Loc, diag::attr_extended_escaping_string, DiagText);
     return None;
   }
-  if (Tok.isMultilineString()) {
-    P.diagnose(Loc, diag::attr_multiline_string, DiagText);
-    return None;
-  }
 
   SmallVector<Lexer::StringSegment, 1> Segments;
   P.L->getStringLiteralSegments(Tok, Segments);
diff --git a/lib/Sema/TypeChecker.h b/lib/Sema/TypeChecker.h
index 06e277b34ca07..6dc3db39a9520 100644
--- a/lib/Sema/TypeChecker.h
+++ b/lib/Sema/TypeChecker.h
@@ -2160,7 +2160,7 @@ class EncodedDiagnosticMessage {
 public:
   /// \param S A string with an encoded message
   EncodedDiagnosticMessage(StringRef S)
-      : Message(Lexer::getEncodedStringSegment(S, Buf)) {}
+      : Message(Lexer::getEncodedStringSegment(S, Buf, true, true, ~0U)) {}
 
   /// The unescaped message to display to the user.
   const StringRef Message;
diff --git a/test/Parse/diagnose_availability.swift b/test/Parse/diagnose_availability.swift
index ad41d26644a7e..277ee7b6c9dac 100644
--- a/test/Parse/diagnose_availability.swift
+++ b/test/Parse/diagnose_availability.swift
@@ -66,11 +66,19 @@ func swiftMessage() {}
 // expected-error@-1{{'message' cannot be an interpolated string literal}}
 func interpolatedMessage() {}
 
-// expected-error@+1{{'message' cannot be a multiline string literal}}
 @available(*, unavailable, message: """
   foobar message.
   """)
 func multilineMessage() {}
+multilineMessage()
+// expected-error@-1{{'multilineMessage()' is unavailable: foobar message.}}
+// expected-note@-3{{'multilineMessage()' has been explicitly marked unavailable here}}
+
+@available(*, unavailable, message: " ")
+func emptyMessage() {}
+emptyMessage()
+// expected-error@-1{{'emptyMessage()' is unavailable:  }}
+// expected-note@-3{{'emptyMessage()' has been explicitly marked unavailable here}}
 
 // expected-error@+1{{'message' cannot be an extended escaping string literal}}
 @available(*, unavailable, message: #"""