diff --git a/include/clang/Frontend/TextDiagnosticPrinter.h b/include/clang/Frontend/TextDiagnosticPrinter.h
index d727e48..d09e51f 100644
--- a/include/clang/Frontend/TextDiagnosticPrinter.h
+++ b/include/clang/Frontend/TextDiagnosticPrinter.h
@@ -37,11 +37,19 @@
   unsigned LastCaretDiagnosticWasNote : 1;
   unsigned OwnsOutputStream : 1;
 
+  /// A string to prefix to error messages.
+  std::string Prefix;
+
 public:
   TextDiagnosticPrinter(llvm::raw_ostream &os, const DiagnosticOptions &diags,
                         bool OwnsOutputStream = false);
   virtual ~TextDiagnosticPrinter();
 
+  /// setPrefix - Set the diagnostic printer prefix string, which will be
+  /// printed at the start of any diagnostics. If empty, no prefix string is
+  /// used.
+  void setPrefix(std::string Value) { Prefix = Value; }
+
   void BeginSourceFile(const LangOptions &LO, const Preprocessor *PP) {
     LangOpts = &LO;
   }
diff --git a/lib/Frontend/TextDiagnosticPrinter.cpp b/lib/Frontend/TextDiagnosticPrinter.cpp
index 3c64332..d2aa548 100644
--- a/lib/Frontend/TextDiagnosticPrinter.cpp
+++ b/lib/Frontend/TextDiagnosticPrinter.cpp
@@ -31,7 +31,7 @@
   llvm::raw_ostream::MAGENTA;
 static const enum llvm::raw_ostream::Colors errorColor = llvm::raw_ostream::RED;
 static const enum llvm::raw_ostream::Colors fatalColor = llvm::raw_ostream::RED;
-// used for changing only the bold attribute
+// Used for changing only the bold attribute.
 static const enum llvm::raw_ostream::Colors savedColor =
   llvm::raw_ostream::SAVEDCOLOR;
 
@@ -682,6 +682,9 @@
   // file+line+column number prefix is.
   uint64_t StartOfLocationInfo = OS.tell();
 
+  if (!Prefix.empty())
+    OS << Prefix << ": ";
+
   // If the location is specified, print out a file/line/col and include trace
   // if enabled.
   if (Info.getLocation().isValid()) {