Add a new ForwardingDiagnosticConsumer type.

Author: graydon

include/swift/AST/DiagnosticConsumer.h

@@ -25,6 +25,7 @@
 
 namespace swift {
   class DiagnosticArgument;
+  class DiagnosticEngine;
   class SourceManager;
   enum class DiagID : uint32_t;
 
@@ -122,6 +123,19 @@ class NullDiagnosticConsumer : public DiagnosticConsumer {
                         const DiagnosticInfo &Info) override;
 };
 
+/// \brief DiagnosticConsumer that forwards diagnostics to the consumers of
+// another DiagnosticEngine.
+class ForwardingDiagnosticConsumer : public DiagnosticConsumer {
+  DiagnosticEngine &TargetEngine;
+public:
+  ForwardingDiagnosticConsumer(DiagnosticEngine &Target);
+  void handleDiagnostic(SourceManager &SM, SourceLoc Loc,
+                        DiagnosticKind Kind,
+                        StringRef FormatString,
+                        ArrayRef<DiagnosticArgument> FormatArgs,
+                        const DiagnosticInfo &Info) override;
+};
+
 /// \brief DiagnosticConsumer that funnels diagnostics in certain files to
 /// particular sub-consumers.
 ///

include/swift/AST/DiagnosticEngine.h

@@ -604,6 +604,11 @@ namespace swift {
       return Result;
     }
 
+    /// \brief Return all \c DiagnosticConsumers.
+    ArrayRef<DiagnosticConsumer *> getConsumers() {
+      return Consumers;
+    }
+
     /// \brief Emit a diagnostic using a preformatted array of diagnostic
     /// arguments.
     ///

lib/AST/DiagnosticConsumer.cpp

@@ -222,3 +222,21 @@ void NullDiagnosticConsumer::handleDiagnostic(
     llvm::dbgs() << "\n";
   });
 }
+
+ForwardingDiagnosticConsumer::ForwardingDiagnosticConsumer(DiagnosticEngine &Target)
+  : TargetEngine(Target) {}
+
+void ForwardingDiagnosticConsumer::handleDiagnostic(
+    SourceManager &SM, SourceLoc Loc, DiagnosticKind Kind,
+    StringRef FormatString, ArrayRef<DiagnosticArgument> FormatArgs,
+    const DiagnosticInfo &Info) {
+  LLVM_DEBUG({
+    llvm::dbgs() << "ForwardingDiagnosticConsumer received diagnostic: ";
+    DiagnosticEngine::formatDiagnosticText(llvm::dbgs(), FormatString,
+                                           FormatArgs);
+    llvm::dbgs() << "\n";
+  });
+  for (auto *C : TargetEngine.getConsumers()) {
+    C->handleDiagnostic(SM, Loc, Kind, FormatString, FormatArgs, Info);
+  }
+}