1 files changed, 9 insertions, 2 deletions
diff --git a/src/state.cc b/src/state.cc
index 0a68f21..d4b9a71 100644
--- a/src/state.cc
+++ b/src/state.cc
@@ -133,10 +133,17 @@ void State::AddIn(Edge* edge, StringPiece path, uint64_t slash_bits) {
   node->AddOutEdge(edge);
 }
 
-bool State::AddOut(Edge* edge, StringPiece path, uint64_t slash_bits) {
+bool State::AddOut(Edge* edge, StringPiece path, uint64_t slash_bits,
+                   std::string* err) {
   Node* node = GetNode(path, slash_bits);
-  if (node->in_edge())
+  if (Edge* other = node->in_edge()) {
+    if (other == edge) {
+      *err = path.AsString() + " is defined as an output multiple times";
+    } else {
+      *err = "multiple rules generate " + path.AsString();
+    }
     return false;
+  }
   edge->outputs_.push_back(node);
   node->set_in_edge(edge);
   node->set_generated_by_dep_loader(false);