Add an opt-in flag to make duplicate edges an error (`-w dupbuild=err`).

This is step 1 on #931.  Duplicated edges will become an error by default in
the future.

1 files changed, 10 insertions, 4 deletions

diff --git a/src/manifest_parser.cc b/src/manifest_parser.cc
index 638d751..e8c0436 100644
--- a/src/manifest_parser.cc
+++ b/src/manifest_parser.cc
@@ -24,8 +24,10 @@
 #include "util.h"
 #include "version.h"
 
-ManifestParser::ManifestParser(State* state, FileReader* file_reader)
-  : state_(state), file_reader_(file_reader), quiet_(false) {
+ManifestParser::ManifestParser(State* state, FileReader* file_reader,
+                               bool dupe_edge_should_err)
+    : state_(state), file_reader_(file_reader),
+      dupe_edge_should_err_(dupe_edge_should_err), quiet_(false) {
   env_ = &state->bindings_;
 }
 
@@ -329,10 +331,14 @@ bool ManifestParser::ParseEdge(string* err) {
     if (!CanonicalizePath(&path, &slash_bits, &path_err))
       return lexer_.Error(path_err, err);
     if (!state_->AddOut(edge, path, slash_bits)) {
-      if (!quiet_) {
+      if (dupe_edge_should_err_) {
+        lexer_.Error("multiple rules generate " + path + " [-w dupbuild=err]",
+                     err);
+        return false;
+      } else if (!quiet_) {
         Warning("multiple rules generate %s. "
                 "builds involving this target will not be correct; "
-                "continuing anyway",
+                "continuing anyway [-w dupbuild=warn]",
                 path.c_str());
       }
     }