Improve GN duplicate name error.

tools/gn/ninja_build_writer.cc

Index: tools/gn/ninja_build_writer.cc
diff --git a/tools/gn/ninja_build_writer.cc b/tools/gn/ninja_build_writer.cc
index 43916763db56231856526a099441e7b9d7b1cbdc..d27ebdcce66420b813f64cafbbdffc505809911b 100644
--- a/tools/gn/ninja_build_writer.cc
+++ b/tools/gn/ninja_build_writer.cc
@@ -76,6 +76,44 @@ std::string GetSelfInvocationCommand(const BuildSettings* build_settings) {
   return cmdline.GetCommandLineString();
 #endif
 }
+
+OutputFile GetTargetOutputFile(const Target* target) {
+  OutputFile result(target->dependency_output_file());
+
+  // The output files may have leading "./" so normalize those away.
+  NormalizePath(&result.value());
+  return result;
+}
+
+// Given an output that appears more than once, generates an error message
+// that describes the problem and which targets generate it.
+Err GetDuplicateOutputError(const std::vector<const Target*>& all_targets,
+                            const OutputFile& bad_output) {
+  std::vector<const Target*> matches;
+  for (const Target* target : all_targets) {
+    if (GetTargetOutputFile(target) == bad_output)
+      matches.push_back(target);
+  }
+
+  // There should always be at least two targets generating this file for this
+  // function to be called in the first place.
+  DCHECK(matches.size() >= 2);
+  std::string matches_string;
+  for (const Target* target : matches)
+    matches_string += "  " + target->label().GetUserVisibleName(false) + "\n";
+
+  Err result(matches[0]->defined_from(), "Duplicate output file.",
+      "Two or more targets generate the same output:\n  " +
+      bad_output.value() + "\n"
+      "This is normally the result of either overriding the output name or\n"
+      "having two shared libraries or executables in different directories\n"
+      "with the same name (since all such targets will be written to the root\n"
+      "output directory).\n\nCollisions:\n" + matches_string);
+  for (size_t i = 1; i < matches.size(); i++)
+    result.AppendSubErr(Err(matches[i]->defined_from(), "Collision."));
+  return result;
+}
+
 }  // namespace
 
 NinjaBuildWriter::NinjaBuildWriter(
@@ -242,11 +280,9 @@ bool NinjaBuildWriter::WritePhonyAndAllRules(Err* err) {
 
   for (const auto& target : default_toolchain_targets_) {
     const Label& label = target->label();
-    OutputFile target_file(target->dependency_output_file());
-    // The output files may have leading "./" so normalize those away.
-    NormalizePath(&target_file.value());
+    OutputFile target_file = GetTargetOutputFile(target);
     if (!target_files.insert(target_file.value()).second) {
-      *err = Err(Location(), "Duplicate rules for " + target_file.value());
+      *err = GetDuplicateOutputError(default_toolchain_targets_, target_file);
       return false;
     }