Add toolchains to dependency error messages in GN

tools/gn/command_gen.cc

Index: tools/gn/command_gen.cc
diff --git a/tools/gn/command_gen.cc b/tools/gn/command_gen.cc
index eceb8df09f867c28ed8a45733e8a01b418ee016a..56d3e98524b74a3102b91638a118f03e4e6f24f5 100644
--- a/tools/gn/command_gen.cc
+++ b/tools/gn/command_gen.cc
@@ -71,18 +71,34 @@ void PrintInvalidGeneratedInput(const Builder* builder,
                                 const std::vector<const Target*>& targets) {
   std::string err;
 
+  // Only show the toolchain labels (which can be confusing) if something
+  // isn't the default.
+  bool show_toolchains = false;
+  const Label& default_toolchain =
+      targets[0]->settings()->default_toolchain_label();
+  for (const Target* target : targets) {
+    if (target->settings()->toolchain_label() != default_toolchain) {
+      show_toolchains = true;
+      break;
+    }
+  }
+
+  const Target* generator = FindTargetThatGeneratesFile(builder, file);
+  if (generator &&
+      generator->settings()->toolchain_label() != default_toolchain)
+    show_toolchains = true;
+
   const std::string target_str = targets.size() > 1 ? "targets" : "target";
   err += "The file:\n";
   err += "  " + file.value() + "\n";
   err += "is listed as an input or source for the " + target_str + ":\n";
   for (const Target* target : targets)
-    err += "  " + target->label().GetUserVisibleName(false) + "\n";
+    err += "  " + target->label().GetUserVisibleName(show_toolchains) + "\n";
 
-  const Target* generator = FindTargetThatGeneratesFile(builder, file);
   if (generator) {
     err += "but this file was not generated by any dependencies of the " +
         target_str + ". The target\nthat generates the file is:\n  ";
-    err += generator->label().GetUserVisibleName(false);
+    err += generator->label().GetUserVisibleName(show_toolchains);
   } else {
     err += "but no targets in the build generate that file.";
   }