Check for inputs not generated by deps

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 76f1f36f7c4ee841a692db6f812416025bb9a7c7..eceb8df09f867c28ed8a45733e8a01b418ee016a 100644
--- a/tools/gn/command_gen.cc
+++ b/tools/gn/command_gen.cc
@@ -6,6 +6,7 @@
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/strings/string_number_conversions.h"
+#include "base/strings/stringprintf.h"
 #include "base/timer/elapsed_timer.h"
 #include "tools/gn/build_settings.h"
 #include "tools/gn/commands.h"
@@ -44,6 +45,89 @@ void ItemResolvedCallback(base::subtle::Atomic32* write_counter,
   }
 }
 
+// Returns a pointer to the target with the given file as an output, or null
+// if no targets generate the file. This is brute force since this is an
+// error condition and performance shouldn't matter.
+const Target* FindTargetThatGeneratesFile(const Builder* builder,
+                                          const SourceFile& file) {
+  std::vector<const Target*> targets = builder->GetAllResolvedTargets();
+  if (targets.empty())
+    return nullptr;
+
+  OutputFile output_file(targets[0]->settings()->build_settings(), file);
+  for (const Target* target : targets) {
+    for (const auto& cur_output : target->computed_outputs()) {
+      if (cur_output == output_file)
+        return target;
+    }
+  }
+  return nullptr;
+}
+
+// Prints an error that the given file was present as a source or input in
+// the given target(s) but was not generated by any of its dependencies.
+void PrintInvalidGeneratedInput(const Builder* builder,
+                                const SourceFile& file,
+                                const std::vector<const Target*>& targets) {
+  std::string err;
+
+  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";
+
+  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);
+  } else {
+    err += "but no targets in the build generate that file.";
+  }
+
+  Err(Location(), "Input to " + target_str + " not generated by a dependency.",
+      err).PrintToStdout();
+}
+
+bool CheckForInvalidGeneratedInputs(Setup* setup) {
+  std::multimap<SourceFile, const Target*> unknown_inputs =
+      g_scheduler->GetUnknownGeneratedInputs();
+  if (unknown_inputs.empty())
+    return true;  // No bad files.
+
+  int errors_found = 0;
+  auto cur = unknown_inputs.begin();
+  while (cur != unknown_inputs.end()) {
+    errors_found++;
+    auto end_of_range = unknown_inputs.upper_bound(cur->first);
+
+    // Package the values more conveniently for printing.
+    SourceFile bad_input = cur->first;
+    std::vector<const Target*> targets;
+    while (cur != end_of_range)
+      targets.push_back((cur++)->second);
+
+    PrintInvalidGeneratedInput(setup->builder(), bad_input, targets);
+    OutputString("\n");
+  }
+
+  OutputString(
+      "If you have generated inputs, there needs to be a dependency path "
+      "between the\ntwo targets in addition to just listing the files. For "
+      "indirect dependencies,\nthe intermediate ones must be public_deps. "
+      "data_deps don't count since they're\nonly runtime dependencies. If "
+      "you think a dependency chain exists, it might be\nbecause the chain "
+      "is private. Try \"gn path\" to analyze.\n");
+
+  if (errors_found > 1) {
+    OutputString(base::StringPrintf("\n%d generated input errors found.\n",
+                                    errors_found), DECORATION_YELLOW);
+  }
+  return false;
+}
+
 }  // namespace
 
 const char kGen[] = "gen";
@@ -107,6 +191,9 @@ int RunGen(const std::vector<std::string>& args) {
     return 1;
   }
 
+  if (!CheckForInvalidGeneratedInputs(setup))
+    return 1;
+
   base::TimeDelta elapsed_time = timer.Elapsed();
 
   if (!base::CommandLine::ForCurrentProcess()->HasSwitch(switches::kQuiet)) {