Refactor GN source expansions.

tools/gn/target_generator.cc

Index: tools/gn/target_generator.cc
diff --git a/tools/gn/target_generator.cc b/tools/gn/target_generator.cc
index 12d09fca1527092d16ff2b249a47e72ce739fe14..182bf30b6894a4cc2dd173b153d4f20fdcfc5ade 100644
--- a/tools/gn/target_generator.cc
+++ b/tools/gn/target_generator.cc
@@ -222,20 +222,53 @@ void TargetGenerator::FillOutputs() {
   if (!value)
     return;
 
-  std::vector<std::string>& outputs = target_->action_values().outputs();
-  if (!ExtractListOfStringValues(*value, &outputs, err_))
+  SubstitutionList& outputs = target_->action_values().outputs();
+  if (!outputs.Parse(*value, err_))
     return;
 
+  // Check the substitutions used are valid for this purpose.
+  if (!EnsureValidSourcesSubstitutions(outputs.required_types(),
+                                       value->origin(), err_))
+    return
+
   // Validate that outputs are in the output dir.
-  bool allow_templates = target_->output_type() == Target::ACTION_FOREACH ||
-                         target_->output_type() == Target::COPY_FILES;
-  CHECK(outputs.size() == value->list_value().size());
-  for (size_t i = 0; i < outputs.size(); i++) {
+  CHECK(outputs.list().size() == value->list_value().size());
+  for (size_t i = 0; i < outputs.list().size(); i++) {
+    if (!EnsureSubstitutionIsInOutputDir(outputs.list()[i],
+                                         value->list_value()[i]))
+      return;
+  }
+}
+
+bool TargetGenerator::EnsureSubstitutionIsInOutputDir(
+    const SubstitutionPattern& pattern,
+    const Value& original_value) {
+  if (pattern.ranges().empty()) {
+    // Pattern is empty, error out (this prevents weirdness below).
+    *err_ = Err(original_value, "This has an empty value in it.");
+    return false;
+  }
+
+  if (pattern.ranges()[0].type == SUBSTITUTION_LITERAL) {
+    // If the first thing is a literal, it must start with the output dir.
     if (!EnsureStringIsInOutputDir(
             GetBuildSettings()->build_dir(),
-            outputs[i], value->list_value()[i], allow_templates, err_))
-      return;
+            pattern.ranges()[0].literal, original_value, err_))
+      return false;
+  } else {
+    // Otherwise, the first subrange must be a pattern that expands to
+    // something in the output directory.
+    if (!SubstitutionIsInOutputDir(pattern.ranges()[0].type)) {
+      *err_ = Err(original_value,
+          "File is not inside output directory.",
+          "The given file should be in the output directory. Normally you\n"
+          "would specify\n\"$target_out_dir/foo\" or "
+          "\"{{source_gen_dir}}/foo\".");
+      return false;
+    }
   }
+
+  return true;
 }
 
 void TargetGenerator::FillGenericConfigs(const char* var_name,