Add not_needed function

tools/gn/scope.cc

Index: tools/gn/scope.cc
diff --git a/tools/gn/scope.cc b/tools/gn/scope.cc
index f2fe7521dfafc8e94f5a6c35ff2723a9ec960c9f..be76c48277217e4a2e1ea37b8f7664a361f638cb 100644
--- a/tools/gn/scope.cc
+++ b/tools/gn/scope.cc
@@ -205,18 +205,31 @@ const Template* Scope::GetTemplate(const std::string& name) const {
   return nullptr;
 }
 
-void Scope::MarkUsed(const base::StringPiece& ident) {
+void Scope::MarkUsed(const base::StringPiece& ident, Err* err) {
   RecordMap::iterator found = values_.find(ident);
   if (found == values_.end()) {
     NOTREACHED();
     return;
   }
+  if (found->second.unusable) {
+    *err = Err(found->second.value.origin(), "Variable was marked as unused.",
+               "You marked the variable \"" + found->first.as_string() +
+                   "\" as unused, but it is being used.");
+    return;
+  }
   found->second.used = true;
 }
 
-void Scope::MarkAllUsed() {
-  for (auto& cur : values_)
+void Scope::MarkAllUsed(Err* err) {
+  for (auto& cur : values_) {
+    if (cur.second.unusable) {
+      *err = Err(cur.second.value.origin(), "Variable was marked as unused.",
+                 "You marked the variable \"" + cur.first.as_string() +
+                     "\" as unused, but it is being used.");
+      return;
+    }
     cur.second.used = true;
+  }
 }
 
 void Scope::MarkUnused(const base::StringPiece& ident) {
@@ -228,6 +241,38 @@ void Scope::MarkUnused(const base::StringPiece& ident) {
   found->second.used = false;
 }
 
+void Scope::MarkAllUnusable(Err* err, std::set<std::string> excluded_values) {
+  for (auto& cur : values_) {
+    if (!excluded_values.empty() &&
+        excluded_values.find(cur.first.as_string()) != excluded_values.end()) {
+      continue;  // Skip this excluded value.
+    }
+
+    if (cur.second.used) {
+      *err = Err(cur.second.value.origin(), "Variable was marked as unused.",
+                 "You marked the variable \"" + cur.first.as_string() +
+                     "\" as unused, but it is being used.");
+      return;
+    }
+    cur.second.unusable = true;
+  }
+}
+
+void Scope::MarkUnusable(const base::StringPiece& ident, Err* err) {
+  RecordMap::iterator found = values_.find(ident);
+  if (found == values_.end()) {
+    NOTREACHED();
+    return;
+  }
+  if (found->second.used) {
+    *err = Err(found->second.value.origin(), "Variable was marked as unused.",
+               "You marked the variable \"" + ident.as_string() +
+                   "\" as unused, but it is being used.");
+    return;
+  }
+  found->second.unusable = true;
+}
+
 bool Scope::IsSetButUnused(const base::StringPiece& ident) const {
   RecordMap::const_iterator found = values_.find(ident);
   if (found != values_.end()) {
@@ -240,7 +285,7 @@ bool Scope::IsSetButUnused(const base::StringPiece& ident) const {
 
 bool Scope::CheckForUnusedVars(Err* err) const {
   for (const auto& pair : values_) {
-    if (!pair.second.used) {
+    if (!pair.second.unusable && !pair.second.used) {
       std::string help = "You set the variable \"" + pair.first.as_string() +
           "\" here and it was unused before it went\nout of scope.";
 
@@ -301,7 +346,7 @@ bool Scope::NonRecursiveMergeTo(Scope* dest,
     dest->values_[current_name] = pair.second;
 
     if (options.mark_dest_used)
-      dest->MarkUsed(current_name);
+      dest->MarkUsed(current_name, err);
   }
 
   // Target defaults are owning pointers.