Improve and polish ported extender code

- Added some more documentation from dart sass
- Use references where possible to avoid copies
- Move extend check to extender for less API surface

src/ast.hpp

@@ -228,6 +228,8 @@ namespace Sass {
     void clear()            { return elements_.clear(); }
     T last() const          { return elements_.back(); }
     T first() const         { return elements_.front(); }
+    T& last() { return elements_.back(); }
+    T& first() { return elements_.front(); }
 
     bool operator== (const Vectorized<T>& rhs) const {
       // Abort early if sizes do not match

src/context.cpp

@@ -654,22 +654,18 @@ namespace Sass {
     }
     // expand and eval the tree
     root = expand(root);
+
+    Extension unsatisfied;
+    // check that all extends were used
+    if (extender.checkForUnsatisfiedExtends(unsatisfied)) {
+      throw Exception::UnsatisfiedExtend(traces, unsatisfied);
+    }
+
     // check nesting
     check_nesting(root);
     // merge and bubble certain rules
     root = cssize(root);
 
-    ExtSmplSelSet originals = extender.getSimpleSelectors();
-    for (auto target : extender.extensions) {
-      SimpleSelector* key = target.first;
-      ExtSelExtMapEntry& val = target.second;
-      if (originals.find(key) == originals.end()) {
-        const Extension& extension = val.front().second;
-        if (extension.isOptional) continue;
-        throw Exception::UnsatisfiedExtend(traces, extension);
-      }
-    }
-
     // clean up by removing empty placeholders
     // ToDo: maybe we can do this somewhere else?
     Remove_Placeholders remove_placeholders;

src/expand.cpp

@@ -67,24 +67,30 @@ namespace Sass {
     return selector_stack;
   }
 
-  SelectorListObj Expand::selector()
+  SelectorListObj& Expand::selector()
   {
     if (selector_stack.size() > 0) {
-      auto asd = selector_stack.back();
-      if (asd.isNull()) return {};
-      return asd;
+      auto& sel = selector_stack.back();
+      if (sel.isNull()) return sel;
+      return sel;
     }
-    return {};
+    // Avoid the need to return copies
+    // We always want an empty first item
+    selector_stack.push_back({});
+    return selector_stack.back();;
   }
 
-  SelectorListObj Expand::original()
+  SelectorListObj& Expand::original()
   {
     if (originalStack.size() > 0) {
-      auto asd = originalStack.back();
-      if (asd.isNull()) return {};
-      return asd;
-    }
-    return {};
+      auto& sel = originalStack.back();
+      if (sel.isNull()) return sel;
+      return sel;
+    }
+    // Avoid the need to return copies
+    // We always want an empty first item
+    originalStack.push_back({});
+    return originalStack.back();
   }
 
   SelectorListObj Expand::popFromSelectorStack()

src/expand.hpp

@@ -19,8 +19,8 @@ namespace Sass {
   public:
 
     Env* environment();
-    SelectorListObj selector();
-    SelectorListObj original();
+    SelectorListObj& selector();
+    SelectorListObj& original();
     SelectorListObj popFromSelectorStack();
     SelectorStack getSelectorStack();
     void pushToSelectorStack(SelectorListObj selector);