[Sass] Add an @extend node that registers extends.

lib/sass/engine.rb

@@ -9,6 +9,7 @@ require 'sass/tree/directive_node'
 require 'sass/tree/variable_node'
 require 'sass/tree/mixin_def_node'
 require 'sass/tree/mixin_node'
+require 'sass/tree/extend_node'
 require 'sass/tree/if_node'
 require 'sass/tree/while_node'
 require 'sass/tree/for_node'

lib/sass/tree/extend_node.rb

@@ -0,0 +1,22 @@
+require 'sass/tree/node'
+
+module Sass::Tree
+  class ExtendNode < Node
+    def initialize(selector)
+      @selector = selector
+      super()
+    end
+
+    protected
+
+    def perform!(environment)
+      @resolved_selector = run_interp(@selector, environment)
+      super
+    end
+
+    def _cssize(extends, parent)
+      extends[@resolved_selector] ||= []
+      extends[@resolved_selector] += parent.resolved_rules
+    end
+  end
+end

lib/sass/tree/mixin_node.rb

@@ -22,6 +22,18 @@ module Sass::Tree
 
     protected
 
+    # Returns an error message if the given child node is invalid,
+    # and false otherwise.
+    #
+    # {ExtendNode}s are valid within {MixinNode}s.
+    #
+    # @param child [Tree::Node] A potential child node
+    # @return [Boolean, String] Whether or not the child node is valid,
+    #   as well as the error message to display if it is invalid
+    def invalid_child?(child)
+      super unless child.is_a?(ExtendNode)
+    end
+
     def to_src(tabs, opts, fmt)
       args = '(' + @args.map {|a| a.to_sass}.join(", ") + ')' unless @args.empty?
       "#{'  ' * tabs}#{fmt == :sass ? '+' : '@include '}#{@name}#{args}#{semi fmt}\n"

lib/sass/tree/node.rb

@@ -328,8 +328,8 @@ module Sass
       # Returns an error message if the given child node is invalid,
       # and false otherwise.
       #
-      # By default, all child nodes except those only allowed at root level
-      # ({Tree::MixinDefNode}, {Tree::ImportNode}) are valid.
+      # By default, all child nodes except those only allowed under specific nodes
+      # ({Tree::MixinDefNode}, {Tree::ImportNode}, {Tree::ExtendNode}) are valid.
       # This is expected to be overriden by subclasses
       # for which some children are invalid.
       #
@@ -342,6 +342,8 @@ module Sass
           "Mixins may only be defined at the root of a document."
         when Tree::ImportNode
           "Import directives may only be used at the root of a document."
+        when Tree::ExtendNode
+          "Extend directives may only be used within rules."
         end
       end
 

lib/sass/tree/rule_node.rb

@@ -235,6 +235,18 @@ module Sass::Tree
       super
     end
 
+    # Returns an error message if the given child node is invalid,
+    # and false otherwise.
+    #
+    # {ExtendNode}s are valid within {RuleNode}s.
+    #
+    # @param child [Tree::Node] A potential child node
+    # @return [Boolean, String] Whether or not the child node is valid,
+    #   as well as the error message to display if it is invalid
+    def invalid_child?(child)
+      super unless child.is_a?(ExtendNode)
+    end
+
     private
 
     def resolve_parent_refs(super_rules)