@content support.

lib/src/ast/sass.dart

@@ -24,6 +24,7 @@ export 'sass/node.dart';
 export 'sass/statement.dart';
 export 'sass/statement/at_rule.dart';
 export 'sass/statement/comment.dart';
+export 'sass/statement/content.dart';
 export 'sass/statement/declaration.dart';
 export 'sass/statement/extend_rule.dart';
 export 'sass/statement/function_declaration.dart';

lib/src/ast/sass/statement/content.dart

@@ -0,0 +1,19 @@
+// Copyright 2016 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import 'package:source_span/source_span.dart';
+
+import '../../../visitor/interface/statement.dart';
+import '../statement.dart';
+
+class Content implements Statement {
+  final FileSpan span;
+
+  Content({this.span});
+
+  /*=T*/ accept/*<T>*/(StatementVisitor/*<T>*/ visitor) =>
+      visitor.visitContent(this);
+
+  String toString() => "@content;";
+}

lib/src/ast/sass/statement/mixin_declaration.dart

@@ -10,8 +10,10 @@ import '../argument_declaration.dart';
 import '../statement.dart';
 
 class MixinDeclaration extends CallableDeclaration {
+  final bool hasContent;
+
   MixinDeclaration(String name, ArgumentDeclaration arguments,
-      Iterable<Statement> children, {FileSpan span})
+      Iterable<Statement> children, {this.hasContent: false, FileSpan span})
       : super(name, arguments, children, span: span);
 
   /*=T*/ accept/*<T>*/(StatementVisitor/*<T>*/ visitor) =>

lib/src/environment.dart

@@ -2,6 +2,7 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'ast/sass.dart';
 import 'callable.dart';
 import 'functions.dart';
 import 'value.dart';
@@ -24,6 +25,14 @@ class Environment {
   // Note: this is not necessarily complete
   final Map<String, int> _mixinIndices;
 
+  /// The content block passed to the lexically-current mixin, if any.
+  List<Statement> get contentBlock => _contentBlock;
+  List<Statement> _contentBlock;
+
+  /// The environment for [_contentBlock].
+  Environment get contentEnvironment => _contentEnvironment;
+  Environment _contentEnvironment;
+
   Environment()
       : _variables = [normalizedMap()],
         _variableIndices = normalizedMap(),
@@ -35,7 +44,8 @@ class Environment {
   }
 
   Environment._(this._variables, this._variableIndices, this._functions,
-      this._functionIndices, this._mixins, this._mixinIndices);
+      this._functionIndices, this._mixins, this._mixinIndices,
+      this._contentBlock, this._contentEnvironment);
 
   Environment closure() => new Environment._(
       _variables.toList(),
@@ -43,7 +53,9 @@ class Environment {
       _functions.toList(),
       new Map.from(_functionIndices),
       _mixins.toList(),
-      new Map.from(_mixinIndices));
+      new Map.from(_mixinIndices),
+      _contentBlock,
+      _contentEnvironment);
 
   Value getVariable(String name) =>
       _variables[_variableIndices[name] ?? 0][name];
@@ -99,6 +111,17 @@ class Environment {
     _mixins[_mixins.length - 1][callable.name] = callable;
   }
 
+  void withContent(List<Statement> block, Environment environment,
+      void callback()) {
+    var oldBlock = _contentBlock;
+    var oldEnvironment = _contentEnvironment;
+    _contentBlock = block;
+    _contentEnvironment = environment;
+    callback();
+    _contentBlock = oldBlock;
+    _contentEnvironment = oldEnvironment;
+  }
+
   /*=T*/ scope/*<T>*/(/*=T*/ callback()) {
     // TODO: avoid creating a new scope if no variables are declared.
     _variables.add(normalizedMap());

lib/src/parser.dart

@@ -23,7 +23,11 @@ final _prefixedSelectorPseudoClasses =
 class Parser {
   final SpanScanner _scanner;
 
-  bool _inMixin = false;
+  var _inMixin = false;
+
+  var _inContentBlock = false;
+
+  bool _mixinHasContent;
 
   Parser(String contents, {url})
       : _scanner = new SpanScanner(contents, sourceUrl: url);
@@ -116,6 +120,7 @@ class Parser {
     _ignoreComments();
 
     switch (name) {
+      case "content": return _content(start);
       case "extend":
         return new ExtendRule(_almostAnyValue(),
             span: _scanner.spanFrom(start));
@@ -140,6 +145,19 @@ class Parser {
         span: _scanner.spanFrom(start));
   }
 
+  Content _content(LineScannerState start) {
+    if (_inMixin) {
+      _mixinHasContent = true;
+      return new Content(span: _scanner.spanFrom(start));
+    }
+
+    _scanner.error(
+        "@content is only allowed within mixin declarations.",
+        position: start.position,
+        length: "@content".length);
+    return null;
+  }
+
   Include _include(LineScannerState start) {
     var name = _identifier();
     _ignoreComments();
@@ -150,9 +168,9 @@ class Parser {
 
     List<Statement> children;
     if (_scanner.peekChar() == $lbrace) {
-      _inMixin = true;
+      _inContentBlock = true;
       children = _ruleChildren();
-      _inMixin = false;
+      _inContentBlock = false;
     }
 
     return new Include(name, arguments,
@@ -166,7 +184,7 @@ class Parser {
         ? _argumentDeclaration()
         : new ArgumentDeclaration.empty(span: _scanner.emptySpan);
 
-    if (_inMixin) {
+    if (_inMixin || _inContentBlock) {
       throw new StringScannerException(
           "Mixins may not contain mixin declarations.",
           _scanner.spanFrom(start), _scanner.string);
@@ -174,10 +192,12 @@ class Parser {
 
     _ignoreComments();
     _inMixin = true;
+    _mixinHasContent = false;
     var children = _ruleChildren();
     _inMixin = false;
 
     return new MixinDeclaration(name, arguments, children,
+        hasContent: _mixinHasContent,
         span: _scanner.spanFrom(start));
   }
 
@@ -186,7 +206,7 @@ class Parser {
     _ignoreComments();
     var arguments = _argumentDeclaration();
 
-    if (_inMixin) {
+    if (_inMixin || _inContentBlock) {
       throw new StringScannerException(
           "Mixins may not contain function declarations.",
           _scanner.spanFrom(start), _scanner.string);

lib/src/visitor/interface/statement.dart

@@ -6,6 +6,7 @@ import '../../ast/sass.dart';
 
 abstract class StatementVisitor<T> {
   T visitComment(Comment node) => null;
+  T visitContent(Content node) => null;
   T visitExtendRule(ExtendRule node) => null;
   T visitReturn(Return node) => null;
   T visitVariableDeclaration(VariableDeclaration node) => null;

lib/src/visitor/perform.dart

@@ -64,6 +64,17 @@ class PerformVisitor extends StatementVisitor
     _parent.addChild(new CssComment(node.text, span: node.span));
   }
 
+  void visitContent(Content node) {
+    var block = _environment.contentBlock;
+    if (block == null) return;
+
+    _withEnvironment(_environment.contentEnvironment, () {
+      for (var statement in block) {
+        statement.accept(this);
+      }
+    });
+  }
+
   void visitDeclaration(Declaration node) {
     var name = _interpolationToValue(node.name);
     if (_declarationName != null) {
@@ -131,16 +142,27 @@ class PerformVisitor extends StatementVisitor
   void visitInclude(Include node) {
     var mixin = _environment.getMixin(node.name) as UserDefinedCallable;
     if (mixin == null) throw node.span.message("Undefined mixin.");
-    if (node.children != null) {
+
+    if (node.children != null &&
+        !(mixin.declaration as MixinDeclaration).hasContent) {
       throw node.span.message("Mixin doesn't accept a content block.");
     }
 
-    _runUserDefinedCallable(node, mixin, () {
+    Value callback() {
       for (var statement in mixin.declaration.children) {
         statement.accept(this);
       }
       return null;
-    });
+    }
+
+    if (node.children == null) {
+      _runUserDefinedCallable(node, mixin, callback);
+    } else {
+      var environment = _environment.closure();
+      _runUserDefinedCallable(node, mixin, () {
+        _environment.withContent(node.children, environment, callback);
+      });
+    }
   }
 
   void visitMixinDeclaration(MixinDeclaration node) {