From ce1ea0d4a30ab289e8d3cdcfb7344746eb77f2fc Mon Sep 17 00:00:00 2001
From: Natalie Weizenbaum <nex342@gmail.com>
Date: Sat, 2 Dec 2017 14:30:47 -0800
Subject: [PATCH 1/3] Add a ParentStatement class for Sass statements with
 children

---
 lib/src/ast/sass/statement/at_root_rule.dart  |  8 +++-----
 lib/src/ast/sass/statement/at_rule.dart       | 11 +++--------
 lib/src/ast/sass/statement/declaration.dart   | 11 ++++-------
 lib/src/ast/sass/statement/each_rule.dart     |  8 +++-----
 lib/src/ast/sass/statement/for_rule.dart      | 10 ++++------
 lib/src/ast/sass/statement/media_rule.dart    |  8 +++-----
 lib/src/ast/sass/statement/parent.dart        | 13 +++++++++++++
 lib/src/ast/sass/statement/style_rule.dart    |  8 +++-----
 lib/src/ast/sass/statement/stylesheet.dart    |  8 +++-----
 lib/src/ast/sass/statement/supports_rule.dart |  8 +++-----
 lib/src/ast/sass/statement/while_rule.dart    |  8 +++-----
 11 files changed, 45 insertions(+), 56 deletions(-)
 create mode 100644 lib/src/ast/sass/statement/parent.dart

diff --git a/lib/src/ast/sass/statement/at_root_rule.dart b/lib/src/ast/sass/statement/at_root_rule.dart
index 3df4ce0b..1335510b 100644
--- a/lib/src/ast/sass/statement/at_root_rule.dart
+++ b/lib/src/ast/sass/statement/at_root_rule.dart
@@ -7,22 +7,20 @@ import 'package:source_span/source_span.dart';
 import '../../../visitor/interface/statement.dart';
 import '../interpolation.dart';
 import '../statement.dart';
+import 'parent.dart';
 
 /// An `@at-root` rule.
 ///
 /// This moves it contents "up" the tree through parent nodes.
-class AtRootRule implements Statement {
+class AtRootRule extends ParentStatement {
   /// The query specifying which statements this should move its contents
   /// through.
   final Interpolation query;
 
-  /// The statements contained in [this].
-  final List<Statement> children;
-
   final FileSpan span;
 
   AtRootRule(Iterable<Statement> children, this.span, {this.query})
-      : children = new List.from(children);
+      : super(new List.unmodifiable(children));
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitAtRootRule(this);
 
diff --git a/lib/src/ast/sass/statement/at_rule.dart b/lib/src/ast/sass/statement/at_rule.dart
index c7e00a53..7ea3ff33 100644
--- a/lib/src/ast/sass/statement/at_rule.dart
+++ b/lib/src/ast/sass/statement/at_rule.dart
@@ -8,9 +8,10 @@ import '../../../utils.dart';
 import '../../../visitor/interface/statement.dart';
 import '../interpolation.dart';
 import '../statement.dart';
+import 'parent.dart';
 
 /// An unknown at-rule.
-class AtRule implements Statement {
+class AtRule extends ParentStatement {
   /// The name of this rule.
   final String name;
 
@@ -20,18 +21,12 @@ class AtRule implements Statement {
   /// The value of this rule.
   final Interpolation value;
 
-  /// The children of this rule.
-  ///
-  /// If [children] is empty, [this] was declared with empty brackets. If
-  /// [children] is null, it was declared without brackets.
-  final List<Statement> children;
-
   final FileSpan span;
 
   AtRule(String name, this.span, {this.value, Iterable<Statement> children})
       : name = name,
         normalizedName = unvendor(name),
-        children = children == null ? null : new List.unmodifiable(children);
+        super(children == null ? null : new List.unmodifiable(children));
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitAtRule(this);
 
diff --git a/lib/src/ast/sass/statement/declaration.dart b/lib/src/ast/sass/statement/declaration.dart
index c00a1bac..0feaf841 100644
--- a/lib/src/ast/sass/statement/declaration.dart
+++ b/lib/src/ast/sass/statement/declaration.dart
@@ -8,24 +8,21 @@ import '../../../visitor/interface/statement.dart';
 import '../expression.dart';
 import '../interpolation.dart';
 import '../statement.dart';
+import 'parent.dart';
 
 /// A declaration (that is, a `name: value` pair).
-class Declaration implements Statement {
+class Declaration extends ParentStatement {
   /// The name of this declaration.
   final Interpolation name;
 
   /// The value of this declaration.
   final Expression value;
 
-  /// The children of this declaration.
-  ///
-  /// This is `null` if the declaration has no children.
-  final List<Statement> children;
-
   final FileSpan span;
 
   Declaration(this.name, this.span, {this.value, Iterable<Statement> children})
-      : children = children == null ? null : new List.unmodifiable(children);
+      : super(children =
+            children == null ? null : new List.unmodifiable(children));
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitDeclaration(this);
 
diff --git a/lib/src/ast/sass/statement/each_rule.dart b/lib/src/ast/sass/statement/each_rule.dart
index 94a2a227..fe5566ed 100644
--- a/lib/src/ast/sass/statement/each_rule.dart
+++ b/lib/src/ast/sass/statement/each_rule.dart
@@ -7,26 +7,24 @@ import 'package:source_span/source_span.dart';
 import '../../../visitor/interface/statement.dart';
 import '../expression.dart';
 import '../statement.dart';
+import 'parent.dart';
 
 /// An `@each` rule.
 ///
 /// This iterates over values in a list or map.
-class EachRule implements Statement {
+class EachRule extends ParentStatement {
   /// The variables assigned for each iteration.
   final List<String> variables;
 
   /// The expression whose value this iterates through.
   final Expression list;
 
-  /// The child statements executed for each iteration.
-  final List<Statement> children;
-
   final FileSpan span;
 
   EachRule(Iterable<String> variables, this.list, Iterable<Statement> children,
       this.span)
       : variables = new List.unmodifiable(variables),
-        children = new List.unmodifiable(children);
+        super(new List.unmodifiable(children));
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitEachRule(this);
 
diff --git a/lib/src/ast/sass/statement/for_rule.dart b/lib/src/ast/sass/statement/for_rule.dart
index eb936096..e3661ee5 100644
--- a/lib/src/ast/sass/statement/for_rule.dart
+++ b/lib/src/ast/sass/statement/for_rule.dart
@@ -7,11 +7,12 @@ import 'package:source_span/source_span.dart';
 import '../../../visitor/interface/statement.dart';
 import '../expression.dart';
 import '../statement.dart';
+import 'parent.dart';
 
 /// A `@for` rule.
 ///
 /// This iterates a set number of times.
-class ForRule implements Statement {
+class ForRule extends ParentStatement {
   /// The name of the variable that will contain the index value.
   final String variable;
 
@@ -24,16 +25,13 @@ class ForRule implements Statement {
   /// Whether [to] is exclusive.
   final bool isExclusive;
 
-  /// The child statements executed for each iteration.
-  final List<Statement> children;
-
   final FileSpan span;
 
   ForRule(this.variable, this.from, this.to, Iterable<Statement> children,
       this.span,
       {bool exclusive: true})
-      : children = new List.unmodifiable(children),
-        isExclusive = exclusive;
+      : isExclusive = exclusive,
+        super(new List.unmodifiable(children));
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitForRule(this);
 
diff --git a/lib/src/ast/sass/statement/media_rule.dart b/lib/src/ast/sass/statement/media_rule.dart
index 52263833..2bb63bda 100644
--- a/lib/src/ast/sass/statement/media_rule.dart
+++ b/lib/src/ast/sass/statement/media_rule.dart
@@ -7,21 +7,19 @@ import 'package:source_span/source_span.dart';
 import '../../../visitor/interface/statement.dart';
 import '../interpolation.dart';
 import '../statement.dart';
+import 'parent.dart';
 
 /// A `@media` rule.
-class MediaRule implements Statement {
+class MediaRule extends ParentStatement {
   /// The query that determines on which platforms the styles will be in effect.
   ///
   /// This is only parsed after the interpolation has been resolved.
   final Interpolation query;
 
-  /// The contents of this rule.
-  final List<Statement> children;
-
   final FileSpan span;
 
   MediaRule(this.query, Iterable<Statement> children, this.span)
-      : children = new List.unmodifiable(children);
+      : super(new List.unmodifiable(children));
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitMediaRule(this);
 
diff --git a/lib/src/ast/sass/statement/parent.dart b/lib/src/ast/sass/statement/parent.dart
new file mode 100644
index 00000000..84d3f2ad
--- /dev/null
+++ b/lib/src/ast/sass/statement/parent.dart
@@ -0,0 +1,13 @@
+// Copyright 2017 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 '../statement.dart';
+
+/// A [Statement] that can have child statements.
+abstract class ParentStatement implements Statement {
+  /// The child statements of this statement.
+  final List<Statement> children;
+
+  ParentStatement(this.children);
+}
diff --git a/lib/src/ast/sass/statement/style_rule.dart b/lib/src/ast/sass/statement/style_rule.dart
index afdde4d2..7732aff7 100644
--- a/lib/src/ast/sass/statement/style_rule.dart
+++ b/lib/src/ast/sass/statement/style_rule.dart
@@ -7,23 +7,21 @@ import 'package:source_span/source_span.dart';
 import '../../../visitor/interface/statement.dart';
 import '../interpolation.dart';
 import '../statement.dart';
+import 'parent.dart';
 
 /// A style rule.
 ///
 /// This applies style declarations to elements that match a given selector.
-class StyleRule implements Statement {
+class StyleRule extends ParentStatement {
   /// The selector to which the declaration will be applied.
   ///
   /// This is only parsed after the interpolation has been resolved.
   final Interpolation selector;
 
-  /// The declarations associated with the selector.
-  final List<Statement> children;
-
   final FileSpan span;
 
   StyleRule(this.selector, Iterable<Statement> children, this.span)
-      : children = new List.unmodifiable(children);
+      : super(new List.unmodifiable(children));
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitStyleRule(this);
 
diff --git a/lib/src/ast/sass/statement/stylesheet.dart b/lib/src/ast/sass/statement/stylesheet.dart
index 4abc7c9e..f3bbfefd 100644
--- a/lib/src/ast/sass/statement/stylesheet.dart
+++ b/lib/src/ast/sass/statement/stylesheet.dart
@@ -8,18 +8,16 @@ import '../../../visitor/interface/statement.dart';
 import '../../../parse/sass.dart';
 import '../../../parse/scss.dart';
 import '../statement.dart';
+import 'parent.dart';
 
 /// A Sass stylesheet.
 ///
 /// This is the root Sass node. It contains top-level statements.
-class Stylesheet implements Statement {
-  /// The top-level statements of this Sass stylesheet.
-  final List<Statement> children;
-
+class Stylesheet extends ParentStatement {
   final FileSpan span;
 
   Stylesheet(Iterable<Statement> children, this.span)
-      : children = new List.unmodifiable(children);
+      : super(new List.unmodifiable(children));
 
   /// Parses an indented-syntax stylesheet from [contents].
   ///
diff --git a/lib/src/ast/sass/statement/supports_rule.dart b/lib/src/ast/sass/statement/supports_rule.dart
index 98d1c60e..e065c866 100644
--- a/lib/src/ast/sass/statement/supports_rule.dart
+++ b/lib/src/ast/sass/statement/supports_rule.dart
@@ -7,19 +7,17 @@ import 'package:source_span/source_span.dart';
 import '../../../visitor/interface/statement.dart';
 import '../statement.dart';
 import '../supports_condition.dart';
+import 'parent.dart';
 
 /// A `@supports` rule.
-class SupportsRule implements Statement {
+class SupportsRule extends ParentStatement {
   /// The condition that selects what browsers this rule targets.
   final SupportsCondition condition;
 
-  /// The contents of this rule.
-  final List<Statement> children;
-
   final FileSpan span;
 
   SupportsRule(this.condition, Iterable<Statement> children, this.span)
-      : children = new List.from(children);
+      : super(new List.unmodifiable(children));
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitSupportsRule(this);
 
diff --git a/lib/src/ast/sass/statement/while_rule.dart b/lib/src/ast/sass/statement/while_rule.dart
index c496aaf7..302c4a6f 100644
--- a/lib/src/ast/sass/statement/while_rule.dart
+++ b/lib/src/ast/sass/statement/while_rule.dart
@@ -7,22 +7,20 @@ import 'package:source_span/source_span.dart';
 import '../../../visitor/interface/statement.dart';
 import '../expression.dart';
 import '../statement.dart';
+import 'parent.dart';
 
 /// A `@while` rule.
 ///
 /// This repeatedly executes a block of code as long as a statement evaluates to
 /// `true`.
-class WhileRule implements Statement {
+class WhileRule extends ParentStatement {
   /// The condition that determines whether the block will be executed.
   final Expression condition;
 
-  /// The code to execute repeatedly.
-  final List<Statement> children;
-
   final FileSpan span;
 
   WhileRule(this.condition, Iterable<Statement> children, this.span)
-      : children = new List.unmodifiable(children);
+      : super(new List.unmodifiable(children));
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitWhileRule(this);
 

From 3de6680bcb12951f3573adc1a650a05e92887175 Mon Sep 17 00:00:00 2001
From: Natalie Weizenbaum <nex342@gmail.com>
Date: Sat, 2 Dec 2017 14:34:03 -0800
Subject: [PATCH 2/3] Add an explicit IfClause class

---
 lib/src/ast/sass/statement/if_rule.dart | 41 ++++++++++++++++++-------
 lib/src/parse/stylesheet.dart           |  8 ++---
 lib/src/visitor/async_evaluate.dart     |  9 +++---
 lib/src/visitor/evaluate.dart           | 11 ++++---
 4 files changed, 45 insertions(+), 24 deletions(-)

diff --git a/lib/src/ast/sass/statement/if_rule.dart b/lib/src/ast/sass/statement/if_rule.dart
index e8ef6839..945b5f6a 100644
--- a/lib/src/ast/sass/statement/if_rule.dart
+++ b/lib/src/ast/sass/statement/if_rule.dart
@@ -3,7 +3,6 @@
 // https://opensource.org/licenses/MIT.
 
 import 'package:source_span/source_span.dart';
-import 'package:tuple/tuple.dart';
 
 import '../../../visitor/interface/statement.dart';
 import '../expression.dart';
@@ -18,30 +17,50 @@ class IfRule implements Statement {
   /// The first clause whose expression evaluates to `true` will have its
   /// statements executed. If no expression evaluates to `true`, `lastClause`
   /// will be executed if it's not `null`.
-  final List<Tuple2<Expression, List<Statement>>> clauses;
+  final List<IfClause> clauses;
 
   /// The final, unconditional `@else` clause.
   ///
   /// This is `null` if there is no unconditional `@else`.
-  final List<Statement> lastClause;
+  final IfClause lastClause;
 
   final FileSpan span;
 
-  IfRule(Iterable<Tuple2<Expression, Iterable<Statement>>> clauses, this.span,
-      {Iterable<Statement> lastClause})
-      : clauses = new List.unmodifiable(clauses.map((pair) =>
-            new Tuple2(pair.item1, new List.unmodifiable(pair.item2)))),
-        lastClause =
-            lastClause == null ? null : new List.unmodifiable(lastClause);
+  IfRule(Iterable<IfClause> clauses, this.span, {this.lastClause})
+      : clauses = new List.unmodifiable(clauses) {
+    assert(clauses.every((clause) => clause.expression != null));
+    assert(lastClause?.expression == null);
+  }
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitIfRule(this);
 
   String toString() {
     var first = true;
-    return clauses.map((pair) {
+    return clauses.map((clause) {
       var name = first ? 'if' : 'else';
       first = false;
-      return '@$name ${pair.item1} {${pair.item2.join(" ")}}';
+      return '@$name ${clause.expression} {${clause.children.join(" ")}}';
     }).join(' ');
   }
 }
+
+/// A single clause in an `@if` rule.
+class IfClause {
+  /// The expression to evaluate to determine whether to run this rule, or
+  /// `null` if this is the final unconditional `@else` clause.
+  final Expression expression;
+
+  /// The statements to evaluate if this clause matches.
+  final List<Statement> children;
+
+  IfClause(this.expression, Iterable<Statement> children)
+      : children = new List.unmodifiable(children);
+
+  IfClause.last(Iterable<Statement> children)
+      : expression = null,
+        children = new List.unmodifiable(children);
+
+  String toString() =>
+      (expression == null ? "@else" : "@if $expression") +
+      " {${children.join(' ')}}";
+}
diff --git a/lib/src/parse/stylesheet.dart b/lib/src/parse/stylesheet.dart
index 25445e4d..824f786f 100644
--- a/lib/src/parse/stylesheet.dart
+++ b/lib/src/parse/stylesheet.dart
@@ -671,16 +671,16 @@ abstract class StylesheetParser extends Parser {
     var expression = _expression();
     var children = this.children(child);
 
-    var clauses = [new Tuple2(expression, children)];
-    List<Statement> lastClause;
+    var clauses = [new IfClause(expression, children)];
+    IfClause lastClause;
 
     while (scanElse(ifIndentation)) {
       whitespace();
       if (scanIdentifier("if")) {
         whitespace();
-        clauses.add(new Tuple2(_expression(), this.children(child)));
+        clauses.add(new IfClause(_expression(), this.children(child)));
       } else {
-        lastClause = this.children(child);
+        lastClause = new IfClause.last(this.children(child));
         break;
       }
     }
diff --git a/lib/src/visitor/async_evaluate.dart b/lib/src/visitor/async_evaluate.dart
index 432a2277..113c22d6 100644
--- a/lib/src/visitor/async_evaluate.dart
+++ b/lib/src/visitor/async_evaluate.dart
@@ -634,16 +634,17 @@ class _EvaluateVisitor
 
   Future<Value> visitIfRule(IfRule node) async {
     var clause = node.lastClause;
-    for (var pair in node.clauses) {
-      if ((await pair.item1.accept(this)).isTruthy) {
-        clause = pair.item2;
+    for (var clauseToCheck in node.clauses) {
+      if ((await clauseToCheck.expression.accept(this)).isTruthy) {
+        clause = clauseToCheck;
         break;
       }
     }
     if (clause == null) return null;
 
     return await _environment.scope(
-        () => _handleReturn<Statement>(clause, (child) => child.accept(this)),
+        () => _handleReturn<Statement>(
+            clause.children, (child) => child.accept(this)),
         semiGlobal: true);
   }
 
diff --git a/lib/src/visitor/evaluate.dart b/lib/src/visitor/evaluate.dart
index 3dd4d288..aadfaebb 100644
--- a/lib/src/visitor/evaluate.dart
+++ b/lib/src/visitor/evaluate.dart
@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/synchronize.dart for details.
 //
-// Checksum: ae64ba442752642066f0e9e038f5f2c1bbfa866b
+// Checksum: 33e96540f3e5999ed172d168221b27910e8672b1
 
 import 'dart:math' as math;
 
@@ -629,16 +629,17 @@ class _EvaluateVisitor
 
   Value visitIfRule(IfRule node) {
     var clause = node.lastClause;
-    for (var pair in node.clauses) {
-      if (pair.item1.accept(this).isTruthy) {
-        clause = pair.item2;
+    for (var clauseToCheck in node.clauses) {
+      if (clauseToCheck.expression.accept(this).isTruthy) {
+        clause = clauseToCheck;
         break;
       }
     }
     if (clause == null) return null;
 
     return _environment.scope(
-        () => _handleReturn<Statement>(clause, (child) => child.accept(this)),
+        () => _handleReturn<Statement>(
+            clause.children, (child) => child.accept(this)),
         semiGlobal: true);
   }
 

From 3de531b9bc5eaf6729233977086c4354a0ed277f Mon Sep 17 00:00:00 2001
From: Natalie Weizenbaum <nex342@gmail.com>
Date: Sat, 2 Dec 2017 14:36:52 -0800
Subject: [PATCH 3/3] Don't create scopes when nothing is declared

This seems to provide a fairly minimal speed boost, but it's more than
nothing. It's also a pretty easy change now that we have
ParentStatement.
---
 lib/src/ast/sass/statement/if_rule.dart | 19 +++++++--
 lib/src/ast/sass/statement/parent.dart  | 13 ++++++-
 lib/src/async_environment.dart          | 32 ++++++++++++---
 lib/src/environment.dart                | 33 +++++++++++++---
 lib/src/visitor/async_evaluate.dart     | 50 ++++++++++++++----------
 lib/src/visitor/evaluate.dart           | 52 +++++++++++++++----------
 6 files changed, 142 insertions(+), 57 deletions(-)

diff --git a/lib/src/ast/sass/statement/if_rule.dart b/lib/src/ast/sass/statement/if_rule.dart
index 945b5f6a..23063843 100644
--- a/lib/src/ast/sass/statement/if_rule.dart
+++ b/lib/src/ast/sass/statement/if_rule.dart
@@ -7,6 +7,9 @@ import 'package:source_span/source_span.dart';
 import '../../../visitor/interface/statement.dart';
 import '../expression.dart';
 import '../statement.dart';
+import 'function_rule.dart';
+import 'mixin_rule.dart';
+import 'variable_declaration.dart';
 
 /// An `@if` rule.
 ///
@@ -53,12 +56,20 @@ class IfClause {
   /// The statements to evaluate if this clause matches.
   final List<Statement> children;
 
-  IfClause(this.expression, Iterable<Statement> children)
-      : children = new List.unmodifiable(children);
+  /// Whether any of [children] is a variable, function, or mixin declaration.
+  final bool hasDeclarations;
+
+  IfClause(Expression expression, Iterable<Statement> children)
+      : this._(expression, new List.unmodifiable(children));
 
   IfClause.last(Iterable<Statement> children)
-      : expression = null,
-        children = new List.unmodifiable(children);
+      : this._(null, new List.unmodifiable(children));
+
+  IfClause._(this.expression, this.children)
+      : hasDeclarations = children.any((child) =>
+            child is VariableDeclaration ||
+            child is FunctionRule ||
+            child is MixinRule);
 
   String toString() =>
       (expression == null ? "@else" : "@if $expression") +
diff --git a/lib/src/ast/sass/statement/parent.dart b/lib/src/ast/sass/statement/parent.dart
index 84d3f2ad..98b8641a 100644
--- a/lib/src/ast/sass/statement/parent.dart
+++ b/lib/src/ast/sass/statement/parent.dart
@@ -3,11 +3,22 @@
 // https://opensource.org/licenses/MIT.
 
 import '../statement.dart';
+import 'function_rule.dart';
+import 'mixin_rule.dart';
+import 'variable_declaration.dart';
 
 /// A [Statement] that can have child statements.
 abstract class ParentStatement implements Statement {
   /// The child statements of this statement.
   final List<Statement> children;
 
-  ParentStatement(this.children);
+  /// Whether any of [children] is a variable, function, or mixin declaration.
+  final bool hasDeclarations;
+
+  ParentStatement(this.children)
+      : hasDeclarations = children?.any((child) =>
+                child is VariableDeclaration ||
+                child is FunctionRule ||
+                child is MixinRule) ??
+            false;
 }
diff --git a/lib/src/async_environment.dart b/lib/src/async_environment.dart
index 57b172c2..9ab42679 100644
--- a/lib/src/async_environment.dart
+++ b/lib/src/async_environment.dart
@@ -82,11 +82,11 @@ class AsyncEnvironment {
   bool get inMixin => _inMixin;
   var _inMixin = false;
 
-  /// Whether the environment is currently in a semi-global scope.
+  /// Whether the environment is currently in a global or semi-global scope.
   ///
   /// A semi-global scope can assign to global variables, but it doesn't declare
   /// them by default.
-  var _inSemiGlobalScope = false;
+  var _inSemiGlobalScope = true;
 
   AsyncEnvironment()
       : _variables = [normalizedMap()],
@@ -279,10 +279,32 @@ class AsyncEnvironment {
   /// Variables, functions, and mixins declared in a given scope are
   /// inaccessible outside of it. If [semiGlobal] is passed, this scope can
   /// assign to global variables without a `!global` declaration.
-  Future<T> scope<T>(Future<T> callback(), {bool semiGlobal: false}) async {
-    semiGlobal = semiGlobal && (_inSemiGlobalScope || _variables.length == 1);
+  ///
+  /// If [when] is false, this doesn't create a new scope and instead just
+  /// executes [callback] and returns its result.
+  Future<T> scope<T>(Future<T> callback(),
+      {bool semiGlobal: false, bool when: true}) async {
+    if (!when) {
+      // We still have to track semi-globalness so that
+      //
+      //     div {
+      //       @if ... {
+      //         $x: y;
+      //       }
+      //     }
+      //
+      // doesn't assign to the global scope.
+      var wasInSemiGlobalScope = _inSemiGlobalScope;
+      _inSemiGlobalScope = semiGlobal;
+      try {
+        return await callback();
+      } finally {
+        _inSemiGlobalScope = wasInSemiGlobalScope;
+      }
+    }
+
+    semiGlobal = semiGlobal && _inSemiGlobalScope;
 
-    // TODO: avoid creating a new scope if no variables are declared.
     var wasInSemiGlobalScope = _inSemiGlobalScope;
     _inSemiGlobalScope = semiGlobal;
     _variables.add(normalizedMap());
diff --git a/lib/src/environment.dart b/lib/src/environment.dart
index f4fa839c..7e4dcde9 100644
--- a/lib/src/environment.dart
+++ b/lib/src/environment.dart
@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_environment.dart.
 // See tool/synchronize.dart for details.
 //
-// Checksum: 97410abbd78c3bbc9899f3ac460cc0736218bfe3
+// Checksum: 4669f41a70664bd5f391c6b8627264a5d0ad8f6c
 
 import 'ast/sass.dart';
 import 'callable.dart';
@@ -85,11 +85,11 @@ class Environment {
   bool get inMixin => _inMixin;
   var _inMixin = false;
 
-  /// Whether the environment is currently in a semi-global scope.
+  /// Whether the environment is currently in a global or semi-global scope.
   ///
   /// A semi-global scope can assign to global variables, but it doesn't declare
   /// them by default.
-  var _inSemiGlobalScope = false;
+  var _inSemiGlobalScope = true;
 
   Environment()
       : _variables = [normalizedMap()],
@@ -282,10 +282,31 @@ class Environment {
   /// Variables, functions, and mixins declared in a given scope are
   /// inaccessible outside of it. If [semiGlobal] is passed, this scope can
   /// assign to global variables without a `!global` declaration.
-  T scope<T>(T callback(), {bool semiGlobal: false}) {
-    semiGlobal = semiGlobal && (_inSemiGlobalScope || _variables.length == 1);
+  ///
+  /// If [when] is false, this doesn't create a new scope and instead just
+  /// executes [callback] and returns its result.
+  T scope<T>(T callback(), {bool semiGlobal: false, bool when: true}) {
+    if (!when) {
+      // We still have to track semi-globalness so that
+      //
+      //     div {
+      //       @if ... {
+      //         $x: y;
+      //       }
+      //     }
+      //
+      // doesn't assign to the global scope.
+      var wasInSemiGlobalScope = _inSemiGlobalScope;
+      _inSemiGlobalScope = semiGlobal;
+      try {
+        return callback();
+      } finally {
+        _inSemiGlobalScope = wasInSemiGlobalScope;
+      }
+    }
+
+    semiGlobal = semiGlobal && _inSemiGlobalScope;
 
-    // TODO: avoid creating a new scope if no variables are declared.
     var wasInSemiGlobalScope = _inSemiGlobalScope;
     _inSemiGlobalScope = semiGlobal;
     _variables.add(normalizedMap());
diff --git a/lib/src/visitor/async_evaluate.dart b/lib/src/visitor/async_evaluate.dart
index 113c22d6..13828776 100644
--- a/lib/src/visitor/async_evaluate.dart
+++ b/lib/src/visitor/async_evaluate.dart
@@ -327,7 +327,7 @@ class _EvaluateVisitor
         for (var child in node.children) {
           await child.accept(this);
         }
-      });
+      }, when: node.hasDeclarations);
       return null;
     }
 
@@ -341,7 +341,7 @@ class _EvaluateVisitor
     }
 
     if (outerCopy != null) root.addChild(outerCopy);
-    await _scopeForAtRoot(innerCopy ?? root, query, included)(() async {
+    await _scopeForAtRoot(node, innerCopy ?? root, query, included)(() async {
       for (var child in node.children) {
         await child.accept(this);
       }
@@ -386,14 +386,14 @@ class _EvaluateVisitor
   /// This returns a callback that adjusts various instance variables for its
   /// duration, based on which rules are excluded by [query]. It always assigns
   /// [_parent] to [newParent].
-  _ScopeCallback _scopeForAtRoot(CssParentNode newParent, AtRootQuery query,
-      List<CssParentNode> included) {
+  _ScopeCallback _scopeForAtRoot(AtRootRule node, CssParentNode newParent,
+      AtRootQuery query, List<CssParentNode> included) {
     var scope = (Future callback()) async {
       // We can't use [_withParent] here because it'll add the node to the tree
       // in the wrong place.
       var oldParent = _parent;
       _parent = newParent;
-      await _environment.scope(callback);
+      await _environment.scope(callback, when: node.hasDeclarations);
       _parent = oldParent;
     };
 
@@ -488,7 +488,7 @@ class _EvaluateVisitor
         for (var child in node.children) {
           await child.accept(this);
         }
-      });
+      }, when: node.hasDeclarations);
       _declarationName = oldDeclarationName;
     }
 
@@ -588,9 +588,11 @@ class _EvaluateVisitor
           for (var child in node.children) {
             await child.accept(this);
           }
-        });
+        }, scopeWhen: false);
       }
-    }, through: (node) => node is CssStyleRule);
+    },
+        through: (node) => node is CssStyleRule,
+        scopeWhen: node.hasDeclarations);
 
     _inUnknownAtRule = wasInUnknownAtRule;
     _inKeyframes = wasInKeyframes;
@@ -645,7 +647,8 @@ class _EvaluateVisitor
     return await _environment.scope(
         () => _handleReturn<Statement>(
             clause.children, (child) => child.accept(this)),
-        semiGlobal: true);
+        semiGlobal: true,
+        when: clause.hasDeclarations);
   }
 
   Future<Value> visitImportRule(ImportRule node) async {
@@ -878,10 +881,12 @@ class _EvaluateVisitor
             for (var child in node.children) {
               await child.accept(this);
             }
-          });
+          }, scopeWhen: false);
         }
       });
-    }, through: (node) => node is CssStyleRule || node is CssMediaRule);
+    },
+        through: (node) => node is CssStyleRule || node is CssMediaRule,
+        scopeWhen: node.hasDeclarations);
 
     return null;
   }
@@ -931,7 +936,9 @@ class _EvaluateVisitor
         for (var child in node.children) {
           await child.accept(this);
         }
-      }, through: (node) => node is CssStyleRule);
+      },
+          through: (node) => node is CssStyleRule,
+          scopeWhen: node.hasDeclarations);
       return null;
     }
 
@@ -955,7 +962,9 @@ class _EvaluateVisitor
           await child.accept(this);
         }
       });
-    }, through: (node) => node is CssStyleRule);
+    },
+        through: (node) => node is CssStyleRule,
+        scopeWhen: node.hasDeclarations);
     _atRootExcludingStyleRule = oldAtRootExcludingStyleRule;
 
     if (!_inStyleRule) {
@@ -992,7 +1001,9 @@ class _EvaluateVisitor
           }
         });
       }
-    }, through: (node) => node is CssStyleRule);
+    },
+        through: (node) => node is CssStyleRule,
+        scopeWhen: node.hasDeclarations);
 
     return null;
   }
@@ -1068,7 +1079,7 @@ class _EvaluateVisitor
         if (result != null) return result;
       }
       return null;
-    }, semiGlobal: true);
+    }, semiGlobal: true, when: node.hasDeclarations);
   }
 
   // ## Expressions
@@ -1250,9 +1261,6 @@ class _EvaluateVisitor
           _verifyArguments(
               positional.length, named, callable.declaration.arguments, span);
 
-          // TODO: if we get here and there are no rest params involved, mark
-          // the callable as fast-path and don't do error checking or extra
-          // allocations for future calls.
           var declaredArguments = callable.declaration.arguments.arguments;
           var minLength = math.min(positional.length, declaredArguments.length);
           for (var i = 0; i < minLength; i++) {
@@ -1633,9 +1641,11 @@ class _EvaluateVisitor
   /// If [through] is passed, [node] is added as a child of the first parent for
   /// which [through] returns `false`. That parent is copied unless it's the
   /// lattermost child of its parent.
+  ///
+  /// Runs [callback] in a new environment scope unless [scopeWhen] is false.
   Future<T> _withParent<S extends CssParentNode, T>(
       S node, Future<T> callback(),
-      {bool through(CssNode node)}) async {
+      {bool through(CssNode node), bool scopeWhen: true}) async {
     var oldParent = _parent;
 
     // Go up through parents that match [through].
@@ -1657,7 +1667,7 @@ class _EvaluateVisitor
 
     parent.addChild(node);
     _parent = node;
-    var result = await _environment.scope(callback);
+    var result = await _environment.scope(callback, when: scopeWhen);
     _parent = oldParent;
 
     return result;
diff --git a/lib/src/visitor/evaluate.dart b/lib/src/visitor/evaluate.dart
index aadfaebb..0b51614e 100644
--- a/lib/src/visitor/evaluate.dart
+++ b/lib/src/visitor/evaluate.dart
@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/synchronize.dart for details.
 //
-// Checksum: 33e96540f3e5999ed172d168221b27910e8672b1
+// Checksum: cdeeec2634c97638aef571043c0be1e99f22d7d5
 
 import 'dart:math' as math;
 
@@ -328,7 +328,7 @@ class _EvaluateVisitor
         for (var child in node.children) {
           child.accept(this);
         }
-      });
+      }, when: node.hasDeclarations);
       return null;
     }
 
@@ -342,7 +342,7 @@ class _EvaluateVisitor
     }
 
     if (outerCopy != null) root.addChild(outerCopy);
-    _scopeForAtRoot(innerCopy ?? root, query, included)(() {
+    _scopeForAtRoot(node, innerCopy ?? root, query, included)(() {
       for (var child in node.children) {
         child.accept(this);
       }
@@ -387,14 +387,14 @@ class _EvaluateVisitor
   /// This returns a callback that adjusts various instance variables for its
   /// duration, based on which rules are excluded by [query]. It always assigns
   /// [_parent] to [newParent].
-  _ScopeCallback _scopeForAtRoot(CssParentNode newParent, AtRootQuery query,
-      List<CssParentNode> included) {
+  _ScopeCallback _scopeForAtRoot(AtRootRule node, CssParentNode newParent,
+      AtRootQuery query, List<CssParentNode> included) {
     var scope = (void callback()) {
       // We can't use [_withParent] here because it'll add the node to the tree
       // in the wrong place.
       var oldParent = _parent;
       _parent = newParent;
-      _environment.scope(callback);
+      _environment.scope(callback, when: node.hasDeclarations);
       _parent = oldParent;
     };
 
@@ -487,7 +487,7 @@ class _EvaluateVisitor
         for (var child in node.children) {
           child.accept(this);
         }
-      });
+      }, when: node.hasDeclarations);
       _declarationName = oldDeclarationName;
     }
 
@@ -583,9 +583,11 @@ class _EvaluateVisitor
           for (var child in node.children) {
             child.accept(this);
           }
-        });
+        }, scopeWhen: false);
       }
-    }, through: (node) => node is CssStyleRule);
+    },
+        through: (node) => node is CssStyleRule,
+        scopeWhen: node.hasDeclarations);
 
     _inUnknownAtRule = wasInUnknownAtRule;
     _inKeyframes = wasInKeyframes;
@@ -640,7 +642,8 @@ class _EvaluateVisitor
     return _environment.scope(
         () => _handleReturn<Statement>(
             clause.children, (child) => child.accept(this)),
-        semiGlobal: true);
+        semiGlobal: true,
+        when: clause.hasDeclarations);
   }
 
   Value visitImportRule(ImportRule node) {
@@ -872,10 +875,12 @@ class _EvaluateVisitor
             for (var child in node.children) {
               child.accept(this);
             }
-          });
+          }, scopeWhen: false);
         }
       });
-    }, through: (node) => node is CssStyleRule || node is CssMediaRule);
+    },
+        through: (node) => node is CssStyleRule || node is CssMediaRule,
+        scopeWhen: node.hasDeclarations);
 
     return null;
   }
@@ -922,7 +927,9 @@ class _EvaluateVisitor
         for (var child in node.children) {
           child.accept(this);
         }
-      }, through: (node) => node is CssStyleRule);
+      },
+          through: (node) => node is CssStyleRule,
+          scopeWhen: node.hasDeclarations);
       return null;
     }
 
@@ -946,7 +953,9 @@ class _EvaluateVisitor
           child.accept(this);
         }
       });
-    }, through: (node) => node is CssStyleRule);
+    },
+        through: (node) => node is CssStyleRule,
+        scopeWhen: node.hasDeclarations);
     _atRootExcludingStyleRule = oldAtRootExcludingStyleRule;
 
     if (!_inStyleRule) {
@@ -983,7 +992,9 @@ class _EvaluateVisitor
           }
         });
       }
-    }, through: (node) => node is CssStyleRule);
+    },
+        through: (node) => node is CssStyleRule,
+        scopeWhen: node.hasDeclarations);
 
     return null;
   }
@@ -1058,7 +1069,7 @@ class _EvaluateVisitor
         if (result != null) return result;
       }
       return null;
-    }, semiGlobal: true);
+    }, semiGlobal: true, when: node.hasDeclarations);
   }
 
   // ## Expressions
@@ -1231,9 +1242,6 @@ class _EvaluateVisitor
           _verifyArguments(
               positional.length, named, callable.declaration.arguments, span);
 
-          // TODO: if we get here and there are no rest params involved, mark
-          // the callable as fast-path and don't do error checking or extra
-          // allocations for future calls.
           var declaredArguments = callable.declaration.arguments.arguments;
           var minLength = math.min(positional.length, declaredArguments.length);
           for (var i = 0; i < minLength; i++) {
@@ -1604,8 +1612,10 @@ class _EvaluateVisitor
   /// If [through] is passed, [node] is added as a child of the first parent for
   /// which [through] returns `false`. That parent is copied unless it's the
   /// lattermost child of its parent.
+  ///
+  /// Runs [callback] in a new environment scope unless [scopeWhen] is false.
   T _withParent<S extends CssParentNode, T>(S node, T callback(),
-      {bool through(CssNode node)}) {
+      {bool through(CssNode node), bool scopeWhen: true}) {
     var oldParent = _parent;
 
     // Go up through parents that match [through].
@@ -1627,7 +1637,7 @@ class _EvaluateVisitor
 
     parent.addChild(node);
     _parent = node;
-    var result = _environment.scope(callback);
+    var result = _environment.scope(callback, when: scopeWhen);
     _parent = oldParent;
 
     return result;