diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4fe47caa4..dd635060a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,44 @@
+## 1.23.0
+
+* **Launch the new Sass module system!** This adds:
+
+  * The [`@use` rule][], which loads Sass files as *modules* and makes their
+    members available only in the current file, with automatic namespacing.
+    
+    [`@use` rule]: https://sass-lang.com/documentation/at-rules/use
+
+  * The [`@forward` rule][], which makes members of another Sass file available
+    to stylesheets that `@use` the current file.
+
+    [`@forward` rule]: https://sass-lang.com/documentation/at-rules/forward
+
+  * Built-in modules named `sass:color`, `sass:list`, `sass:map`, `sass:math`,
+    `sass:meta`, `sass:selector`, and `sass:string` that provide access to all
+    the built-in Sass functions you know and love, with automatic module
+    namespaces.
+
+  * The [`meta.load-css()` mixin][], which includes the CSS contents of a module
+    loaded from a (potentially dynamic) URL.
+
+    [`meta.load-css()` function]: https://sass-lang.com/documentation/modules/meta#load-css
+
+  * The [`meta.module-variables()` function][], which provides access to the
+    variables defined in a given module.
+
+    [`meta.module-variables()` function]: https://sass-lang.com/documentation/modules/meta#module-variables
+
+  * The [`meta.module-functions()` function][], which provides access to the
+    functions defined in a given module.
+
+    [`meta.module-functions()` function]: https://sass-lang.com/documentation/modules/meta#module-functions
+
+  Check out [the Sass blog][migrator blog] for more information on the new
+  module system. You can also use the new [Sass migrator][] to automatically
+  migrate your stylesheets to the new module system!
+
+  [migrator blog]: https://sass-lang.com/blog/7858341-the-module-system-is-launched
+  [Sass migrator]: https://sass-lang.com/documentation/cli/migrator
+
 ## 1.22.12
 
 * **Potentially breaking bug fix:** character sequences consisting of two or
diff --git a/lib/src/ast/sass/statement/use_rule.dart b/lib/src/ast/sass/statement/use_rule.dart
index 97532283b..4041b3306 100644
--- a/lib/src/ast/sass/statement/use_rule.dart
+++ b/lib/src/ast/sass/statement/use_rule.dart
@@ -3,8 +3,12 @@
 // https://opensource.org/licenses/MIT.
 
 import 'package:source_span/source_span.dart';
+import 'package:tuple/tuple.dart';
 
+import '../../../logger.dart';
+import '../../../parse/scss.dart';
 import '../../../visitor/interface/statement.dart';
+import '../expression.dart';
 import '../expression/string.dart';
 import '../statement.dart';
 
@@ -19,12 +23,45 @@ class UseRule implements Statement {
   /// can be accessed without a namespace.
   final String namespace;
 
+  /// A map from variable names to their values and the spans for those
+  /// variables, used to configure the loaded modules.
+  final Map<String, Tuple2<Expression, FileSpan>> configuration;
+
   final FileSpan span;
 
-  UseRule(this.url, this.namespace, this.span);
+  UseRule(this.url, this.namespace, this.span,
+      {Map<String, Tuple2<Expression, FileSpan>> configuration})
+      : configuration = Map.unmodifiable(configuration ?? const {});
+
+  /// Parses a `@use` rule from [contents].
+  ///
+  /// If passed, [url] is the name of the file from which [contents] comes.
+  ///
+  /// Throws a [SassFormatException] if parsing fails.
+  factory UseRule.parse(String contents, {url, Logger logger}) =>
+      ScssParser(contents, url: url, logger: logger).parseUseRule();
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitUseRule(this);
 
-  String toString() => "@use ${StringExpression.quoteText(url.toString())} as "
-      "${namespace ?? "*"};";
+  String toString() {
+    var buffer =
+        StringBuffer("@use ${StringExpression.quoteText(url.toString())}");
+
+    var basename = url.pathSegments.isEmpty ? "" : url.pathSegments.last;
+    var dot = basename.indexOf(".");
+    if (namespace != basename.substring(0, dot == -1 ? basename.length : dot)) {
+      buffer.write(" as ${namespace ?? "*"}");
+    }
+
+    if (configuration.isNotEmpty) {
+      buffer.write(" with (");
+      buffer.write(configuration.entries
+          .map((entry) => "\$${entry.key}: ${entry.value.item1}")
+          .join(", "));
+      buffer.write(")");
+    }
+
+    buffer.write(";");
+    return buffer.toString();
+  }
 }
diff --git a/lib/src/ast/selector/placeholder.dart b/lib/src/ast/selector/placeholder.dart
index b8e53b4b6..8c7a43f83 100644
--- a/lib/src/ast/selector/placeholder.dart
+++ b/lib/src/ast/selector/placeholder.dart
@@ -2,8 +2,7 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-import 'package:charcode/charcode.dart';
-
+import '../../util/character.dart' as character;
 import '../../visitor/interface/selector.dart';
 import '../selector.dart';
 
@@ -20,10 +19,7 @@ class PlaceholderSelector extends SimpleSelector {
 
   /// Returns whether this is a private selector (that is, whether it begins
   /// with `-` or `_`).
-  bool get isPrivate {
-    var start = name.codeUnitAt(0);
-    return start == $dash || start == $underscore;
-  }
+  bool get isPrivate => character.isPrivate(name);
 
   PlaceholderSelector(this.name);
 
diff --git a/lib/src/async_environment.dart b/lib/src/async_environment.dart
index f5a0fc7a7..e470b8c8e 100644
--- a/lib/src/async_environment.dart
+++ b/lib/src/async_environment.dart
@@ -48,9 +48,7 @@ class AsyncEnvironment {
 
   /// A list of variables defined at each lexical scope level.
   ///
-  /// Each scope maps the names of declared variables to their values. These
-  /// maps are *normalized*, meaning that they treat hyphens and underscores in
-  /// its keys interchangeably.
+  /// Each scope maps the names of declared variables to their values.
   ///
   /// The first element is the global scope, and each successive element is
   /// deeper in the tree.
@@ -67,17 +65,12 @@ class AsyncEnvironment {
 
   /// A map of variable names to their indices in [_variables].
   ///
-  /// This map is *normalized*, meaning that it treats hyphens and underscores
-  /// in its keys interchangeably.
-  ///
   /// This map is filled in as-needed, and may not be complete.
   final Map<String, int> _variableIndices;
 
   /// A list of functions defined at each lexical scope level.
   ///
-  /// Each scope maps the names of declared functions to their values. These
-  /// maps are *normalized*, meaning that they treat hyphens and underscores in
-  /// its keys interchangeably.
+  /// Each scope maps the names of declared functions to their values.
   ///
   /// The first element is the global scope, and each successive element is
   /// deeper in the tree.
@@ -85,17 +78,12 @@ class AsyncEnvironment {
 
   /// A map of function names to their indices in [_functions].
   ///
-  /// This map is *normalized*, meaning that it treats hyphens and underscores
-  /// in its keys interchangeably.
-  ///
   /// This map is filled in as-needed, and may not be complete.
   final Map<String, int> _functionIndices;
 
   /// A list of mixins defined at each lexical scope level.
   ///
-  /// Each scope maps the names of declared mixins to their values. These
-  /// maps are *normalized*, meaning that they treat hyphens and underscores in
-  /// its keys interchangeably.
+  /// Each scope maps the names of declared mixins to their values.
   ///
   /// The first element is the global scope, and each successive element is
   /// deeper in the tree.
@@ -103,9 +91,6 @@ class AsyncEnvironment {
 
   /// A map of mixin names to their indices in [_mixins].
   ///
-  /// This map is *normalized*, meaning that it treats hyphens and underscores
-  /// in its keys interchangeably.
-  ///
   /// This map is filled in as-needed, and may not be complete.
   final Map<String, int> _mixinIndices;
 
@@ -211,7 +196,7 @@ class AsyncEnvironment {
   /// with the same name as a variable defined in this environment.
   void addModule(Module module, {String namespace}) {
     if (namespace == null) {
-      _globalModules ??= Set();
+      _globalModules ??= {};
       _globalModules.add(module);
       _allModules.add(module);
 
@@ -715,10 +700,14 @@ class AsyncEnvironment {
     T value;
     for (var module in _globalModules) {
       var valueInModule = callback(module);
-      if (valueInModule != null && value != null) {
-        // TODO(nweiz): List the module URLs.
+      if (valueInModule == null) continue;
+
+      if (value != null) {
         throw SassScriptException(
-            'This $type is available from multiple global modules.');
+            'This $type is available from multiple global modules:\n' +
+                bulletedList(_globalModules
+                    .where((module) => callback(module) != null)
+                    .map((module) => p.prettyUri(module.url))));
       }
 
       value = valueInModule;
diff --git a/lib/src/environment.dart b/lib/src/environment.dart
index caa752a82..19db8373b 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/grind/synchronize.dart for details.
 //
-// Checksum: 0e6357eae4a02ebe64d8cdbf48a14797f98a418c
+// Checksum: 2522d5fbfb9d301b361a9bacac97228de2c6fd68
 //
 // ignore_for_file: unused_import
 
@@ -54,9 +54,7 @@ class Environment {
 
   /// A list of variables defined at each lexical scope level.
   ///
-  /// Each scope maps the names of declared variables to their values. These
-  /// maps are *normalized*, meaning that they treat hyphens and underscores in
-  /// its keys interchangeably.
+  /// Each scope maps the names of declared variables to their values.
   ///
   /// The first element is the global scope, and each successive element is
   /// deeper in the tree.
@@ -73,17 +71,12 @@ class Environment {
 
   /// A map of variable names to their indices in [_variables].
   ///
-  /// This map is *normalized*, meaning that it treats hyphens and underscores
-  /// in its keys interchangeably.
-  ///
   /// This map is filled in as-needed, and may not be complete.
   final Map<String, int> _variableIndices;
 
   /// A list of functions defined at each lexical scope level.
   ///
-  /// Each scope maps the names of declared functions to their values. These
-  /// maps are *normalized*, meaning that they treat hyphens and underscores in
-  /// its keys interchangeably.
+  /// Each scope maps the names of declared functions to their values.
   ///
   /// The first element is the global scope, and each successive element is
   /// deeper in the tree.
@@ -91,17 +84,12 @@ class Environment {
 
   /// A map of function names to their indices in [_functions].
   ///
-  /// This map is *normalized*, meaning that it treats hyphens and underscores
-  /// in its keys interchangeably.
-  ///
   /// This map is filled in as-needed, and may not be complete.
   final Map<String, int> _functionIndices;
 
   /// A list of mixins defined at each lexical scope level.
   ///
-  /// Each scope maps the names of declared mixins to their values. These
-  /// maps are *normalized*, meaning that they treat hyphens and underscores in
-  /// its keys interchangeably.
+  /// Each scope maps the names of declared mixins to their values.
   ///
   /// The first element is the global scope, and each successive element is
   /// deeper in the tree.
@@ -109,9 +97,6 @@ class Environment {
 
   /// A map of mixin names to their indices in [_mixins].
   ///
-  /// This map is *normalized*, meaning that it treats hyphens and underscores
-  /// in its keys interchangeably.
-  ///
   /// This map is filled in as-needed, and may not be complete.
   final Map<String, int> _mixinIndices;
 
@@ -217,7 +202,7 @@ class Environment {
   /// with the same name as a variable defined in this environment.
   void addModule(Module<Callable> module, {String namespace}) {
     if (namespace == null) {
-      _globalModules ??= Set();
+      _globalModules ??= {};
       _globalModules.add(module);
       _allModules.add(module);
 
@@ -719,10 +704,14 @@ class Environment {
     T value;
     for (var module in _globalModules) {
       var valueInModule = callback(module);
-      if (valueInModule != null && value != null) {
-        // TODO(nweiz): List the module URLs.
+      if (valueInModule == null) continue;
+
+      if (value != null) {
         throw SassScriptException(
-            'This $type is available from multiple global modules.');
+            'This $type is available from multiple global modules:\n' +
+                bulletedList(_globalModules
+                    .where((module) => callback(module) != null)
+                    .map((module) => p.prettyUri(module.url))));
       }
 
       value = valueInModule;
diff --git a/lib/src/executable/options.dart b/lib/src/executable/options.dart
index bb4cd6368..9f76ee761 100644
--- a/lib/src/executable/options.dart
+++ b/lib/src/executable/options.dart
@@ -138,8 +138,8 @@ class ExecutableOptions {
     if (!_interactive) return false;
 
     var invalidOptions = [
-      'stdin', 'indented', 'load-path', 'style', 'source-map', //
-      'source-map-urls', 'embed-sources', 'embed-source-map', 'update', 'watch'
+      'stdin', 'indented', 'style', 'source-map', 'source-map-urls', //
+      'embed-sources', 'embed-source-map', 'update', 'watch'
     ];
     for (var option in invalidOptions) {
       if (_options.wasParsed(option)) {
diff --git a/lib/src/executable/repl.dart b/lib/src/executable/repl.dart
index d69910163..642d5984e 100644
--- a/lib/src/executable/repl.dart
+++ b/lib/src/executable/repl.dart
@@ -11,32 +11,38 @@ import 'package:stack_trace/stack_trace.dart';
 import '../ast/sass.dart';
 import '../exception.dart';
 import '../executable/options.dart';
+import '../import_cache.dart';
+import '../importer/filesystem.dart';
 import '../logger/tracking.dart';
 import '../parse/parser.dart';
-import '../value.dart' as internal;
 import '../visitor/evaluate.dart';
 
 /// Runs an interactive SassScript shell according to [options].
 Future<void> repl(ExecutableOptions options) async {
   var repl = Repl(prompt: '>> ');
-  var variables = <String, internal.Value>{};
+  var logger = TrackingLogger(options.logger);
+  var evaluator = Evaluator(
+      importer: FilesystemImporter('.'),
+      importCache:
+          ImportCache(const [], loadPaths: options.loadPaths, logger: logger),
+      logger: logger);
   await for (String line in repl.runAsync()) {
     if (line.trim().isEmpty) continue;
-    var logger = TrackingLogger(options.logger);
     try {
-      VariableDeclaration declaration;
-      Expression expression;
+      if (line.startsWith("@")) {
+        evaluator.use(UseRule.parse(line, logger: logger));
+        continue;
+      }
+
       if (Parser.isVariableDeclarationLike(line)) {
-        declaration = VariableDeclaration.parse(line, logger: logger);
-        expression = declaration.expression;
+        var declaration = VariableDeclaration.parse(line, logger: logger);
+        evaluator.setVariable(declaration);
+        print(evaluator.evaluate(VariableExpression(
+            declaration.name, declaration.span,
+            namespace: declaration.namespace)));
       } else {
-        expression = Expression.parse(line, logger: logger);
+        print(evaluator.evaluate(Expression.parse(line, logger: logger)));
       }
-
-      var result =
-          evaluateExpression(expression, variables: variables, logger: logger);
-      if (declaration != null) variables[declaration.name] = result;
-      print(result);
     } on SassException catch (error, stackTrace) {
       _logError(error, stackTrace, line, repl, options, logger);
     }
@@ -46,10 +52,11 @@ Future<void> repl(ExecutableOptions options) async {
 /// Logs an error from the interactive shell.
 void _logError(SassException error, StackTrace stackTrace, String line,
     Repl repl, ExecutableOptions options, TrackingLogger logger) {
-  // If something was logged after the input, just print the error.
-  if (!options.quiet && (logger.emittedDebug || logger.emittedWarning)) {
-    print("Error: ${error.message}");
-    print(error.span.highlight(color: options.color));
+  // If the error doesn't come from the repl line, or if something was logged
+  // after the user's input, just print the error normally.
+  if (error.span.sourceUrl != null ||
+      (!options.quiet && (logger.emittedDebug || logger.emittedWarning))) {
+    print(error.toString(color: options.color));
     return;
   }
 
diff --git a/lib/src/functions/color.dart b/lib/src/functions/color.dart
index 2928d7f74..fa43d6f58 100644
--- a/lib/src/functions/color.dart
+++ b/lib/src/functions/color.dart
@@ -66,7 +66,7 @@ final global = UnmodifiableListView([
   }),
 
   // ### HSL
-  _hue, _saturation, _lightness, _adjustHue, _complement,
+  _hue, _saturation, _lightness, _complement,
 
   BuiltInCallable.overloaded("hsl", {
     r"$hue, $saturation, $lightness, $alpha": (arguments) =>
@@ -117,6 +117,12 @@ final global = UnmodifiableListView([
     return color.changeHsl(saturation: 0);
   }),
 
+  BuiltInCallable("adjust-hue", r"$color, $degrees", (arguments) {
+    var color = arguments[0].assertColor("color");
+    var degrees = arguments[1].assertNumber("degrees");
+    return color.changeHsl(hue: color.hue + degrees.value);
+  }),
+
   BuiltInCallable("lighten", r"$color, $amount", (arguments) {
     var color = arguments[0].assertColor("color");
     var amount = arguments[1].assertNumber("amount");
@@ -212,7 +218,7 @@ final global = UnmodifiableListView([
 ]);
 
 /// The Sass color module.
-final module = BuiltInModule("color", [
+final module = BuiltInModule("color", functions: [
   // ### RGB
   _red, _green, _blue, _mix,
 
@@ -239,7 +245,8 @@ final module = BuiltInModule("color", [
   }),
 
   // ### HSL
-  _hue, _saturation, _lightness, _adjustHue, _complement,
+  _hue, _saturation, _lightness, _complement,
+  _removedColorFunction("adjust-hue", "hue"),
   _removedColorFunction("lighten", "lightness"),
   _removedColorFunction("darken", "lightness", negative: true),
   _removedColorFunction("saturate", "saturation"),
@@ -355,13 +362,6 @@ final _lightness = BuiltInCallable(
     (arguments) =>
         SassNumber(arguments.first.assertColor("color").lightness, "%"));
 
-final _adjustHue =
-    BuiltInCallable("adjust-hue", r"$color, $degrees", (arguments) {
-  var color = arguments[0].assertColor("color");
-  var degrees = arguments[1].assertNumber("degrees");
-  return color.changeHsl(hue: color.hue + degrees.value);
-});
-
 final _complement = BuiltInCallable("complement", r"$color", (arguments) {
   var color = arguments[0].assertColor("color");
   return color.changeHsl(hue: color.hue + 180);
diff --git a/lib/src/functions/list.dart b/lib/src/functions/list.dart
index f8db21381..7f460b62a 100644
--- a/lib/src/functions/list.dart
+++ b/lib/src/functions/list.dart
@@ -18,7 +18,7 @@ final global = UnmodifiableListView([
 ]);
 
 /// The Sass list module.
-final module = BuiltInModule("list", [
+final module = BuiltInModule("list", functions: [
   _length, _nth, _setNth, _join, _append, _zip, _index, _isBracketed, //
   _separator
 ]);
diff --git a/lib/src/functions/map.dart b/lib/src/functions/map.dart
index c71f3726a..9ca6a3025 100644
--- a/lib/src/functions/map.dart
+++ b/lib/src/functions/map.dart
@@ -21,8 +21,8 @@ final global = UnmodifiableListView([
 ]);
 
 /// The Sass map module.
-final module =
-    BuiltInModule("map", [_get, _merge, _remove, _keys, _values, _hasKey]);
+final module = BuiltInModule("map",
+    functions: [_get, _merge, _remove, _keys, _values, _hasKey]);
 
 final _get = BuiltInCallable("get", r"$map, $key", (arguments) {
   var map = arguments[0].assertMap("map");
diff --git a/lib/src/functions/math.dart b/lib/src/functions/math.dart
index b78706c0d..b0b52ec7a 100644
--- a/lib/src/functions/math.dart
+++ b/lib/src/functions/math.dart
@@ -25,7 +25,7 @@ final global = UnmodifiableListView([
 ]);
 
 /// The Sass math module.
-final module = BuiltInModule("math", [
+final module = BuiltInModule("math", functions: [
   _round, _ceil, _floor, _abs, _max, _min, _randomFunction, _unit,
   _isUnitless, //
   _percentage, _compatible
diff --git a/lib/src/functions/selector.dart b/lib/src/functions/selector.dart
index 2883565bb..0af5cfccf 100644
--- a/lib/src/functions/selector.dart
+++ b/lib/src/functions/selector.dart
@@ -26,7 +26,7 @@ final global = UnmodifiableListView([
 ]);
 
 /// The Sass selector module.
-final module = BuiltInModule("selector", [
+final module = BuiltInModule("selector", functions: [
   _isSuperselector,
   _simpleSelectors,
   _parse,
diff --git a/lib/src/functions/string.dart b/lib/src/functions/string.dart
index 4eb663a76..03b7728d4 100644
--- a/lib/src/functions/string.dart
+++ b/lib/src/functions/string.dart
@@ -29,7 +29,7 @@ final global = UnmodifiableListView([
 ]);
 
 /// The Sass string module.
-final module = BuiltInModule("string", [
+final module = BuiltInModule("string", functions: [
   _unquote, _quote, _toUpperCase, _toLowerCase, _length, _insert, _index, //
   _slice, _uniqueId,
 ]);
diff --git a/lib/src/module/built_in.dart b/lib/src/module/built_in.dart
index 9a2a15f5f..63d098687 100644
--- a/lib/src/module/built_in.dart
+++ b/lib/src/module/built_in.dart
@@ -16,20 +16,28 @@ import '../value.dart';
 class BuiltInModule<T extends AsyncCallable> implements Module<T> {
   final Uri url;
   final Map<String, T> functions;
+  final Map<String, T> mixins;
 
   List<Module<T>> get upstream => const [];
   Map<String, Value> get variables => const {};
   Map<String, AstNode> get variableNodes => const {};
-  Map<String, T> get mixins => const {};
   Extender get extender => Extender.empty;
   CssStylesheet get css => CssStylesheet.empty(url: url);
   bool get transitivelyContainsCss => false;
   bool get transitivelyContainsExtensions => false;
 
-  BuiltInModule(String name, Iterable<T> functions)
+  BuiltInModule(String name, {Iterable<T> functions, Iterable<T> mixins})
       : url = Uri(scheme: "sass", path: name),
-        functions = UnmodifiableMapView(
-            {for (var function in functions) function.name: function});
+        functions = _callableMap(functions),
+        mixins = _callableMap(mixins);
+
+  /// Returns a map from [callables]' names to their values.
+  static Map<String, T> _callableMap<T extends AsyncCallable>(
+          Iterable<T> callables) =>
+      UnmodifiableMapView(callables == null
+          ? {}
+          : UnmodifiableMapView(
+              {for (var callable in callables) callable.name: callable}));
 
   void setVariable(String name, Value value, AstNode nodeWithSpan) {
     throw SassScriptException("Undefined variable.");
diff --git a/lib/src/module/forwarded_view.dart b/lib/src/module/forwarded_view.dart
index ca38098ef..441133b75 100644
--- a/lib/src/module/forwarded_view.dart
+++ b/lib/src/module/forwarded_view.dart
@@ -11,7 +11,6 @@ import '../extend/extender.dart';
 import '../module.dart';
 import '../util/limited_map_view.dart';
 import '../util/prefixed_map_view.dart';
-import '../utils.dart';
 import '../value.dart';
 
 /// A [Module] that exposes members according to a [ForwardRule].
@@ -82,7 +81,7 @@ class ForwardedModuleView<T extends AsyncCallable> implements Module<T> {
     }
 
     if (_rule.prefix != null) {
-      if (!startsWithIgnoreSeparator(name, _rule.prefix)) {
+      if (!name.startsWith(_rule.prefix)) {
         throw SassScriptException("Undefined variable.");
       }
 
diff --git a/lib/src/parse/sass.dart b/lib/src/parse/sass.dart
index eb39aa269..bd2b89f4d 100644
--- a/lib/src/parse/sass.dart
+++ b/lib/src/parse/sass.dart
@@ -172,7 +172,7 @@ class SassParser extends StylesheetParser {
         return null;
 
       case $dollar:
-        return variableDeclaration();
+        return variableDeclarationWithoutNamespace();
         break;
 
       case $slash:
diff --git a/lib/src/parse/scss.dart b/lib/src/parse/scss.dart
index 439033283..aec633d57 100644
--- a/lib/src/parse/scss.dart
+++ b/lib/src/parse/scss.dart
@@ -66,7 +66,7 @@ class ScssParser extends StylesheetParser {
     while (true) {
       switch (scanner.peekChar()) {
         case $dollar:
-          children.add(variableDeclaration());
+          children.add(variableDeclarationWithoutNamespace());
           break;
 
         case $slash:
@@ -107,7 +107,7 @@ class ScssParser extends StylesheetParser {
     while (!scanner.isDone) {
       switch (scanner.peekChar()) {
         case $dollar:
-          statements.add(variableDeclaration());
+          statements.add(variableDeclarationWithoutNamespace());
           break;
 
         case $slash:
diff --git a/lib/src/parse/stylesheet.dart b/lib/src/parse/stylesheet.dart
index 1eed929eb..0a36b7bd7 100644
--- a/lib/src/parse/stylesheet.dart
+++ b/lib/src/parse/stylesheet.dart
@@ -22,14 +22,6 @@ import '../value.dart';
 import '../value/color.dart';
 import 'parser.dart';
 
-/// Whether to parse `@use` rules.
-///
-/// This is set to `false` on Dart Sass's master branch and `true` on the
-/// `feature.use` branch. It allows us to avoid having separate development
-/// tracks as much as possible without shipping `@use` support until we're
-/// ready.
-const _parseUse = false;
-
 /// The base class for both the SCSS and indented syntax parsers.
 ///
 /// Having a base class that's separate from both parsers allows us to make
@@ -109,27 +101,30 @@ abstract class StylesheetParser extends Parser {
     });
   }
 
-  ArgumentDeclaration parseArgumentDeclaration() {
-    return wrapSpanFormatException(() {
-      var declaration = _argumentDeclaration();
-      scanner.expectDone();
-      return declaration;
-    });
-  }
+  ArgumentDeclaration parseArgumentDeclaration() =>
+      _parseSingleProduction(_argumentDeclaration);
 
-  Expression parseExpression() {
-    return wrapSpanFormatException(() {
-      var result = expression();
-      scanner.expectDone();
-      return result;
-    });
-  }
+  Expression parseExpression() => _parseSingleProduction(expression);
 
-  VariableDeclaration parseVariableDeclaration() {
+  VariableDeclaration parseVariableDeclaration() =>
+      _parseSingleProduction(() => lookingAtIdentifier()
+          ? _variableDeclarationWithNamespace()
+          : variableDeclarationWithoutNamespace());
+
+  UseRule parseUseRule() => _parseSingleProduction(() {
+        var start = scanner.state;
+        scanner.expectChar($at, name: "@-rule");
+        expectIdentifier("use");
+        whitespace();
+        return _useRule(start);
+      });
+
+  /// Parses and returns [production] as the entire contents of [scanner].
+  T _parseSingleProduction<T>(T production()) {
     return wrapSpanFormatException(() {
-      var declaration = variableDeclaration();
+      var result = production();
       scanner.expectDone();
-      return declaration;
+      return result;
     });
   }
 
@@ -174,27 +169,38 @@ abstract class StylesheetParser extends Parser {
         whitespace();
         return _mixinRule(start);
 
+      case $rbrace:
+        scanner.error('unmatched "}".', length: 1);
+        return null;
+
       default:
-        _isUseAllowed = false;
         return _inStyleRule || _inUnknownAtRule || _inMixin || _inContentBlock
             ? _declarationOrStyleRule()
-            : _styleRule();
+            : _variableDeclarationOrStyleRule();
     }
   }
 
+  /// Consumes a namespaced variable declaration.
+  VariableDeclaration _variableDeclarationWithNamespace() {
+    var start = scanner.state;
+    var namespace = identifier();
+    scanner.expectChar($dot);
+    return variableDeclarationWithoutNamespace(namespace, start);
+  }
+
   /// Consumes a variable declaration.
+  ///
+  /// This never *consumes* a namespace, but if [namespace] is passed it will be
+  /// used for the declaration.
   @protected
-  VariableDeclaration variableDeclaration() {
+  VariableDeclaration variableDeclarationWithoutNamespace(
+      [String namespace, LineScannerState start]) {
     var precedingComment = lastSilentComment;
     lastSilentComment = null;
-    var start = scanner.state;
+    start ??= scanner.state;
 
-    String namespace;
     var name = variableName();
-    if (scanner.scanChar($dot)) {
-      namespace = name;
-      name = _publicIdentifier();
-    }
+    if (namespace != null) _assertPublic(name, () => scanner.spanFrom(start));
 
     if (plainCss) {
       error("Sass variables aren't allowed in plain CSS.",
@@ -239,31 +245,39 @@ abstract class StylesheetParser extends Parser {
     return declaration;
   }
 
-  /// Consumes a style rule.
-  StyleRule _styleRule() {
-    var wasInStyleRule = _inStyleRule;
-    _inStyleRule = true;
+  /// Consumes a namespaced [VariableDeclaration] or a [StyleRule].
+  Statement _variableDeclarationOrStyleRule() {
+    if (plainCss) return _styleRule();
 
     // The indented syntax allows a single backslash to distinguish a style rule
     // from old-style property syntax. We don't support old property syntax, but
     // we do support the backslash because it's easy to do.
-    if (indented) scanner.scanChar($backslash);
+    if (indented && scanner.scanChar($backslash)) return _styleRule();
+
+    if (!lookingAtIdentifier()) return _styleRule();
 
     var start = scanner.state;
-    var selector = styleRuleSelector();
-    var rule = _withChildren(_statement, start,
-        (children, span) => StyleRule(selector, children, span));
-    _inStyleRule = wasInStyleRule;
-    return rule;
+    var variableOrInterpolation = _variableDeclarationOrInterpolation();
+    if (variableOrInterpolation is VariableDeclaration) {
+      return variableOrInterpolation;
+    } else {
+      return _styleRule(
+          InterpolationBuffer()
+            ..addInterpolation(variableOrInterpolation as Interpolation),
+          start);
+    }
   }
 
-  /// Consumes a [Declaration] or a [StyleRule].
+  /// Consumes a [VariableDeclaration], a [Declaration], or a [StyleRule].
+  ///
+  /// When parsing the children of a style rule, property declarations,
+  /// namespaced variable declarations, and nested style rules can all begin
+  /// with bare identifiers. In order to know which statement type to produce,
+  /// we need to disambiguate them. We use the following criteria:
   ///
-  /// When parsing the contents of a style rule, it can be difficult to tell
-  /// declarations apart from nested style rules. Since we don't thoroughly
-  /// parse selectors until after resolving interpolation, we can share a bunch
-  /// of the parsing of the two, but we need to disambiguate them first. We use
-  /// the following criteria:
+  /// * If the entity starts with an identifier followed by a period and a
+  ///   dollar sign, it's a variable declaration. This is the simplest case,
+  ///   because `.$` is used in and only in variable declarations.
   ///
   /// * If the entity doesn't start with an identifier followed by a colon,
   ///   it's a selector. There are some additional mostly-unimportant cases
@@ -284,7 +298,9 @@ abstract class StylesheetParser extends Parser {
   ///   parsed as a selector and never as a property with nested properties
   ///   beneath it.
   Statement _declarationOrStyleRule() {
-    if (plainCss && _inStyleRule && !_inUnknownAtRule) return _declaration();
+    if (plainCss && _inStyleRule && !_inUnknownAtRule) {
+      return _propertyOrVariableDeclaration();
+    }
 
     // The indented syntax allows a single backslash to distinguish a style rule
     // from old-style property syntax. We don't support old property syntax, but
@@ -293,38 +309,18 @@ abstract class StylesheetParser extends Parser {
 
     var start = scanner.state;
     var declarationOrBuffer = _declarationOrBuffer();
-
-    if (declarationOrBuffer is Declaration) return declarationOrBuffer;
-
-    var buffer = declarationOrBuffer as InterpolationBuffer;
-    buffer.addInterpolation(styleRuleSelector());
-    var selectorSpan = scanner.spanFrom(start);
-
-    var wasInStyleRule = _inStyleRule;
-    _inStyleRule = true;
-
-    if (buffer.isEmpty) scanner.error('expected "}".');
-
-    return _withChildren(_statement, start, (children, span) {
-      if (indented && children.isEmpty) {
-        warn("This selector doesn't have any properties and won't be rendered.",
-            selectorSpan);
-      }
-
-      _inStyleRule = wasInStyleRule;
-
-      return StyleRule(buffer.interpolation(selectorSpan), children,
-          scanner.spanFrom(start));
-    });
+    return declarationOrBuffer is Statement
+        ? declarationOrBuffer
+        : _styleRule(declarationOrBuffer as InterpolationBuffer, start);
   }
 
-  /// Tries to parse a declaration, and returns the value parsed so far if it
-  /// fails.
+  /// Tries to parse a variable or property declaration, and returns the value
+  /// parsed so far if it fails.
   ///
   /// This can return either an [InterpolationBuffer], indicating that it
   /// couldn't consume a declaration and that selector parsing should be
-  /// attempted; or it can return a [Declaration], indicating that it
-  /// successfully consumed a declaration.
+  /// attempted; or it can return a [Declaration] or a [VariableDeclaration],
+  /// indicating that it successfully consumed a declaration.
   dynamic _declarationOrBuffer() {
     var start = scanner.state;
     var nameBuffer = InterpolationBuffer();
@@ -332,16 +328,28 @@ abstract class StylesheetParser extends Parser {
     // Allow the "*prop: val", ":prop: val", "#prop: val", and ".prop: val"
     // hacks.
     var first = scanner.peekChar();
+    var startsWithPunctuation = false;
     if (first == $colon ||
         first == $asterisk ||
         first == $dot ||
         (first == $hash && scanner.peekChar(1) != $lbrace)) {
+      startsWithPunctuation = true;
       nameBuffer.writeCharCode(scanner.readChar());
       nameBuffer.write(rawText(whitespace));
     }
 
     if (!_lookingAtInterpolatedIdentifier()) return nameBuffer;
-    nameBuffer.addInterpolation(interpolatedIdentifier());
+
+    var variableOrInterpolation = startsWithPunctuation
+        ? interpolatedIdentifier()
+        : _variableDeclarationOrInterpolation();
+    if (variableOrInterpolation is VariableDeclaration) {
+      return variableOrInterpolation;
+    } else {
+      nameBuffer.addInterpolation(variableOrInterpolation as Interpolation);
+    }
+
+    _isUseAllowed = false;
     if (scanner.matches("/*")) nameBuffer.write(rawText(loudComment));
 
     var midBuffer = StringBuffer();
@@ -425,13 +433,70 @@ abstract class StylesheetParser extends Parser {
     }
   }
 
-  /// Consumes a property declaration.
+  /// Tries to parse a namespaced [VariableDeclaration], and returns the value
+  /// parsed so far if it fails.
+  ///
+  /// This can return either an [Interpolation], indicating that it couldn't
+  /// consume a variable declaration and that property declaration or selector
+  /// parsing should be attempted; or it can return a [VariableDeclaration],
+  /// indicating that it successfully consumed a variable declaration.
+  dynamic _variableDeclarationOrInterpolation() {
+    if (!lookingAtIdentifier()) return interpolatedIdentifier();
+
+    var start = scanner.state;
+    var identifier = this.identifier();
+    if (scanner.matches(".\$")) {
+      scanner.readChar();
+      return variableDeclarationWithoutNamespace(identifier, start);
+    } else {
+      var buffer = InterpolationBuffer()..write(identifier);
+
+      // Parse the rest of an interpolated identifier if one exists, so callers
+      // don't have to.
+      if (_lookingAtInterpolatedIdentifierBody()) {
+        buffer.addInterpolation(interpolatedIdentifier());
+      }
+
+      return buffer.interpolation(scanner.spanFrom(start));
+    }
+  }
+
+  /// Consumes a [StyleRule], optionally with a [buffer] that may contain some
+  /// text that has already been parsed.
+  StyleRule _styleRule([InterpolationBuffer buffer, LineScannerState start]) {
+    _isUseAllowed = false;
+    start ??= scanner.state;
+
+    var interpolation = styleRuleSelector();
+    if (buffer != null) {
+      buffer.addInterpolation(interpolation);
+      interpolation = buffer.interpolation(scanner.spanFrom(start));
+    }
+    if (interpolation.contents.isEmpty) scanner.error('expected "}".');
+
+    var wasInStyleRule = _inStyleRule;
+    _inStyleRule = true;
+
+    return _withChildren(_statement, start, (children, span) {
+      if (indented && children.isEmpty) {
+        warn("This selector doesn't have any properties and won't be rendered.",
+            interpolation.span);
+      }
+
+      _inStyleRule = wasInStyleRule;
+
+      return StyleRule(interpolation, children, scanner.spanFrom(start));
+    });
+  }
+
+  /// Consumes either a property declaration or a namespaced variable
+  /// declaration.
   ///
   /// This is only used in contexts where declarations are allowed but style
   /// rules are not, such as nested declarations. Otherwise,
   /// [_declarationOrStyleRule] is used instead.
   @protected
-  Declaration _declaration() {
+  Statement _propertyOrVariableDeclaration() {
     var start = scanner.state;
 
     Interpolation name;
@@ -447,6 +512,13 @@ abstract class StylesheetParser extends Parser {
       nameBuffer.write(rawText(whitespace));
       nameBuffer.addInterpolation(interpolatedIdentifier());
       name = nameBuffer.interpolation(scanner.spanFrom(start));
+    } else if (!plainCss) {
+      var variableOrInterpolation = _variableDeclarationOrInterpolation();
+      if (variableOrInterpolation is VariableDeclaration) {
+        return variableOrInterpolation;
+      } else {
+        name = variableOrInterpolation as Interpolation;
+      }
     } else {
       name = interpolatedIdentifier();
     }
@@ -482,7 +554,7 @@ abstract class StylesheetParser extends Parser {
   /// Consumes a statement that's allowed within a declaration.
   Statement _declarationChild() {
     if (scanner.peekChar() == $at) return _declarationAtRule();
-    return _declaration();
+    return _propertyOrVariableDeclaration();
   }
 
   // ## At Rules
@@ -600,23 +672,30 @@ abstract class StylesheetParser extends Parser {
     }
   }
 
-  /// Consumes an at-rule allowed within a function.
-  Statement _functionAtRule() {
+  /// Consumes a statement allowed within a function.
+  Statement _functionChild() {
     if (scanner.peekChar() != $at) {
-      var position = scanner.position;
-      Statement statement;
+      var state = scanner.state;
       try {
-        statement = _declarationOrStyleRule();
-      } on SourceSpanFormatException catch (_) {
-        // If we can't parse a valid declaration or style rule, throw a more
-        // generic error message.
-        scanner.error("expected @-rule", position: position);
-      }
+        return _variableDeclarationWithNamespace();
+      } on SourceSpanFormatException catch (variableDeclarationError) {
+        scanner.state = state;
+
+        // If a variable declaration failed to parse, it's possible the user
+        // thought they could write a style rule or property declaration in a
+        // function. If so, throw a more helpful error message.
+        Statement statement;
+        try {
+          statement = _declarationOrStyleRule();
+        } on SourceSpanFormatException catch (_) {
+          throw variableDeclarationError;
+        }
 
-      error(
-          "@function rules may not contain "
-          "${statement is StyleRule ? "style rules" : "declarations"}.",
-          statement.span);
+        error(
+            "@function rules may not contain "
+            "${statement is StyleRule ? "style rules" : "declarations"}.",
+            statement.span);
+      }
     }
 
     var start = scanner.state;
@@ -624,21 +703,21 @@ abstract class StylesheetParser extends Parser {
       case "debug":
         return _debugRule(start);
       case "each":
-        return _eachRule(start, _functionAtRule);
+        return _eachRule(start, _functionChild);
       case "else":
         return _disallowedAtRule(start);
       case "error":
         return _errorRule(start);
       case "for":
-        return _forRule(start, _functionAtRule);
+        return _forRule(start, _functionChild);
       case "if":
-        return _ifRule(start, _functionAtRule);
+        return _ifRule(start, _functionChild);
       case "return":
         return _returnRule(start);
       case "warn":
         return _warnRule(start);
       case "while":
-        return _whileRule(start, _functionAtRule);
+        return _whileRule(start, _functionChild);
       default:
         return _disallowedAtRule(start);
     }
@@ -810,7 +889,7 @@ abstract class StylesheetParser extends Parser {
 
     whitespace();
     return _withChildren(
-        _functionAtRule,
+        _functionChild,
         start,
         (children, span) => FunctionRule(name, arguments, children, span,
             comment: precedingComment));
@@ -864,7 +943,7 @@ abstract class StylesheetParser extends Parser {
     String prefix;
     if (scanIdentifier("as")) {
       whitespace();
-      prefix = identifier();
+      prefix = identifier(normalize: true);
       scanner.expectChar($asterisk);
       whitespace();
     }
@@ -885,12 +964,7 @@ abstract class StylesheetParser extends Parser {
 
     expectStatementSeparator("@forward rule");
     var span = scanner.spanFrom(start);
-    if (!_parseUse) {
-      error(
-          "@forward is coming soon, but it's not supported in this version of "
-          "Dart Sass.",
-          span);
-    } else if (!_isUseAllowed) {
+    if (!_isUseAllowed) {
       error("@forward rules must be written before any other rules.", span);
     }
 
@@ -921,7 +995,7 @@ abstract class StylesheetParser extends Parser {
         if (scanner.peekChar() == $dollar) {
           variables.add(variableName());
         } else {
-          identifiers.add(identifier());
+          identifiers.add(identifier(normalize: true));
         }
       });
       whitespace();
@@ -1274,35 +1348,73 @@ relase. For details, see http://bit.ly/moz-document.
     var url = _urlString();
     whitespace();
 
-    String namespace;
-    if (scanIdentifier("as")) {
-      whitespace();
-      namespace = scanner.scanChar($asterisk) ? null : identifier();
-    } else {
-      var basename = url.pathSegments.isEmpty ? "" : url.pathSegments.last;
-      var dot = basename.indexOf(".");
-      namespace = basename.substring(0, dot == -1 ? basename.length : dot);
+    var namespace = _useNamespace(url, start);
+    whitespace();
+    var configuration = _useConfiguration();
 
-      try {
-        namespace = Parser.parseIdentifier(namespace, logger: logger);
-      } on SassFormatException {
-        error('Invalid Sass identifier "$namespace"', scanner.spanFrom(start));
-      }
-    }
     expectStatementSeparator("@use rule");
 
     var span = scanner.spanFrom(start);
-    if (!_parseUse) {
-      error(
-          "@use is coming soon, but it's not supported in this version of "
-          "Dart Sass.",
-          span);
-    } else if (!_isUseAllowed) {
+    if (!_isUseAllowed) {
       error("@use rules must be written before any other rules.", span);
     }
     expectStatementSeparator("@use rule");
 
-    return UseRule(url, namespace, span);
+    return UseRule(url, namespace, span, configuration: configuration);
+  }
+
+  /// Parses the namespace of a `@use` rule from an `as` clause, or returns the
+  /// default namespace from its URL.
+  String _useNamespace(Uri url, LineScannerState start) {
+    if (scanIdentifier("as")) {
+      whitespace();
+      return scanner.scanChar($asterisk) ? null : identifier();
+    }
+
+    var basename = url.pathSegments.isEmpty ? "" : url.pathSegments.last;
+    var dot = basename.indexOf(".");
+    var namespace = basename.substring(0, dot == -1 ? basename.length : dot);
+    try {
+      return Parser.parseIdentifier(namespace, logger: logger);
+    } on SassFormatException {
+      error('Invalid Sass identifier "$namespace"', scanner.spanFrom(start));
+    }
+  }
+
+  /// Returns the map from variable names to expressions from a `@use` rule's
+  /// `with` clause.
+  ///
+  /// Returns `null` if there is no `with` clause.
+  Map<String, Tuple2<Expression, FileSpan>> _useConfiguration() {
+    if (!scanIdentifier("with")) return null;
+
+    var configuration = <String, Tuple2<Expression, FileSpan>>{};
+    whitespace();
+    scanner.expectChar($lparen);
+
+    while (true) {
+      whitespace();
+
+      var variableStart = scanner.state;
+      var name = variableName();
+      whitespace();
+      scanner.expectChar($colon);
+      whitespace();
+      var expression = _expressionUntilComma();
+      var span = scanner.spanFrom(variableStart);
+
+      if (configuration.containsKey(name)) {
+        error("The same variable may only be configured once.", span);
+      }
+      configuration[name] = Tuple2(expression, span);
+
+      if (!scanner.scanChar($comma)) break;
+      whitespace();
+      if (!_lookingAtExpression()) break;
+    }
+
+    scanner.expectChar($rparen);
+    return configuration;
   }
 
   /// Consumes a `@warn` rule.
@@ -2335,29 +2447,14 @@ relase. For details, see http://bit.ly/moz-document.
   /// Consumes a variable expression.
   VariableExpression _variable() {
     var start = scanner.state;
-
-    // We can't use [variableName] here because it always normalizes the
-    // identifier, but we don't want the variable namespace to end up
-    // normalized.
-    scanner.expectChar($dollar);
-
-    String namespace;
-    var name = identifier();
-    if (scanner.peekChar() == $dot && scanner.peekChar(1) != $dot) {
-      scanner.readChar();
-      namespace = name;
-      name = _publicIdentifier();
-    } else {
-      name = name.replaceAll("_", "-");
-    }
+    var name = variableName();
 
     if (plainCss) {
       error("Sass variables aren't allowed in plain CSS.",
           scanner.spanFrom(start));
     }
 
-    return VariableExpression(name, scanner.spanFrom(start),
-        namespace: namespace);
+    return VariableExpression(name, scanner.spanFrom(start));
   }
 
   /// Consumes a selector expression.
@@ -2467,20 +2564,26 @@ relase. For details, see http://bit.ly/moz-document.
     switch (scanner.peekChar()) {
       case $dot:
         if (scanner.peekChar(1) == $dot) return StringExpression(identifier);
-
-        var namespace = identifier.asPlain;
         scanner.readChar();
-        var beforeName = scanner.state;
-        var name =
-            Interpolation([_publicIdentifier()], scanner.spanFrom(beforeName));
 
-        if (namespace == null) {
+        if (plain == null) {
           error("Interpolation isn't allowed in namespaces.", identifier.span);
         }
 
+        if (scanner.peekChar() == $dollar) {
+          var name = variableName();
+          _assertPublic(name, () => scanner.spanFrom(start));
+          return VariableExpression(name, scanner.spanFrom(start),
+              namespace: plain);
+        }
+
+        var beforeName = scanner.state;
+        var name =
+            Interpolation([_publicIdentifier()], scanner.spanFrom(beforeName));
+
         return FunctionExpression(
             name, _argumentInvocation(), scanner.spanFrom(start),
-            namespace: namespace);
+            namespace: plain);
 
       case $lparen:
         return FunctionExpression(
@@ -3340,16 +3443,19 @@ relase. For details, see http://bit.ly/moz-document.
   String _publicIdentifier() {
     var start = scanner.state;
     var result = identifier(normalize: true);
-
-    var first = result.codeUnitAt(0);
-    if (first == $dash || first == $underscore) {
-      error("Private members can't be accessed from outside their modules.",
-          scanner.spanFrom(start));
-    }
-
+    _assertPublic(result, () => scanner.spanFrom(start));
     return result;
   }
 
+  /// Throws an error if [identifier] isn't public.
+  ///
+  /// Calls [span] to provide the span for an error if one occurs.
+  void _assertPublic(String identifier, FileSpan span()) {
+    if (!isPrivate(identifier)) return;
+    error("Private members can't be accessed from outside their modules.",
+        span());
+  }
+
   // ## Abstract Methods
 
   /// Whether this is parsing the indented syntax.
diff --git a/lib/src/util/character.dart b/lib/src/util/character.dart
index cdc1c57e0..13d2e937d 100644
--- a/lib/src/util/character.dart
+++ b/lib/src/util/character.dart
@@ -65,6 +65,14 @@ bool isSimpleSelectorStart(int character) =>
     character == $percent ||
     character == $colon;
 
+/// Returns whether [identifier] is module-private.
+///
+/// Assumes [identifier] is a valid Sass identifier.
+bool isPrivate(String identifier) {
+  var first = identifier.codeUnitAt(0);
+  return first == $dash || first == $underscore;
+}
+
 /// Returns the value of [character] as a hex digit.
 ///
 /// Assumes that [character] is a hex digit.
diff --git a/lib/src/util/limited_map_view.dart b/lib/src/util/limited_map_view.dart
index 1253f77f4..f7c370f35 100644
--- a/lib/src/util/limited_map_view.dart
+++ b/lib/src/util/limited_map_view.dart
@@ -6,13 +6,17 @@ import 'dart:collection';
 
 import 'package:collection/collection.dart';
 
-/// An unmodifiable view of a map that only allows certain keys to be accessed.
+/// A mostly-unmodifiable view of a map that only allows certain keys to be
+/// accessed.
 ///
 /// Whether or not the underlying map contains keys that aren't allowed, this
 /// view will behave as though it doesn't contain them.
 ///
 /// The underlying map's values may change independently of this view, but its
 /// set of keys may not.
+///
+/// This is unmodifiable *except for the [remove] method*, which is used for
+/// `@used with` to mark configured variables as used.
 class LimitedMapView<K, V> extends UnmodifiableMapBase<K, V> {
   /// The wrapped map.
   final Map<K, V> _map;
@@ -39,4 +43,6 @@ class LimitedMapView<K, V> extends UnmodifiableMapBase<K, V> {
 
   V operator [](Object key) => _keys.contains(key) ? _map[key] : null;
   bool containsKey(Object key) => _keys.contains(key);
+
+  V remove(Object key) => _keys.contains(key) ? _map.remove(key) : null;
 }
diff --git a/lib/src/util/prefixed_map_view.dart b/lib/src/util/prefixed_map_view.dart
index a50b600a7..29c0b3565 100644
--- a/lib/src/util/prefixed_map_view.dart
+++ b/lib/src/util/prefixed_map_view.dart
@@ -21,17 +21,13 @@ class PrefixedMapView<V> extends UnmodifiableMapBase<String, V> {
   /// Creates a new prefixed map view.
   PrefixedMapView(this._map, this._prefix);
 
-  V operator [](Object key) => key is String && _startsWith(key, _prefix)
+  V operator [](Object key) => key is String && key.startsWith(_prefix)
       ? _map[key.substring(_prefix.length)]
       : null;
 
-  bool containsKey(Object key) => key is String && _startsWith(key, _prefix)
+  bool containsKey(Object key) => key is String && key.startsWith(_prefix)
       ? _map.containsKey(key.substring(_prefix.length))
       : false;
-
-  /// Returns whether [string] begins with [prefix].
-  bool _startsWith(String string, String prefix) =>
-      string.length >= prefix.length && string.startsWith(prefix);
 }
 
 /// The implementation of [PrefixedMapViews.keys].
diff --git a/lib/src/util/unprefixed_map_view.dart b/lib/src/util/unprefixed_map_view.dart
new file mode 100644
index 000000000..e11de4532
--- /dev/null
+++ b/lib/src/util/unprefixed_map_view.dart
@@ -0,0 +1,49 @@
+// Copyright 2019 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 'dart:collection';
+
+/// A mostly-unmodifiable view of a map with string keys that only allows keys
+/// with a given prefix to be accessed, and presents them as though they didn't
+/// have that prefix.
+///
+/// Whether or not the underlying map contains keys without the given prefix,
+/// this view will behave as though it doesn't contain them.
+///
+/// This is unmodifiable *except for the [remove] method*, which is used for
+/// `@used with` to mark configured variables as used.
+class UnprefixedMapView<V> extends UnmodifiableMapBase<String, V> {
+  /// The wrapped map.
+  final Map<String, V> _map;
+
+  /// The prefix to remove from the map keys.
+  final String _prefix;
+
+  Iterable<String> get keys => _UnprefixedKeys(this);
+
+  /// Creates a new unprefixed map view.
+  UnprefixedMapView(this._map, this._prefix);
+
+  V operator [](Object key) => key is String ? _map[_prefix + key] : null;
+
+  bool containsKey(Object key) =>
+      key is String ? _map.containsKey(_prefix + key) : false;
+
+  V remove(Object key) => key is String ? _map.remove(_prefix + key) : null;
+}
+
+/// The implementation of [UnprefixedMapViews.keys].
+class _UnprefixedKeys extends IterableBase<String> {
+  /// The view whose keys are being iterated over.
+  final UnprefixedMapView<Object> _view;
+
+  Iterator<String> get iterator => _view._map.keys
+      .where((key) => key.startsWith(_view._prefix))
+      .map((key) => key.substring(_view._prefix.length))
+      .iterator;
+
+  _UnprefixedKeys(this._view);
+
+  bool contains(Object key) => _view.containsKey(key);
+}
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 8cd061491..dbcf12d94 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -9,6 +9,7 @@ import 'package:charcode/charcode.dart';
 import 'package:collection/collection.dart';
 import 'package:source_span/source_span.dart';
 import 'package:stack_trace/stack_trace.dart';
+import 'package:term_glyph/term_glyph.dart' as glyph;
 
 import 'ast/node.dart';
 import 'util/character.dart';
@@ -37,6 +38,15 @@ String pluralize(String name, int number, {String plural}) {
   return '${name}s';
 }
 
+/// Returns a bulleted list of items in [bullets].
+String bulletedList(Iterable<String> bullets) {
+  return bullets.map((element) {
+    var lines = element.split("\n");
+    return "${glyph.bullet} ${lines.first}" +
+        (lines.length > 1 ? "\n" + indent(lines.skip(1).join("\n"), 2) : "");
+  }).join("\n");
+}
+
 /// Returns the number of times [codeUnit] appears in [string].
 int countOccurrences(String string, int codeUnit) {
   var count = 0;
@@ -258,25 +268,6 @@ bool startsWithIgnoreCase(String string, String prefix) {
   return true;
 }
 
-/// Returns whether [string] begins with [prefix] if `-` and `_` are
-/// considered equivalent.
-bool startsWithIgnoreSeparator(String string, String prefix) {
-  if (string.length < prefix.length) return false;
-  for (var i = 0; i < prefix.length; i++) {
-    var stringCodeUnit = string.codeUnitAt(i);
-    var prefixCodeUnit = prefix.codeUnitAt(i);
-    if (stringCodeUnit == prefixCodeUnit) continue;
-    if (stringCodeUnit == $dash) {
-      if (prefixCodeUnit != $underscore) return false;
-    } else if (stringCodeUnit == $underscore) {
-      if (prefixCodeUnit != $dash) return false;
-    } else {
-      return false;
-    }
-  }
-  return true;
-}
-
 /// Destructively updates every element of [list] with the result of [function].
 void mapInPlace<T>(List<T> list, T function(T element)) {
   for (var i = 0; i < list.length; i++) {
diff --git a/lib/src/visitor/async_evaluate.dart b/lib/src/visitor/async_evaluate.dart
index 2f8ba210e..79ca86a07 100644
--- a/lib/src/visitor/async_evaluate.dart
+++ b/lib/src/visitor/async_evaluate.dart
@@ -37,6 +37,8 @@ import '../module/built_in.dart';
 import '../parse/keyframe_selector.dart';
 import '../syntax.dart';
 import '../util/fixed_length_list_builder.dart';
+import '../util/limited_map_view.dart';
+import '../util/unprefixed_map_view.dart';
 import '../utils.dart';
 import '../value.dart';
 import '../warn.dart';
@@ -85,24 +87,35 @@ Future<EvaluateResult> evaluateAsync(Stylesheet stylesheet,
             sourceMap: sourceMap)
         .run(importer, stylesheet);
 
-/// Evaluates a single [expression]
-///
-/// The [functions] are available as global functions when evaluating
-/// [expression].
-///
-/// The [variables] are available as global variables when evaluating
-/// [expression].
-///
-/// Warnings are emitted using [logger], or printed to standard error by
-/// default.
-///
-/// Throws a [SassRuntimeException] if evaluation fails.
-Future<Value> evaluateExpressionAsync(Expression expression,
-        {Iterable<AsyncCallable> functions,
-        Map<String, Value> variables,
-        Logger logger}) =>
-    _EvaluateVisitor(functions: functions, logger: logger, sourceMap: false)
-        .runExpression(expression, variables: variables);
+/// A class that can evaluate multiple independent statements and expressions
+/// in the context of a single module.
+class AsyncEvaluator {
+  /// The visitor that evaluates each expression and statement.
+  final _EvaluateVisitor _visitor;
+
+  /// The importer to use to resolve `@use` rules in [_importer].
+  final AsyncImporter _importer;
+
+  /// Creates an evaluator.
+  ///
+  /// Arguments are the same as for [evaluateAsync].
+  AsyncEvaluator(
+      {AsyncImportCache importCache,
+      AsyncImporter importer,
+      Iterable<AsyncCallable> functions,
+      Logger logger})
+      : _visitor = _EvaluateVisitor(
+            importCache: importCache, functions: functions, logger: logger),
+        _importer = importer;
+
+  Future<void> use(UseRule use) => _visitor.runStatement(_importer, use);
+
+  Future<Value> evaluate(Expression expression) =>
+      _visitor.runExpression(_importer, expression);
+
+  Future<void> setVariable(VariableDeclaration declaration) =>
+      _visitor.runStatement(_importer, declaration);
+}
 
 /// A visitor that executes Sass code to produce a CSS tree.
 class _EvaluateVisitor
@@ -238,18 +251,32 @@ class _EvaluateVisitor
   /// module.
   Extender _extender;
 
+  /// A map from variable names to the values that override their `!default`
+  /// definitions in this module.
+  ///
+  /// If this is empty, that indicates that the current module is not confiured.
+  /// Note that it may be unmodifiable when empty, in which case [Map.remove]
+  /// must not be called.
+  var _configuration = const <String, _ConfiguredValue>{};
+
+  /// Creates a new visitor.
+  ///
+  /// Most arguments are the same as those to [evaluateAsync].
   _EvaluateVisitor(
       {AsyncImportCache importCache,
       NodeImporter nodeImporter,
       Iterable<AsyncCallable> functions,
       Logger logger,
-      bool sourceMap})
+      bool sourceMap = false})
       : _importCache = nodeImporter == null
             ? importCache ?? AsyncImportCache.none(logger: logger)
             : null,
         _nodeImporter = nodeImporter,
         _logger = logger ?? const Logger.stderr(),
-        _sourceMap = sourceMap {
+        _sourceMap = sourceMap,
+        // The default environment is overridden in [_execute] for full
+        // stylesheets, but for [AsyncEvaluator] this environment is used.
+        _environment = AsyncEnvironment(sourceMap: sourceMap) {
     var metaFunctions = [
       // These functions are defined in the context of the evaluator because
       // they need access to the [_environment] or other local state.
@@ -380,7 +407,39 @@ class _EvaluateVisitor
       })
     ];
 
-    var metaModule = BuiltInModule("meta", [...meta.global, ...metaFunctions]);
+    var metaMixins = [
+      AsyncBuiltInCallable("load-css", r"$module, $with: null",
+          (arguments) async {
+        var url = Uri.parse(arguments[0].assertString("module").text);
+        var withMap = arguments[1].realNull?.assertMap("with")?.contents;
+
+        Map<String, _ConfiguredValue> configuration;
+        if (withMap != null) {
+          configuration = <String, _ConfiguredValue>{};
+          var span = _callableNode.span;
+          withMap.forEach((variable, value) {
+            var name =
+                variable.assertString("with key").text.replaceAll("_", "-");
+            if (configuration.containsKey(name)) {
+              throw "The variable \$$name was configured twice.";
+            }
+
+            configuration[name] = _ConfiguredValue(value, span);
+          });
+        }
+
+        await _loadModule(url, "load-css()", _callableNode,
+            (module) => _combineCss(module, clone: true).accept(this),
+            baseUrl: _callableNode.span?.sourceUrl,
+            configuration: configuration,
+            namesInErrors: true);
+        return null;
+      })
+    ];
+
+    var metaModule = BuiltInModule("meta",
+        functions: [...meta.global, ...metaFunctions], mixins: metaMixins);
+
     for (var module in [...coreModules, metaModule]) {
       _builtInModules[module.url] = module;
     }
@@ -411,18 +470,13 @@ class _EvaluateVisitor
     });
   }
 
-  Future<Value> runExpression(Expression expression,
-      {Map<String, Value> variables}) {
-    return _withWarnCallback(() async {
-      _environment = AsyncEnvironment(sourceMap: _sourceMap);
-
-      for (var name in variables?.keys ?? const <String>[]) {
-        _environment.setVariable(name, variables[name], null, global: true);
-      }
+  Future<Value> runExpression(AsyncImporter importer, Expression expression) =>
+      _withWarnCallback(() => _withFakeStylesheet(
+          importer, expression, () => expression.accept(this)));
 
-      return expression.accept(this);
-    });
-  }
+  Future<void> runStatement(AsyncImporter importer, Statement statement) =>
+      _withWarnCallback(() => _withFakeStylesheet(
+          importer, statement, () => statement.accept(this)));
 
   /// Runs [callback] with a definition for the top-level `warn` function.
   T _withWarnCallback<T>(T callback()) {
@@ -433,39 +487,89 @@ class _EvaluateVisitor
         callback);
   }
 
+  /// Runs [callback] with [importer] as [_importer] and a fake [_stylesheet]
+  /// with [nodeForSpan]'s source span.
+  Future<T> _withFakeStylesheet<T>(AsyncImporter importer, AstNode nodeForSpan,
+      FutureOr<T> callback()) async {
+    var oldImporter = _importer;
+    _importer = importer;
+    var oldStylesheet = _stylesheet;
+    _stylesheet = Stylesheet(const [], nodeForSpan.span);
+
+    try {
+      return await callback();
+    } finally {
+      _importer = oldImporter;
+      _stylesheet = oldStylesheet;
+    }
+  }
+
   /// Loads the module at [url] and passes it to [callback].
   ///
+  /// This first tries loading [url] relative to [baseUrl], which defaults to
+  /// `_stylesheet.span.sourceUrl`.
+  ///
+  /// The [configuration] overrides values for `!default` variables defined in
+  /// the module or modules it forwards and/or imports. If it's not passed, the
+  /// current configuration is used instead. Throws a [SassRuntimeException] if
+  /// a configured variable is not declared with `!default`.
+  ///
+  /// If [namesInErrors] is `true`, this includes the names of modules or
+  /// configured variables in errors relating to them. This should only be
+  /// `true` if the names won't be obvious from the source span.
+  ///
   /// The [stackFrame] and [nodeForSpan] are used for the name and location of
   /// the stack frame for the duration of the [callback].
   Future<void> _loadModule(Uri url, String stackFrame, AstNode nodeForSpan,
-      void callback(Module module)) async {
+      void callback(Module module),
+      {Uri baseUrl,
+      Map<String, _ConfiguredValue> configuration,
+      bool namesInErrors = false}) async {
+    configuration ??= const {};
+
     var builtInModule = _builtInModules[url];
     if (builtInModule != null) {
+      if (configuration.isNotEmpty || _configuration.isNotEmpty) {
+        throw _exception(
+            namesInErrors
+                ? "Built-in module $url can't be configured."
+                : "Built-in modules can't be configured.",
+            nodeForSpan.span);
+      }
+
       callback(builtInModule);
       return;
     }
 
     await _withStackFrame(stackFrame, nodeForSpan, () async {
-      var result = await inUseRuleAsync(
-          () => _loadStylesheet(url.toString(), nodeForSpan.span));
+      var result = await inUseRuleAsync(() =>
+          _loadStylesheet(url.toString(), nodeForSpan.span, baseUrl: baseUrl));
       var importer = result.item1;
       var stylesheet = result.item2;
 
       var canonicalUrl = stylesheet.span.sourceUrl;
       if (_activeModules.contains(canonicalUrl)) {
-        throw _exception("Module loop: this module is already being loaded.");
+        throw _exception(namesInErrors
+            ? "Module loop: ${p.prettyUri(canonicalUrl)} is already being "
+                "loaded."
+            : "Module loop: this module is already being loaded.");
       }
       _activeModules.add(canonicalUrl);
 
       Module module;
       try {
-        module = await _execute(importer, stylesheet);
+        module = await _execute(importer, stylesheet,
+            configuration: configuration, namesInErrors: namesInErrors);
       } finally {
         _activeModules.remove(canonicalUrl);
       }
 
       try {
         await callback(module);
+      } on SassRuntimeException {
+        rethrow;
+      } on SassException catch (error) {
+        throw _exception(error.message, error.span);
       } on SassScriptException catch (error) {
         throw _exception(error.message);
       }
@@ -473,62 +577,102 @@ class _EvaluateVisitor
   }
 
   /// Executes [stylesheet], loaded by [importer], to produce a module.
-  Future<Module> _execute(AsyncImporter importer, Stylesheet stylesheet) {
+  ///
+  /// The [configuration] overrides values for `!default` variables defined in
+  /// the module or modules it forwards and/or imports. If it's not passed, the
+  /// current configuration is used instead. Throws a [SassRuntimeException] if
+  /// a configured variable is not declared with `!default`.
+  ///
+  /// If [namesInErrors] is `true`, this includes the names of modules or
+  /// configured variables in errors relating to them. This should only be
+  /// `true` if the names won't be obvious from the source span.
+  Future<Module> _execute(AsyncImporter importer, Stylesheet stylesheet,
+      {Map<String, _ConfiguredValue> configuration,
+      bool namesInErrors = false}) async {
+    configuration ??= const {};
     var url = stylesheet.span.sourceUrl;
-    return putIfAbsentAsync(_modules, url, () async {
-      var environment = AsyncEnvironment(sourceMap: _sourceMap);
-      CssStylesheet css;
-      var extender = Extender();
-      await _withEnvironment(environment, () async {
-        var oldImporter = _importer;
-        var oldStylesheet = _stylesheet;
-        var oldRoot = _root;
-        var oldParent = _parent;
-        var oldEndOfImports = _endOfImports;
-        var oldOutOfOrderImports = _outOfOrderImports;
-        var oldExtender = _extender;
-        var oldStyleRule = _styleRule;
-        var oldMediaQueries = _mediaQueries;
-        var oldDeclarationName = _declarationName;
-        var oldInUnknownAtRule = _inUnknownAtRule;
-        var oldAtRootExcludingStyleRule = _atRootExcludingStyleRule;
-        var oldInKeyframes = _inKeyframes;
-        _importer = importer;
-        _stylesheet = stylesheet;
-        _root = ModifiableCssStylesheet(stylesheet.span);
-        _parent = _root;
-        _endOfImports = 0;
-        _outOfOrderImports = null;
-        _extender = extender;
-        _styleRule = null;
-        _mediaQueries = null;
-        _declarationName = null;
-        _inUnknownAtRule = false;
-        _atRootExcludingStyleRule = false;
-        _inKeyframes = false;
 
-        await visitStylesheet(stylesheet);
-        css = _outOfOrderImports == null
-            ? _root
-            : CssStylesheet(_addOutOfOrderImports(), stylesheet.span);
+    var alreadyLoaded = _modules[url];
+    if (alreadyLoaded != null) {
+      if (configuration.isNotEmpty || _configuration.isNotEmpty) {
+        throw _exception(namesInErrors
+            ? "${p.prettyUri(url)} was already loaded, so it can't be "
+                "configured using \"with\"."
+            : "This module was already loaded, so it can't be configured using "
+                "\"with\".");
+      }
 
-        _importer = oldImporter;
-        _stylesheet = oldStylesheet;
-        _root = oldRoot;
-        _parent = oldParent;
-        _endOfImports = oldEndOfImports;
-        _outOfOrderImports = oldOutOfOrderImports;
-        _extender = oldExtender;
-        _styleRule = oldStyleRule;
-        _mediaQueries = oldMediaQueries;
-        _declarationName = oldDeclarationName;
-        _inUnknownAtRule = oldInUnknownAtRule;
-        _atRootExcludingStyleRule = oldAtRootExcludingStyleRule;
-        _inKeyframes = oldInKeyframes;
-      });
+      return alreadyLoaded;
+    }
+
+    var environment = AsyncEnvironment(sourceMap: _sourceMap);
+    CssStylesheet css;
+    var extender = Extender();
+    await _withEnvironment(environment, () async {
+      var oldImporter = _importer;
+      var oldStylesheet = _stylesheet;
+      var oldRoot = _root;
+      var oldParent = _parent;
+      var oldEndOfImports = _endOfImports;
+      var oldOutOfOrderImports = _outOfOrderImports;
+      var oldExtender = _extender;
+      var oldStyleRule = _styleRule;
+      var oldMediaQueries = _mediaQueries;
+      var oldDeclarationName = _declarationName;
+      var oldInUnknownAtRule = _inUnknownAtRule;
+      var oldAtRootExcludingStyleRule = _atRootExcludingStyleRule;
+      var oldInKeyframes = _inKeyframes;
+      var oldConfiguration = _configuration;
+      _importer = importer;
+      _stylesheet = stylesheet;
+      _root = ModifiableCssStylesheet(stylesheet.span);
+      _parent = _root;
+      _endOfImports = 0;
+      _outOfOrderImports = null;
+      _extender = extender;
+      _styleRule = null;
+      _mediaQueries = null;
+      _declarationName = null;
+      _inUnknownAtRule = false;
+      _atRootExcludingStyleRule = false;
+      _inKeyframes = false;
+
+      if (configuration.isNotEmpty) _configuration = Map.of(configuration);
+
+      await visitStylesheet(stylesheet);
+      css = _outOfOrderImports == null
+          ? _root
+          : CssStylesheet(_addOutOfOrderImports(), stylesheet.span);
+
+      _importer = oldImporter;
+      _stylesheet = oldStylesheet;
+      _root = oldRoot;
+      _parent = oldParent;
+      _endOfImports = oldEndOfImports;
+      _outOfOrderImports = oldOutOfOrderImports;
+      _extender = oldExtender;
+      _styleRule = oldStyleRule;
+      _mediaQueries = oldMediaQueries;
+      _declarationName = oldDeclarationName;
+      _inUnknownAtRule = oldInUnknownAtRule;
+      _atRootExcludingStyleRule = oldAtRootExcludingStyleRule;
+      _inKeyframes = oldInKeyframes;
 
-      return environment.toModule(css, extender);
+      if (configuration.isNotEmpty && _configuration.isNotEmpty) {
+        throw _exception(
+            namesInErrors
+                ? "\$${_configuration.keys.first} was not declared with "
+                    "!default in the @used module."
+                : "This variable was not declared with !default in the @used "
+                    "module.",
+            _configuration.values.first.configurationSpan);
+      }
+      _configuration = oldConfiguration;
     });
+
+    var module = environment.toModule(css, extender);
+    _modules[url] = module;
+    return module;
   }
 
   /// Returns a copy of [_root.children] with [_outOfOrderImports] inserted
@@ -1059,10 +1203,31 @@ class _EvaluateVisitor
   }
 
   Future<Value> visitForwardRule(ForwardRule node) async {
+    // Only allow variables that are visible through the `@forward` to be
+    // configured. These views support [Map.remove] so we can mark when a
+    // configuration variable is used by removing it even when the underlying
+    // map is wrapped.
+    var oldConfiguration = _configuration;
+    if (_configuration.isNotEmpty) {
+      if (node.prefix != null) {
+        _configuration = UnprefixedMapView(_configuration, node.prefix);
+      }
+
+      if (node.shownVariables != null) {
+        _configuration =
+            LimitedMapView.whitelist(_configuration, node.shownVariables);
+      } else if (node.hiddenVariables != null &&
+          node.hiddenVariables.isNotEmpty) {
+        _configuration =
+            LimitedMapView.blacklist(_configuration, node.hiddenVariables);
+      }
+    }
+
     await _loadModule(node.url, "@forward", node, (module) {
       _environment.forwardModule(module, node);
     });
 
+    _configuration = oldConfiguration;
     return null;
   }
 
@@ -1186,8 +1351,12 @@ class _EvaluateVisitor
 
   /// Loads the [Stylesheet] identified by [url], or throws a
   /// [SassRuntimeException] if loading fails.
+  ///
+  /// This first tries loading [url] relative to [baseUrl], which defaults to
+  /// `_stylesheet.span.sourceUrl`.
   Future<Tuple2<AsyncImporter, Stylesheet>> _loadStylesheet(
-      String url, FileSpan span) async {
+      String url, FileSpan span,
+      {Uri baseUrl}) async {
     try {
       assert(_importSpan == null);
       _importSpan = span;
@@ -1197,7 +1366,7 @@ class _EvaluateVisitor
         if (stylesheet != null) return Tuple2(null, stylesheet);
       } else {
         var tuple = await _importCache.import(
-            Uri.parse(url), _importer, _stylesheet.span?.sourceUrl);
+            Uri.parse(url), _importer, baseUrl ?? _stylesheet?.span?.sourceUrl);
         if (tuple != null) return tuple;
       }
 
@@ -1275,30 +1444,41 @@ class _EvaluateVisitor
 
   Future<Value> visitIncludeRule(IncludeRule node) async {
     var mixin = _addExceptionSpan(node,
-            () => _environment.getMixin(node.name, namespace: node.namespace))
-        as UserDefinedCallable<AsyncEnvironment>;
+        () => _environment.getMixin(node.name, namespace: node.namespace));
     if (mixin == null) {
       throw _exception("Undefined mixin.", node.span);
     }
 
-    if (node.content != null && !(mixin.declaration as MixinRule).hasContent) {
-      throw _exception("Mixin doesn't accept a content block.", node.span);
-    }
+    if (mixin is AsyncBuiltInCallable) {
+      if (node.content != null) {
+        throw _exception("Mixin doesn't accept a content block.", node.span);
+      }
 
-    var contentCallable = node.content == null
-        ? null
-        : UserDefinedCallable(node.content, _environment.closure());
-    await _runUserDefinedCallable(node.arguments, mixin, node, () async {
-      await _environment.withContent(contentCallable, () async {
-        await _environment.asMixin(() async {
-          for (var statement in mixin.declaration.children) {
-            await statement.accept(this);
-          }
+      await _runBuiltInCallable(node.arguments, mixin, node);
+    } else if (mixin is UserDefinedCallable<AsyncEnvironment>) {
+      if (node.content != null &&
+          !(mixin.declaration as MixinRule).hasContent) {
+        throw _exception("Mixin doesn't accept a content block.", node.span);
+      }
+
+      var contentCallable = node.content == null
+          ? null
+          : UserDefinedCallable(node.content, _environment.closure());
+
+      await _runUserDefinedCallable(node.arguments, mixin, node, () async {
+        await _environment.withContent(contentCallable, () async {
+          await _environment.asMixin(() async {
+            for (var statement in mixin.declaration.children) {
+              await statement.accept(this);
+            }
+          });
+          return null;
         });
         return null;
       });
-      return null;
-    });
+    } else {
+      throw UnsupportedError("Unknown callable type $mixin.");
+    }
 
     return null;
   }
@@ -1546,6 +1726,23 @@ class _EvaluateVisitor
 
   Future<Value> visitVariableDeclaration(VariableDeclaration node) async {
     if (node.isGuarded) {
+      if (node.namespace == null && _environment.atRoot) {
+        // Explicitly check whether [_configuration] is empty because if it is,
+        // it may be a constant map which doesn't support `remove()`.
+        //
+        // See also dart-lang/sdk#38540.
+        var override =
+            _configuration.isEmpty ? null : _configuration.remove(node.name);
+        if (override != null) {
+          _addExceptionSpan(node, () {
+            _environment.setVariable(
+                node.name, override.value, override.assignmentNode,
+                global: true);
+          });
+          return null;
+        }
+      }
+
       var value = _addExceptionSpan(node,
           () => _environment.getVariable(node.name, namespace: node.namespace));
       if (value != null && value != sassNull) return null;
@@ -1579,7 +1776,16 @@ class _EvaluateVisitor
   Future<Value> visitUseRule(UseRule node) async {
     await _loadModule(node.url, "@use", node, (module) {
       _environment.addModule(module, namespace: node.namespace);
-    });
+    },
+        configuration: node.configuration.isEmpty
+            ? null
+            : {
+                for (var entry in node.configuration.entries)
+                  entry.key: _ConfiguredValue(
+                      (await entry.value.item1.accept(this)).withoutSlash(),
+                      entry.value.item2,
+                      _expressionNode(entry.value.item1))
+              });
 
     return null;
   }
@@ -1868,8 +2074,13 @@ class _EvaluateVisitor
   Future<Value> _runFunctionCallable(ArgumentInvocation arguments,
       AsyncCallable callable, AstNode nodeWithSpan) async {
     if (callable is AsyncBuiltInCallable) {
-      return (await _runBuiltInCallable(arguments, callable, nodeWithSpan))
-          .withoutSlash();
+      var result = await _runBuiltInCallable(arguments, callable, nodeWithSpan);
+      if (result == null) {
+        throw _exception(
+            "Custom functions may not return Dart's null.", nodeWithSpan.span);
+      }
+
+      return result.withoutSlash();
     } else if (callable is UserDefinedCallable<AsyncEnvironment>) {
       return (await _runUserDefinedCallable(arguments, callable, nodeWithSpan,
               () async {
@@ -1959,7 +2170,8 @@ class _EvaluateVisitor
     Value result;
     try {
       result = await callback(evaluated.positional);
-      if (result == null) throw "Custom functions may not return Dart's null.";
+    } on SassRuntimeException {
+      rethrow;
     } catch (error) {
       String message;
       try {
@@ -2754,3 +2966,19 @@ class _ArgumentResults {
   _ArgumentResults(this.positional, this.named, this.separator,
       {this.positionalNodes, this.namedNodes});
 }
+
+/// A variable value that's been configured using `@use ... with`.
+class _ConfiguredValue {
+  /// The value of the variable.
+  final Value value;
+
+  /// The span where the variable's configuration was written.
+  final FileSpan configurationSpan;
+
+  /// The [AstNode] where the variable's value originated.
+  ///
+  /// This is used to generate source maps.
+  final AstNode assignmentNode;
+
+  _ConfiguredValue(this.value, this.configurationSpan, [this.assignmentNode]);
+}
diff --git a/lib/src/visitor/evaluate.dart b/lib/src/visitor/evaluate.dart
index 5896b779a..ebd2d7dc6 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/grind/synchronize.dart for details.
 //
-// Checksum: d79b881dbbe875fafde129b908aab5e7ff532a87
+// Checksum: d520e2c69342b6e9c9ce0ddf6e75ac74bdf80574
 //
 // ignore_for_file: unused_import
 
@@ -46,6 +46,8 @@ import '../module/built_in.dart';
 import '../parse/keyframe_selector.dart';
 import '../syntax.dart';
 import '../util/fixed_length_list_builder.dart';
+import '../util/limited_map_view.dart';
+import '../util/unprefixed_map_view.dart';
 import '../utils.dart';
 import '../value.dart';
 import '../warn.dart';
@@ -93,24 +95,35 @@ EvaluateResult evaluate(Stylesheet stylesheet,
             sourceMap: sourceMap)
         .run(importer, stylesheet);
 
-/// Evaluates a single [expression]
-///
-/// The [functions] are available as global functions when evaluating
-/// [expression].
-///
-/// The [variables] are available as global variables when evaluating
-/// [expression].
-///
-/// Warnings are emitted using [logger], or printed to standard error by
-/// default.
-///
-/// Throws a [SassRuntimeException] if evaluation fails.
-Value evaluateExpression(Expression expression,
-        {Iterable<Callable> functions,
-        Map<String, Value> variables,
-        Logger logger}) =>
-    _EvaluateVisitor(functions: functions, logger: logger, sourceMap: false)
-        .runExpression(expression, variables: variables);
+/// A class that can evaluate multiple independent statements and expressions
+/// in the context of a single module.
+class Evaluator {
+  /// The visitor that evaluates each expression and statement.
+  final _EvaluateVisitor _visitor;
+
+  /// The importer to use to resolve `@use` rules in [_importer].
+  final Importer _importer;
+
+  /// Creates an evaluator.
+  ///
+  /// Arguments are the same as for [evaluate].
+  Evaluator(
+      {ImportCache importCache,
+      Importer importer,
+      Iterable<Callable> functions,
+      Logger logger})
+      : _visitor = _EvaluateVisitor(
+            importCache: importCache, functions: functions, logger: logger),
+        _importer = importer;
+
+  void use(UseRule use) => _visitor.runStatement(_importer, use);
+
+  Value evaluate(Expression expression) =>
+      _visitor.runExpression(_importer, expression);
+
+  void setVariable(VariableDeclaration declaration) =>
+      _visitor.runStatement(_importer, declaration);
+}
 
 /// A visitor that executes Sass code to produce a CSS tree.
 class _EvaluateVisitor
@@ -246,18 +259,32 @@ class _EvaluateVisitor
   /// module.
   Extender _extender;
 
+  /// A map from variable names to the values that override their `!default`
+  /// definitions in this module.
+  ///
+  /// If this is empty, that indicates that the current module is not confiured.
+  /// Note that it may be unmodifiable when empty, in which case [Map.remove]
+  /// must not be called.
+  var _configuration = const <String, _ConfiguredValue>{};
+
+  /// Creates a new visitor.
+  ///
+  /// Most arguments are the same as those to [evaluate].
   _EvaluateVisitor(
       {ImportCache importCache,
       NodeImporter nodeImporter,
       Iterable<Callable> functions,
       Logger logger,
-      bool sourceMap})
+      bool sourceMap = false})
       : _importCache = nodeImporter == null
             ? importCache ?? ImportCache.none(logger: logger)
             : null,
         _nodeImporter = nodeImporter,
         _logger = logger ?? const Logger.stderr(),
-        _sourceMap = sourceMap {
+        _sourceMap = sourceMap,
+        // The default environment is overridden in [_execute] for full
+        // stylesheets, but for [AsyncEvaluator] this environment is used.
+        _environment = Environment(sourceMap: sourceMap) {
     var metaFunctions = [
       // These functions are defined in the context of the evaluator because
       // they need access to the [_environment] or other local state.
@@ -387,7 +414,38 @@ class _EvaluateVisitor
       })
     ];
 
-    var metaModule = BuiltInModule("meta", [...meta.global, ...metaFunctions]);
+    var metaMixins = [
+      BuiltInCallable("load-css", r"$module, $with: null", (arguments) {
+        var url = Uri.parse(arguments[0].assertString("module").text);
+        var withMap = arguments[1].realNull?.assertMap("with")?.contents;
+
+        Map<String, _ConfiguredValue> configuration;
+        if (withMap != null) {
+          configuration = <String, _ConfiguredValue>{};
+          var span = _callableNode.span;
+          withMap.forEach((variable, value) {
+            var name =
+                variable.assertString("with key").text.replaceAll("_", "-");
+            if (configuration.containsKey(name)) {
+              throw "The variable \$$name was configured twice.";
+            }
+
+            configuration[name] = _ConfiguredValue(value, span);
+          });
+        }
+
+        _loadModule(url, "load-css()", _callableNode,
+            (module) => _combineCss(module, clone: true).accept(this),
+            baseUrl: _callableNode.span?.sourceUrl,
+            configuration: configuration,
+            namesInErrors: true);
+        return null;
+      })
+    ];
+
+    var metaModule = BuiltInModule("meta",
+        functions: [...meta.global, ...metaFunctions], mixins: metaMixins);
+
     for (var module in [...coreModules, metaModule]) {
       _builtInModules[module.url] = module;
     }
@@ -418,17 +476,13 @@ class _EvaluateVisitor
     });
   }
 
-  Value runExpression(Expression expression, {Map<String, Value> variables}) {
-    return _withWarnCallback(() {
-      _environment = Environment(sourceMap: _sourceMap);
-
-      for (var name in variables?.keys ?? const <String>[]) {
-        _environment.setVariable(name, variables[name], null, global: true);
-      }
+  Value runExpression(Importer importer, Expression expression) =>
+      _withWarnCallback(() => _withFakeStylesheet(
+          importer, expression, () => expression.accept(this)));
 
-      return expression.accept(this);
-    });
-  }
+  void runStatement(Importer importer, Statement statement) =>
+      _withWarnCallback(() => _withFakeStylesheet(
+          importer, statement, () => statement.accept(this)));
 
   /// Runs [callback] with a definition for the top-level `warn` function.
   T _withWarnCallback<T>(T callback()) {
@@ -439,39 +493,89 @@ class _EvaluateVisitor
         callback);
   }
 
+  /// Runs [callback] with [importer] as [_importer] and a fake [_stylesheet]
+  /// with [nodeForSpan]'s source span.
+  T _withFakeStylesheet<T>(
+      Importer importer, AstNode nodeForSpan, T callback()) {
+    var oldImporter = _importer;
+    _importer = importer;
+    var oldStylesheet = _stylesheet;
+    _stylesheet = Stylesheet(const [], nodeForSpan.span);
+
+    try {
+      return callback();
+    } finally {
+      _importer = oldImporter;
+      _stylesheet = oldStylesheet;
+    }
+  }
+
   /// Loads the module at [url] and passes it to [callback].
   ///
+  /// This first tries loading [url] relative to [baseUrl], which defaults to
+  /// `_stylesheet.span.sourceUrl`.
+  ///
+  /// The [configuration] overrides values for `!default` variables defined in
+  /// the module or modules it forwards and/or imports. If it's not passed, the
+  /// current configuration is used instead. Throws a [SassRuntimeException] if
+  /// a configured variable is not declared with `!default`.
+  ///
+  /// If [namesInErrors] is `true`, this includes the names of modules or
+  /// configured variables in errors relating to them. This should only be
+  /// `true` if the names won't be obvious from the source span.
+  ///
   /// The [stackFrame] and [nodeForSpan] are used for the name and location of
   /// the stack frame for the duration of the [callback].
   void _loadModule(Uri url, String stackFrame, AstNode nodeForSpan,
-      void callback(Module<Callable> module)) {
+      void callback(Module<Callable> module),
+      {Uri baseUrl,
+      Map<String, _ConfiguredValue> configuration,
+      bool namesInErrors = false}) {
+    configuration ??= const {};
+
     var builtInModule = _builtInModules[url];
     if (builtInModule != null) {
+      if (configuration.isNotEmpty || _configuration.isNotEmpty) {
+        throw _exception(
+            namesInErrors
+                ? "Built-in module $url can't be configured."
+                : "Built-in modules can't be configured.",
+            nodeForSpan.span);
+      }
+
       callback(builtInModule);
       return;
     }
 
     _withStackFrame(stackFrame, nodeForSpan, () {
-      var result =
-          inUseRule(() => _loadStylesheet(url.toString(), nodeForSpan.span));
+      var result = inUseRule(() =>
+          _loadStylesheet(url.toString(), nodeForSpan.span, baseUrl: baseUrl));
       var importer = result.item1;
       var stylesheet = result.item2;
 
       var canonicalUrl = stylesheet.span.sourceUrl;
       if (_activeModules.contains(canonicalUrl)) {
-        throw _exception("Module loop: this module is already being loaded.");
+        throw _exception(namesInErrors
+            ? "Module loop: ${p.prettyUri(canonicalUrl)} is already being "
+                "loaded."
+            : "Module loop: this module is already being loaded.");
       }
       _activeModules.add(canonicalUrl);
 
       Module<Callable> module;
       try {
-        module = _execute(importer, stylesheet);
+        module = _execute(importer, stylesheet,
+            configuration: configuration, namesInErrors: namesInErrors);
       } finally {
         _activeModules.remove(canonicalUrl);
       }
 
       try {
         callback(module);
+      } on SassRuntimeException {
+        rethrow;
+      } on SassException catch (error) {
+        throw _exception(error.message, error.span);
       } on SassScriptException catch (error) {
         throw _exception(error.message);
       }
@@ -479,62 +583,102 @@ class _EvaluateVisitor
   }
 
   /// Executes [stylesheet], loaded by [importer], to produce a module.
-  Module<Callable> _execute(Importer importer, Stylesheet stylesheet) {
+  ///
+  /// The [configuration] overrides values for `!default` variables defined in
+  /// the module or modules it forwards and/or imports. If it's not passed, the
+  /// current configuration is used instead. Throws a [SassRuntimeException] if
+  /// a configured variable is not declared with `!default`.
+  ///
+  /// If [namesInErrors] is `true`, this includes the names of modules or
+  /// configured variables in errors relating to them. This should only be
+  /// `true` if the names won't be obvious from the source span.
+  Module<Callable> _execute(Importer importer, Stylesheet stylesheet,
+      {Map<String, _ConfiguredValue> configuration,
+      bool namesInErrors = false}) {
+    configuration ??= const {};
     var url = stylesheet.span.sourceUrl;
-    return _modules.putIfAbsent(url, () {
-      var environment = Environment(sourceMap: _sourceMap);
-      CssStylesheet css;
-      var extender = Extender();
-      _withEnvironment(environment, () {
-        var oldImporter = _importer;
-        var oldStylesheet = _stylesheet;
-        var oldRoot = _root;
-        var oldParent = _parent;
-        var oldEndOfImports = _endOfImports;
-        var oldOutOfOrderImports = _outOfOrderImports;
-        var oldExtender = _extender;
-        var oldStyleRule = _styleRule;
-        var oldMediaQueries = _mediaQueries;
-        var oldDeclarationName = _declarationName;
-        var oldInUnknownAtRule = _inUnknownAtRule;
-        var oldAtRootExcludingStyleRule = _atRootExcludingStyleRule;
-        var oldInKeyframes = _inKeyframes;
-        _importer = importer;
-        _stylesheet = stylesheet;
-        _root = ModifiableCssStylesheet(stylesheet.span);
-        _parent = _root;
-        _endOfImports = 0;
-        _outOfOrderImports = null;
-        _extender = extender;
-        _styleRule = null;
-        _mediaQueries = null;
-        _declarationName = null;
-        _inUnknownAtRule = false;
-        _atRootExcludingStyleRule = false;
-        _inKeyframes = false;
 
-        visitStylesheet(stylesheet);
-        css = _outOfOrderImports == null
-            ? _root
-            : CssStylesheet(_addOutOfOrderImports(), stylesheet.span);
+    var alreadyLoaded = _modules[url];
+    if (alreadyLoaded != null) {
+      if (configuration.isNotEmpty || _configuration.isNotEmpty) {
+        throw _exception(namesInErrors
+            ? "${p.prettyUri(url)} was already loaded, so it can't be "
+                "configured using \"with\"."
+            : "This module was already loaded, so it can't be configured using "
+                "\"with\".");
+      }
 
-        _importer = oldImporter;
-        _stylesheet = oldStylesheet;
-        _root = oldRoot;
-        _parent = oldParent;
-        _endOfImports = oldEndOfImports;
-        _outOfOrderImports = oldOutOfOrderImports;
-        _extender = oldExtender;
-        _styleRule = oldStyleRule;
-        _mediaQueries = oldMediaQueries;
-        _declarationName = oldDeclarationName;
-        _inUnknownAtRule = oldInUnknownAtRule;
-        _atRootExcludingStyleRule = oldAtRootExcludingStyleRule;
-        _inKeyframes = oldInKeyframes;
-      });
+      return alreadyLoaded;
+    }
+
+    var environment = Environment(sourceMap: _sourceMap);
+    CssStylesheet css;
+    var extender = Extender();
+    _withEnvironment(environment, () {
+      var oldImporter = _importer;
+      var oldStylesheet = _stylesheet;
+      var oldRoot = _root;
+      var oldParent = _parent;
+      var oldEndOfImports = _endOfImports;
+      var oldOutOfOrderImports = _outOfOrderImports;
+      var oldExtender = _extender;
+      var oldStyleRule = _styleRule;
+      var oldMediaQueries = _mediaQueries;
+      var oldDeclarationName = _declarationName;
+      var oldInUnknownAtRule = _inUnknownAtRule;
+      var oldAtRootExcludingStyleRule = _atRootExcludingStyleRule;
+      var oldInKeyframes = _inKeyframes;
+      var oldConfiguration = _configuration;
+      _importer = importer;
+      _stylesheet = stylesheet;
+      _root = ModifiableCssStylesheet(stylesheet.span);
+      _parent = _root;
+      _endOfImports = 0;
+      _outOfOrderImports = null;
+      _extender = extender;
+      _styleRule = null;
+      _mediaQueries = null;
+      _declarationName = null;
+      _inUnknownAtRule = false;
+      _atRootExcludingStyleRule = false;
+      _inKeyframes = false;
+
+      if (configuration.isNotEmpty) _configuration = Map.of(configuration);
+
+      visitStylesheet(stylesheet);
+      css = _outOfOrderImports == null
+          ? _root
+          : CssStylesheet(_addOutOfOrderImports(), stylesheet.span);
+
+      _importer = oldImporter;
+      _stylesheet = oldStylesheet;
+      _root = oldRoot;
+      _parent = oldParent;
+      _endOfImports = oldEndOfImports;
+      _outOfOrderImports = oldOutOfOrderImports;
+      _extender = oldExtender;
+      _styleRule = oldStyleRule;
+      _mediaQueries = oldMediaQueries;
+      _declarationName = oldDeclarationName;
+      _inUnknownAtRule = oldInUnknownAtRule;
+      _atRootExcludingStyleRule = oldAtRootExcludingStyleRule;
+      _inKeyframes = oldInKeyframes;
 
-      return environment.toModule(css, extender);
+      if (configuration.isNotEmpty && _configuration.isNotEmpty) {
+        throw _exception(
+            namesInErrors
+                ? "\$${_configuration.keys.first} was not declared with "
+                    "!default in the @used module."
+                : "This variable was not declared with !default in the @used "
+                    "module.",
+            _configuration.values.first.configurationSpan);
+      }
+      _configuration = oldConfiguration;
     });
+
+    var module = environment.toModule(css, extender);
+    _modules[url] = module;
+    return module;
   }
 
   /// Returns a copy of [_root.children] with [_outOfOrderImports] inserted
@@ -1060,10 +1204,31 @@ class _EvaluateVisitor
   }
 
   Value visitForwardRule(ForwardRule node) {
+    // Only allow variables that are visible through the `@forward` to be
+    // configured. These views support [Map.remove] so we can mark when a
+    // configuration variable is used by removing it even when the underlying
+    // map is wrapped.
+    var oldConfiguration = _configuration;
+    if (_configuration.isNotEmpty) {
+      if (node.prefix != null) {
+        _configuration = UnprefixedMapView(_configuration, node.prefix);
+      }
+
+      if (node.shownVariables != null) {
+        _configuration =
+            LimitedMapView.whitelist(_configuration, node.shownVariables);
+      } else if (node.hiddenVariables != null &&
+          node.hiddenVariables.isNotEmpty) {
+        _configuration =
+            LimitedMapView.blacklist(_configuration, node.hiddenVariables);
+      }
+    }
+
     _loadModule(node.url, "@forward", node, (module) {
       _environment.forwardModule(module, node);
     });
 
+    _configuration = oldConfiguration;
     return null;
   }
 
@@ -1187,7 +1352,11 @@ class _EvaluateVisitor
 
   /// Loads the [Stylesheet] identified by [url], or throws a
   /// [SassRuntimeException] if loading fails.
-  Tuple2<Importer, Stylesheet> _loadStylesheet(String url, FileSpan span) {
+  ///
+  /// This first tries loading [url] relative to [baseUrl], which defaults to
+  /// `_stylesheet.span.sourceUrl`.
+  Tuple2<Importer, Stylesheet> _loadStylesheet(String url, FileSpan span,
+      {Uri baseUrl}) {
     try {
       assert(_importSpan == null);
       _importSpan = span;
@@ -1197,7 +1366,7 @@ class _EvaluateVisitor
         if (stylesheet != null) return Tuple2(null, stylesheet);
       } else {
         var tuple = _importCache.import(
-            Uri.parse(url), _importer, _stylesheet.span?.sourceUrl);
+            Uri.parse(url), _importer, baseUrl ?? _stylesheet?.span?.sourceUrl);
         if (tuple != null) return tuple;
       }
 
@@ -1274,30 +1443,41 @@ class _EvaluateVisitor
 
   Value visitIncludeRule(IncludeRule node) {
     var mixin = _addExceptionSpan(node,
-            () => _environment.getMixin(node.name, namespace: node.namespace))
-        as UserDefinedCallable<Environment>;
+        () => _environment.getMixin(node.name, namespace: node.namespace));
     if (mixin == null) {
       throw _exception("Undefined mixin.", node.span);
     }
 
-    if (node.content != null && !(mixin.declaration as MixinRule).hasContent) {
-      throw _exception("Mixin doesn't accept a content block.", node.span);
-    }
+    if (mixin is BuiltInCallable) {
+      if (node.content != null) {
+        throw _exception("Mixin doesn't accept a content block.", node.span);
+      }
 
-    var contentCallable = node.content == null
-        ? null
-        : UserDefinedCallable(node.content, _environment.closure());
-    _runUserDefinedCallable(node.arguments, mixin, node, () {
-      _environment.withContent(contentCallable, () {
-        _environment.asMixin(() {
-          for (var statement in mixin.declaration.children) {
-            statement.accept(this);
-          }
+      _runBuiltInCallable(node.arguments, mixin, node);
+    } else if (mixin is UserDefinedCallable<Environment>) {
+      if (node.content != null &&
+          !(mixin.declaration as MixinRule).hasContent) {
+        throw _exception("Mixin doesn't accept a content block.", node.span);
+      }
+
+      var contentCallable = node.content == null
+          ? null
+          : UserDefinedCallable(node.content, _environment.closure());
+
+      _runUserDefinedCallable(node.arguments, mixin, node, () {
+        _environment.withContent(contentCallable, () {
+          _environment.asMixin(() {
+            for (var statement in mixin.declaration.children) {
+              statement.accept(this);
+            }
+          });
+          return null;
         });
         return null;
       });
-      return null;
-    });
+    } else {
+      throw UnsupportedError("Unknown callable type $mixin.");
+    }
 
     return null;
   }
@@ -1540,6 +1720,23 @@ class _EvaluateVisitor
 
   Value visitVariableDeclaration(VariableDeclaration node) {
     if (node.isGuarded) {
+      if (node.namespace == null && _environment.atRoot) {
+        // Explicitly check whether [_configuration] is empty because if it is,
+        // it may be a constant map which doesn't support `remove()`.
+        //
+        // See also dart-lang/sdk#38540.
+        var override =
+            _configuration.isEmpty ? null : _configuration.remove(node.name);
+        if (override != null) {
+          _addExceptionSpan(node, () {
+            _environment.setVariable(
+                node.name, override.value, override.assignmentNode,
+                global: true);
+          });
+          return null;
+        }
+      }
+
       var value = _addExceptionSpan(node,
           () => _environment.getVariable(node.name, namespace: node.namespace));
       if (value != null && value != sassNull) return null;
@@ -1573,7 +1770,16 @@ class _EvaluateVisitor
   Value visitUseRule(UseRule node) {
     _loadModule(node.url, "@use", node, (module) {
       _environment.addModule(module, namespace: node.namespace);
-    });
+    },
+        configuration: node.configuration.isEmpty
+            ? null
+            : {
+                for (var entry in node.configuration.entries)
+                  entry.key: _ConfiguredValue(
+                      entry.value.item1.accept(this).withoutSlash(),
+                      entry.value.item2,
+                      _expressionNode(entry.value.item1))
+              });
 
     return null;
   }
@@ -1857,8 +2063,13 @@ class _EvaluateVisitor
   Value _runFunctionCallable(
       ArgumentInvocation arguments, Callable callable, AstNode nodeWithSpan) {
     if (callable is BuiltInCallable) {
-      return _runBuiltInCallable(arguments, callable, nodeWithSpan)
-          .withoutSlash();
+      var result = _runBuiltInCallable(arguments, callable, nodeWithSpan);
+      if (result == null) {
+        throw _exception(
+            "Custom functions may not return Dart's null.", nodeWithSpan.span);
+      }
+
+      return result.withoutSlash();
     } else if (callable is UserDefinedCallable<Environment>) {
       return _runUserDefinedCallable(arguments, callable, nodeWithSpan, () {
         for (var statement in callable.declaration.children) {
@@ -1946,7 +2157,8 @@ class _EvaluateVisitor
     Value result;
     try {
       result = callback(evaluated.positional);
-      if (result == null) throw "Custom functions may not return Dart's null.";
+    } on SassRuntimeException {
+      rethrow;
     } catch (error) {
       String message;
       try {
@@ -2703,3 +2915,19 @@ class _ArgumentResults {
   _ArgumentResults(this.positional, this.named, this.separator,
       {this.positionalNodes, this.namedNodes});
 }
+
+/// A variable value that's been configured using `@use ... with`.
+class _ConfiguredValue {
+  /// The value of the variable.
+  final Value value;
+
+  /// The span where the variable's configuration was written.
+  final FileSpan configurationSpan;
+
+  /// The [AstNode] where the variable's value originated.
+  ///
+  /// This is used to generate source maps.
+  final AstNode assignmentNode;
+
+  _ConfiguredValue(this.value, this.configurationSpan, [this.assignmentNode]);
+}
diff --git a/pubspec.yaml b/pubspec.yaml
index e0eb14acf..7047712d0 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: sass
-version: 1.22.12
+version: 1.23.0
 description: A Sass implementation in Dart.
 author: Sass Team
 homepage: https://github.com/sass/dart-sass
diff --git a/test/cli/shared/repl.dart b/test/cli/shared/repl.dart
index a609e5869..2705d3610 100644
--- a/test/cli/shared/repl.dart
+++ b/test/cli/shared/repl.dart
@@ -5,6 +5,7 @@
 import 'dart:async';
 
 import 'package:test/test.dart';
+import 'package:test_descriptor/test_descriptor.dart' as d;
 import 'package:test_process/test_process.dart';
 
 /// Defines test that are shared between the Dart and Node.js CLI test suites.
@@ -13,7 +14,6 @@ void sharedTests(Future<TestProcess> runSass(Iterable<String> arguments)) {
     var invalidArgs = [
       '--stdin',
       '--indented',
-      '--load-path=x',
       '--style=compressed',
       '--source-map',
       '--source-map-urls=absolute',
@@ -78,6 +78,91 @@ void sharedTests(Future<TestProcess> runSass(Iterable<String> arguments)) {
     await sass.kill();
   });
 
+  group("with @use", () {
+    test("uses variables from the @used module", () async {
+      await d.file("other.scss", r"$var: 12;").create();
+
+      var sass = await runSass(["--interactive"]);
+      sass.stdin.writeln("@use 'other'");
+      await expectLater(sass.stdout, emits(">> @use 'other'"));
+
+      sass.stdin.writeln(r"other.$var");
+      await expectLater(sass.stdout, emitsInOrder([r">> other.$var", "12"]));
+
+      await sass.kill();
+    });
+
+    test("uses functions from the @used module", () async {
+      await d.file("other.scss", r"@function foo() {@return 12}").create();
+
+      var sass = await runSass(["--interactive"]);
+      sass.stdin.writeln("@use 'other'");
+      await expectLater(sass.stdout, emits(">> @use 'other'"));
+
+      sass.stdin.writeln(r"other.foo()");
+      await expectLater(sass.stdout, emitsInOrder([">> other.foo()", "12"]));
+
+      await sass.kill();
+    });
+
+    test("uses a built-in module", () async {
+      var sass = await runSass(["--interactive"]);
+      sass.stdin.writeln("@use 'sass:math'");
+      await expectLater(sass.stdout, emits(">> @use 'sass:math'"));
+
+      sass.stdin.writeln(r"math.abs(-1)");
+      await expectLater(sass.stdout, emitsInOrder([">> math.abs(-1)", "1"]));
+
+      await sass.kill();
+    });
+
+    test("loads a module from the load path", () async {
+      await d.dir("dir", [d.file("other.scss", r"$var: 12;")]).create();
+
+      var sass = await runSass(["--load-path=dir", "--interactive"]);
+      sass.stdin.writeln("@use 'other'");
+      await expectLater(sass.stdout, emits(">> @use 'other'"));
+
+      sass.stdin.writeln(r"other.$var");
+      await expectLater(sass.stdout, emitsInOrder([r">> other.$var", "12"]));
+
+      await sass.kill();
+    });
+
+    test("loads a module in the global scope", () async {
+      await d.file("other.scss", r"$var: 12;").create();
+
+      var sass = await runSass(["--interactive"]);
+      sass.stdin.writeln("@use 'other' as *");
+      await expectLater(sass.stdout, emits(">> @use 'other' as *"));
+
+      sass.stdin.writeln(r"$var");
+      await expectLater(sass.stdout, emitsInOrder([r">> $var", "12"]));
+
+      await sass.kill();
+    });
+
+    test("loads a module with configuration", () async {
+      await d.file("other.scss", r"""
+        $var: 12 !default;
+        $derived: $var + 13;
+      """).create();
+
+      var sass = await runSass(["--interactive"]);
+      sass.stdin.writeln(r"@use 'other' with ($var: 1)");
+      await expectLater(sass.stdout, emits(r">> @use 'other' with ($var: 1)"));
+
+      sass.stdin.writeln(r"other.$var");
+      await expectLater(sass.stdout, emitsInOrder([r">> other.$var", "1"]));
+
+      sass.stdin.writeln(r"other.$derived");
+      await expectLater(
+          sass.stdout, emitsInOrder([r">> other.$derived", "14"]));
+
+      await sass.kill();
+    });
+  });
+
   group("gracefully handles", () {
     test("a parse error", () async {
       var sass = await runSass(["--interactive"]);
@@ -129,7 +214,7 @@ void sharedTests(Future<TestProcess> runSass(Iterable<String> arguments)) {
       await expectLater(
           sass.stdout,
           emitsInOrder(
-              [r">> 1 + $x + 3", r"       ^^", "Error: Undefined variable."]));
+              [r">> 1 + $x + 3", "       ^^", "Error: Undefined variable."]));
       await sass.kill();
     });
 
@@ -150,6 +235,70 @@ void sharedTests(Future<TestProcess> runSass(Iterable<String> arguments)) {
       await sass.kill();
     });
 
+    group("with @use", () {
+      test("a module load error", () async {
+        var sass = await runSass(["--no-unicode", "--interactive"]);
+        sass.stdin.writeln('@use "non-existent"');
+        await expectLater(
+            sass.stdout,
+            emitsInOrder([
+              '>> @use "non-existent"',
+              "   ^^^^^^^^^^^^^^^^^^^",
+              "Error: Can't find stylesheet to import."
+            ]));
+        await sass.kill();
+      });
+
+      test("a parse error for @use", () async {
+        var sass = await runSass(["--no-unicode", "--interactive"]);
+        sass.stdin.writeln('@use "other" as');
+        await expectLater(
+            sass.stdout,
+            emitsInOrder([
+              '>> @use "other" as',
+              "                  ^",
+              "Error: Expected identifier."
+            ]));
+        await sass.kill();
+      });
+
+      test("a parse error in a loaded module", () async {
+        await d.file("other.scss", r"$var: 1px +").create();
+
+        var sass = await runSass(["--no-unicode", "--interactive"]);
+        sass.stdin.writeln('@use "other"');
+        await expectLater(
+            sass.stdout,
+            emitsInOrder([
+              '>> @use "other"',
+              "Error: Expected expression.",
+              "  ,",
+              r"1 | $var: 1px +",
+              "  |            ^",
+              "  '"
+            ]));
+        await sass.kill();
+      });
+
+      test("a runtime error in a loaded module", () async {
+        await d.file("other.scss", r"$var: 1px + 1s;").create();
+
+        var sass = await runSass(["--no-unicode", "--interactive"]);
+        sass.stdin.writeln('@use "other"');
+        await expectLater(
+            sass.stdout,
+            emitsInOrder([
+              '>> @use "other"',
+              "Error: Incompatible units s and px.",
+              "  ,",
+              r"1 | $var: 1px + 1s;",
+              "  |       ^^^^^^^^",
+              "  '"
+            ]));
+        await sass.kill();
+      });
+    });
+
     group("and colorizes", () {
       test("an error in the source text", () async {
         var sass = await runSass(["--interactive", "--color"]);
diff --git a/test/source_map_test.dart b/test/source_map_test.dart
index fb698e0f6..8e1d24432 100644
--- a/test/source_map_test.dart
+++ b/test/source_map_test.dart
@@ -12,6 +12,8 @@ import 'package:tuple/tuple.dart';
 import 'package:sass/sass.dart';
 import 'package:sass/src/utils.dart';
 
+import 'dart_api/test_importer.dart';
+
 main() {
   group("maps source to target for", () {
     group("a style rule", () {
@@ -543,6 +545,25 @@ main() {
           """);
         });
 
+        test("a @use rule with a with clause", () {
+          _expectScssSourceMap(r"""
+            $var1: {{1}}new value;
+            @use 'other' with ($var2: $var1);
+
+            {{2}}a {
+              {{3}}b: other.$var2;
+            }
+          """, """
+            {{2}}a {
+              {{3}}b: {{1}}new value;
+            }
+          """,
+              importer: TestImporter(
+                  (url) => Uri.parse("u:$url"),
+                  (_) => ImporterResult(r"$var2: default value !default;",
+                      syntax: Syntax.scss)));
+        });
+
         group("a mixin argument that is", () {
           test("the default value", () {
             _expectScssSourceMap(r"""
@@ -689,13 +710,14 @@ main() {
 ///
 /// This also re-indents the input strings with [_reindent].
 void _expectSourceMap(String sass, String scss, String css,
-    {OutputStyle style}) {
-  _expectSassSourceMap(sass, css, style: style);
-  _expectScssSourceMap(scss, css, style: style);
+    {Importer importer, OutputStyle style}) {
+  _expectSassSourceMap(sass, css, importer: importer, style: style);
+  _expectScssSourceMap(scss, css, importer: importer, style: style);
 }
 
 /// Like [_expectSourceMap], but with only SCSS source.
-void _expectScssSourceMap(String scss, String css, {OutputStyle style}) {
+void _expectScssSourceMap(String scss, String css,
+    {Importer importer, OutputStyle style}) {
   var scssTuple = _extractLocations(_reindent(scss));
   var scssText = scssTuple.item1;
   var scssLocations = _tuplesToMap(scssTuple.item2);
@@ -705,14 +727,15 @@ void _expectScssSourceMap(String scss, String css, {OutputStyle style}) {
   var cssLocations = cssTuple.item2;
 
   SingleMapping scssMap;
-  var scssOutput =
-      compileString(scssText, sourceMap: (map) => scssMap = map, style: style);
+  var scssOutput = compileString(scssText,
+      sourceMap: (map) => scssMap = map, importer: importer, style: style);
   expect(scssOutput, equals(cssText));
   _expectMapMatches(scssMap, scssText, cssText, scssLocations, cssLocations);
 }
 
 /// Like [_expectSourceMap], but with only indented source.
-void _expectSassSourceMap(String sass, String css, {OutputStyle style}) {
+void _expectSassSourceMap(String sass, String css,
+    {Importer importer, OutputStyle style}) {
   var sassTuple = _extractLocations(_reindent(sass));
   var sassText = sassTuple.item1;
   var sassLocations = _tuplesToMap(sassTuple.item2);
@@ -723,7 +746,10 @@ void _expectSassSourceMap(String sass, String css, {OutputStyle style}) {
 
   SingleMapping sassMap;
   var sassOutput = compileString(sassText,
-      indented: true, sourceMap: (map) => sassMap = map, style: style);
+      indented: true,
+      sourceMap: (map) => sassMap = map,
+      importer: importer,
+      style: style);
   expect(sassOutput, equals(cssText));
   _expectMapMatches(sassMap, sassText, cssText, sassLocations, cssLocations);
 }