Don't crash on a fully-interpolated CSS variable name (#177)

Closes #176

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 * Properly parse numbers with exponents.
 
+* Don't crash when evaluating CSS variables whose names are entirely
+  interpolated (for example, `#{--foo}: ...`).
+
 ## 1.0.0-beta.2
 
 * Add support for the `::slotted()` pseudo-element.

lib/src/ast/css/declaration.dart

@@ -19,12 +19,6 @@ class CssDeclaration extends CssNode {
 
   final FileSpan span;
 
-  /// Whether this is a custom property declaration, also known as a CSS
-  /// variable.
-  ///
-  /// Custom property declarations always have unquoted [SassString] values.
-  bool get isCustomProperty => name.value.startsWith("--");
-
   CssDeclaration(this.name, this.value, this.span);
 
   T accept<T>(CssVisitor<T> visitor) => visitor.visitDeclaration(this);

lib/src/visitor/serialize.dart

@@ -230,8 +230,8 @@ class _SerializeVisitor implements CssVisitor, ValueVisitor, SelectorVisitor {
     _writeIndentation();
     _buffer.write(node.name.value);
     _buffer.writeCharCode($colon);
-    if (node.isCustomProperty) {
-      _writeCustomPropertyValue(node);
+    if (_shouldReindentValue(node)) {
+      _writeReindentedValue(node);
     } else {
       _buffer.writeCharCode($space);
       _visitValue(node.value);
@@ -239,10 +239,22 @@ class _SerializeVisitor implements CssVisitor, ValueVisitor, SelectorVisitor {
     _buffer.writeCharCode($semicolon);
   }
 
-  /// Emits the value of [node] as a custom property value.
+  /// Returns whether [node]'s value should be re-indented when being written to
+  /// the stylesheet.
   ///
-  /// This re-indents [node]'s value relative to the current indentation.
-  void _writeCustomPropertyValue(CssDeclaration node) {
+  /// We only re-indent custom property values that were parsed as custom
+  /// properties, which we detect as unquoted strings. It's possible to have
+  /// false positives here, since someone could write `#{--foo}: unquoted`, but
+  /// that's unlikely enough that we can spare the extra time a no-op
+  /// reindenting will take.
+  bool _shouldReindentValue(CssDeclaration node) {
+    if (!node.name.value.startsWith("--")) return false;
+    var value = node.value.value;
+    return value is SassString && !value.hasQuotes;
+  }
+
+  /// Emits the value of [node], re-indented relative to the current indentation.
+  void _writeReindentedValue(CssDeclaration node) {
     var value = (node.value.value as SassString).text;
 
     var minimumIndentation = _minimumIndentation(value);

pubspec.yaml

@@ -1,5 +1,5 @@
 name: sass
-version: 1.0.0-beta.2
+version: 1.0.0-dev
 description: A Sass implementation in Dart.
 author: Dart Team <misc@dartlang.org>
 homepage: https://github.com/sass/dart-sass