Properly handle some CSS identifier edge cases (#816)

See sass/sass#2738
Closes #815

CHANGELOG.md

@@ -1,5 +1,16 @@
 ## 1.22.12
 
+* **Potentially breaking bug fix:** character sequences consisting of two or
+  more hyphens followed by a number (such as `--123`), or two or more hyphens on
+  their own (such as `--`), are now parsed as identifiers [in accordance with
+  the CSS spec][ident-token-diagram].
+
+  [ident-token-diagram]: https://drafts.csswg.org/css-syntax-3/#ident-token-diagram
+
+  The sequence `--` was previously parsed as multiple applications of the `-`
+  operator. Since this is unlikely to be used intentionally in practice, we
+  consider this bug fix safe.
+
 ### Command-Line Interface
 
 * Fix a bug where changes in `.css` files would be ignored in `--watch` mode.

lib/src/parse/parser.dart

@@ -147,12 +147,18 @@ class Parser {
   @protected
   String identifier({bool normalize = false, bool unit = false}) {
     // NOTE: this logic is largely duplicated in
-    // StylesheetParser._interpolatedIdentifier and isIdentifier in utils.dart.
-    // Most changes here should be mirrored there.
+    // StylesheetParser.interpolatedIdentifier. Most changes here should be
+    // mirrored there.
 
     var text = StringBuffer();
-    while (scanner.scanChar($dash)) {
+    if (scanner.scanChar($dash)) {
       text.writeCharCode($dash);
+
+      if (scanner.scanChar($dash)) {
+        text.writeCharCode($dash);
+        _identifierBody(text, normalize: normalize, unit: unit);
+        return text.toString();
+      }
     }
 
     var first = scanner.peekChar();
@@ -580,11 +586,7 @@ class Parser {
 
     var second = scanner.peekChar(forward + 1);
     if (second == null) return false;
-    if (isNameStart(second) || second == $backslash) return true;
-    if (second != $dash) return false;
-
-    var third = scanner.peekChar(forward + 2);
-    return third != null && isNameStart(third);
+    return isNameStart(second) || second == $backslash || second == $dash;
   }
 
   /// Returns whether the scanner is immediately before a sequence of characters

lib/src/parse/stylesheet.dart

@@ -2979,8 +2979,14 @@ relase. For details, see http://bit.ly/moz-document.
     var start = scanner.state;
     var buffer = InterpolationBuffer();
 
-    while (scanner.scanChar($dash)) {
+    if (scanner.scanChar($dash)) {
       buffer.writeCharCode($dash);
+
+      if (scanner.scanChar($dash)) {
+        buffer.writeCharCode($dash);
+        _interpolatedIdentifierBody(buffer);
+        return buffer.interpolation(scanner.spanFrom(start));
+      }
     }
 
     var first = scanner.peekChar();
@@ -2996,6 +3002,13 @@ relase. For details, see http://bit.ly/moz-document.
       scanner.error("Expected identifier.");
     }
 
+    _interpolatedIdentifierBody(buffer);
+    return buffer.interpolation(scanner.spanFrom(start));
+  }
+
+  /// Consumes a chunk of a possibly-interpolated CSS identifier after the name
+  /// start, and adds the contents to the [buffer] buffer.
+  void _interpolatedIdentifierBody(InterpolationBuffer buffer) {
     while (true) {
       var next = scanner.peekChar();
       if (next == null) {
@@ -3013,8 +3026,6 @@ relase. For details, see http://bit.ly/moz-document.
         break;
       }
     }
-
-    return buffer.interpolation(scanner.spanFrom(start));
   }
 
   /// Consumes interpolation.
@@ -3261,15 +3272,8 @@ relase. For details, see http://bit.ly/moz-document.
     if (first != $dash) return false;
     var second = scanner.peekChar(1);
     if (second == null) return false;
-    if (isNameStart(second) || second == $backslash) return true;
-
     if (second == $hash) return scanner.peekChar(2) == $lbrace;
-    if (second != $dash) return false;
-
-    var third = scanner.peekChar(2);
-    if (third == null) return false;
-    if (third == $hash) return scanner.peekChar(3) == $lbrace;
-    return isNameStart(third);
+    return isNameStart(second) || second == $backslash || second == $dash;
   }
 
   /// Returns whether the scanner is immediately before a sequence of characters