Add support for plain-CSS @function rules

CHANGELOG.md

@@ -1,3 +1,14 @@
+## 1.94.0
+
+* **Potentially breaking compatibility fix:** `@function` rules whose names
+  begin with `--` are now parsed as unknown at-rules to support the plain CSS
+  `@function` rule. Within this rule, the `result` property is parsed as raw
+  CSS just like custom properties.
+
+* **Potentially breaking compatibility fix:** `@mixin` rules whose names begin
+  with `--` are now errors. These are not yet parsed as unknown at-rules because
+  no browser currently supports CSS mixins.
+
 ## 1.93.2
 
 * No user-visible changes.

lib/src/ast/css/declaration.dart

@@ -33,11 +33,13 @@ abstract interface class CssDeclaration implements CssNode {
   /// Returns whether this is a CSS Custom Property declaration.
   bool get isCustomProperty;
 
-  /// Whether this is was originally parsed as a custom property declaration, as
-  /// opposed to using something like `#{--foo}: ...` to cause it to be parsed
-  /// as a normal Sass declaration.
+  /// Whether this property's value was originally parsed as SassScript, as
+  /// opposed to a custom property which is parsed as an interpolated sequence
+  /// of tokens.
   ///
-  /// If this is `true`, [isCustomProperty] will also be `true` and [value] will
-  /// contain a [SassString].
-  bool get parsedAsCustomProperty;
+  /// If this is `false`, [value] will contain an unquoted [SassString].
+  /// [isCustomProperty] will *usually* be true, but there are other properties
+  /// that may not be parsed as SassScript, like `return` in a plain CSS
+  /// `@function`.
+  bool get parsedAsSassScript;
 }

lib/src/ast/css/modifiable/declaration.dart

@@ -16,7 +16,7 @@ final class ModifiableCssDeclaration extends ModifiableCssNode
     implements CssDeclaration {
   final CssValue<String> name;
   final CssValue<Value> value;
-  final bool parsedAsCustomProperty;
+  final bool parsedAsSassScript;
   final Trace? trace;
   final FileSpan valueSpanForMap;
   final FileSpan span;
@@ -28,19 +28,14 @@ final class ModifiableCssDeclaration extends ModifiableCssNode
     this.name,
     this.value,
     this.span, {
-    required this.parsedAsCustomProperty,
+    required this.parsedAsSassScript,
     this.trace,
     FileSpan? valueSpanForMap,
   }) : valueSpanForMap = valueSpanForMap ?? value.span {
-    if (parsedAsCustomProperty) {
-      if (!isCustomProperty) {
+    if (!parsedAsSassScript) {
+      if (value.value is! SassString) {
         throw ArgumentError(
-          'parsedAsCustomProperty must be false if name doesn\'t begin with '
-          '"--".',
-        );
-      } else if (value.value is! SassString) {
-        throw ArgumentError(
-          'If parsedAsCustomProperty is true, value must contain a SassString '
+          'If parsedAsSassScript is false, value must contain a SassString '
           '(was `$value` of type ${value.value.runtimeType}).',
         );
       }

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

@@ -28,20 +28,31 @@ final class Declaration extends ParentStatement {
 
   final FileSpan span;
 
-  /// Returns whether this is a CSS Custom Property declaration.
+  /// Returns whether this declaration's value was parsed as SassScript.
   ///
-  /// Note that this can return `false` for declarations that will ultimately be
+  /// This is `false` for custom properties as well as the `result` property of
+  /// the plain CSS `@function` rule.
+  ///
+  /// Note that this can return `true` for declarations that will ultimately be
   /// serialized as custom properties if they aren't *parsed as* custom
   /// properties, such as `#{--foo}: ...`.
   ///
-  /// If this is `true`, then `value` will be a [StringExpression].
+  /// If this is `false`, then `value` will be an unquoted [StringExpression].
   ///
   /// @nodoc
   @internal
-  bool get isCustomProperty => name.initialPlain.startsWith('--');
+  final bool parsedAsSassScript;
 
   /// Creates a declaration with no children.
-  Declaration(this.name, this.value, this.span) : super(null);
+  Declaration(this.name, this.value, this.span)
+      : parsedAsSassScript = true,
+        super(null);
+
+  /// Creates a declaration with no children whose value is not parsed as
+  /// SassScript.
+  Declaration.notSassScript(this.name, StringExpression this.value, this.span)
+      : parsedAsSassScript = false,
+        super(null);
 
   /// Creates a declaration with children.
   ///
@@ -51,7 +62,8 @@ final class Declaration extends ParentStatement {
     Iterable<Statement> children,
     this.span, {
     this.value,
-  }) : super(List.unmodifiable(children));
+  })  : parsedAsSassScript = true,
+        super(List.unmodifiable(children));
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitDeclaration(this);
 
@@ -61,7 +73,7 @@ final class Declaration extends ParentStatement {
     buffer.writeCharCode($colon);
 
     if (value != null) {
-      if (!isCustomProperty) buffer.writeCharCode($space);
+      if (parsedAsSassScript) buffer.writeCharCode($space);
       buffer.write("$value");
     }
 

lib/src/deprecation.dart

@@ -15,7 +15,7 @@ enum Deprecation {
   // DO NOT EDIT. This section was generated from the language repo.
   // See tool/grind/generate_deprecations.dart for details.
   //
-  // Checksum: 0d3df25297a4e76b865aee1e908baf355e26d665
+  // Checksum: fc5363081eb9d8e9e7220b9d5c7fd928d67009fe
 
   /// Deprecation for passing a string directly to meta.call().
   callString('call-string',

lib/src/parse/css.dart

@@ -67,7 +67,6 @@ class CssParser extends ScssParser {
       "error" ||
       "extend" ||
       "for" ||
-      "function" ||
       "if" ||
       "include" ||
       "mixin" ||
@@ -76,6 +75,7 @@ class CssParser extends ScssParser {
       "while" =>
         _forbiddenAtRule(start),
       "import" => _cssImportRule(start),
+      "function" => _cssFunctionRule(start, name),
       "media" => mediaRule(start),
       "-moz-document" => mozDocumentRule(start, name),
       "supports" => supportsRule(start),
@@ -132,6 +132,20 @@ class CssParser extends ScssParser {
     ], scanner.spanFrom(start));
   }
 
+  /// Consumes a plain CSS function declaration.
+  ///
+  /// [start] should point before the `@`.
+  Statement _cssFunctionRule(LineScannerState start, Interpolation atRuleName) {
+    whitespace(consumeNewlines: true);
+    if (!scanner.matches('--')) {
+      almostAnyValue();
+      error(
+          "This at-rule isn't allowed in plain CSS.", scanner.spanFrom(start));
+    } else {
+      return unknownAtRule(start, atRuleName);
+    }
+  }
+
   ParenthesizedExpression parentheses() {
     // Expressions are only allowed within calculations, but we verify this at
     // evaluation time.

lib/src/parse/stylesheet.dart

@@ -50,6 +50,9 @@ abstract class StylesheetParser extends Parser {
   /// Whether the parser is currently parsing an unknown rule.
   var _inUnknownAtRule = false;
 
+  /// Whether the parser is currently parsing a plain-CSS `@function` rule.
+  var _inPlainCssFunction = false;
+
   /// Whether the parser is currently parsing a style rule.
   var _inStyleRule = false;
 
@@ -411,14 +414,19 @@ abstract class StylesheetParser extends Parser {
 
     // Parse custom properties as declarations no matter what.
     var name = nameBuffer.interpolation(scanner.spanFrom(start, beforeColon));
-    if (name.initialPlain.startsWith('--')) {
+    var customProperty = name.initialPlain.startsWith('--');
+    if (customProperty ||
+        (_inPlainCssFunction &&
+            (name.asPlain.andThen((name) => equalsIgnoreCase(name, 'result')) ??
+                false))) {
       var value = StringExpression(
         atEndOfStatement()
             ? Interpolation(const [], const [], scanner.emptySpan)
             : _interpolatedDeclarationValue(silentComments: false),
       );
-      expectStatementSeparator("custom property");
-      return Declaration(name, value, scanner.spanFrom(start));
+      expectStatementSeparator(
+          customProperty ? "custom property" : "@function result");
+      return Declaration.notSassScript(name, value, scanner.spanFrom(start));
     }
 
     if (scanner.scanChar($colon)) {
@@ -542,15 +550,12 @@ abstract class StylesheetParser extends Parser {
   /// 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,
+  /// This is only used when nested beneath other declarations. Otherwise,
   /// [_declarationOrStyleRule] is used instead.
   ///
   /// If [parseCustomProperties] is `true`, properties that begin with `--` will
   /// be parsed using custom property parsing rules.
-  Statement _propertyOrVariableDeclaration({
-    bool parseCustomProperties = true,
-  }) {
+  Statement _propertyOrVariableDeclaration() {
     var start = scanner.state;
 
     Interpolation name;
@@ -574,12 +579,9 @@ abstract class StylesheetParser extends Parser {
     whitespace(consumeNewlines: false);
     scanner.expectChar($colon);
 
-    if (parseCustomProperties && name.initialPlain.startsWith('--')) {
-      var value = StringExpression(
-        _interpolatedDeclarationValue(silentComments: false),
-      );
-      expectStatementSeparator("custom property");
-      return Declaration(name, value, scanner.spanFrom(start));
+    if (name.initialPlain.startsWith('--')) {
+      error('Declarations whose names begin with "--" may not be nested.',
+          name.span);
     }
 
     whitespace(consumeNewlines: false);
@@ -619,7 +621,7 @@ abstract class StylesheetParser extends Parser {
   /// Consumes a statement that's allowed within a declaration.
   Statement _declarationChild() => scanner.peekChar() == $at
       ? _declarationAtRule()
-      : _propertyOrVariableDeclaration(parseCustomProperties: false);
+      : _propertyOrVariableDeclaration();
 
   // ## At Rules
 
@@ -669,7 +671,7 @@ abstract class StylesheetParser extends Parser {
         if (!root) _disallowedAtRule(start);
         return _forwardRule(start);
       case "function":
-        return _functionRule(start);
+        return _functionRule(start, name);
       case "if":
         return _ifRule(start, child);
       case "import":
@@ -914,24 +916,16 @@ abstract class StylesheetParser extends Parser {
   /// Consumes a function declaration.
   ///
   /// [start] should point before the `@`.
-  FunctionRule _functionRule(LineScannerState start) {
+  Statement _functionRule(LineScannerState start, Interpolation atRuleName) {
     whitespace(consumeNewlines: true);
     var precedingComment = lastSilentComment;
     lastSilentComment = null;
     var beforeName = scanner.state;
-    var name = identifier();
 
-    if (name.startsWith('--')) {
-      warnings.add((
-        deprecation: Deprecation.cssFunctionMixin,
-        message:
-            'Sass @function names beginning with -- are deprecated for forward-'
-            'compatibility with plain CSS functions.\n'
-            '\n'
-            'For details, see https://sass-lang.com/d/css-function-mixin',
-        span: scanner.spanFrom(beforeName),
-      ));
-    } else if (equalsIgnoreCase(name, 'type')) {
+    if (scanner.matches('--')) return unknownAtRule(start, atRuleName);
+
+    var name = identifier();
+    if (equalsIgnoreCase(name, 'type')) {
       error('This name is reserved for the plain-CSS function.',
           scanner.spanFrom(beforeName));
     }
@@ -1428,15 +1422,13 @@ abstract class StylesheetParser extends Parser {
     var name = identifier();
 
     if (name.startsWith('--')) {
-      warnings.add((
-        deprecation: Deprecation.cssFunctionMixin,
-        message:
-            'Sass @mixin names beginning with -- are deprecated for forward-'
-            'compatibility with plain CSS mixins.\n'
-            '\n'
-            'For details, see https://sass-lang.com/d/css-function-mixin',
-        span: scanner.spanFrom(beforeName),
-      ));
+      error(
+        'Sass @mixin names beginning with -- are forbidden for forward-'
+        'compatibility with plain CSS mixins.\n'
+        '\n'
+        'For details, see https://sass-lang.com/d/css-function-mixin',
+        scanner.spanFrom(beforeName),
+      );
     }
 
     whitespace(consumeNewlines: false);
@@ -1736,22 +1728,27 @@ abstract class StylesheetParser extends Parser {
     if (scanner.peekChar() != $exclamation && !atEndOfStatement()) {
       value = _interpolatedDeclarationValue(allowOpenBrace: false);
     }
-
-    AtRule rule;
-    if (lookingAtChildren()) {
-      rule = _withChildren(
-        _statement,
-        start,
-        (children, span) =>
-            AtRule(name, span, value: value, children: children),
-      );
-    } else {
-      expectStatementSeparator();
-      rule = AtRule(name, scanner.spanFrom(start), value: value);
+    var wasInPlainCssFunction = _inPlainCssFunction;
+    if (name.asPlain case var name? when equalsIgnoreCase(name, 'function')) {
+      _inPlainCssFunction = true;
     }
 
-    _inUnknownAtRule = wasInUnknownAtRule;
-    return rule;
+    try {
+      if (lookingAtChildren()) {
+        return _withChildren(
+          _statement,
+          start,
+          (children, span) =>
+              AtRule(name, span, value: value, children: children),
+        );
+      } else {
+        expectStatementSeparator();
+        return AtRule(name, scanner.spanFrom(start), value: value);
+      }
+    } finally {
+      _inUnknownAtRule = wasInUnknownAtRule;
+      _inPlainCssFunction = wasInPlainCssFunction;
+    }
   }
 
   /// Throws a [StringScannerException] indicating that the at-rule starting at