Don't treat underscores as hyphens for the purpose of error checking

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## 1.77.2
 
+* Don't emit deprecation warnings for functions and mixins beginning with `__`.
+
+* Allow user-defined functions whose names begin with `_` and otherwise look
+  like vendor-prefixed functions with special CSS syntax.
+
 ### Command-Line Interface
 
 * Properly handle the `--silence-deprecation` flag.

lib/src/ast/sass/expression/function.dart

@@ -23,6 +23,12 @@ final class FunctionExpression
   /// without a namespace.
   final String? namespace;
 
+  /// The name of the function being invoked, with underscores converted to
+  /// hyphens.
+  ///
+  /// If this function is a plain CSS function, use [originalName] instead.
+  final String name;
+
   /// The name of the function being invoked, with underscores left as-is.
   final String originalName;
 
@@ -31,12 +37,6 @@ final class FunctionExpression
 
   final FileSpan span;
 
-  /// The name of the function being invoked, with underscores converted to
-  /// hyphens.
-  ///
-  /// If this function is a plain CSS function, use [originalName] instead.
-  String get name => originalName.replaceAll('_', '-');
-
   FileSpan get nameSpan {
     if (namespace == null) return span.initialIdentifier();
     return span.withoutNamespace().initialIdentifier();
@@ -46,7 +46,8 @@ final class FunctionExpression
       namespace == null ? null : span.initialIdentifier();
 
   FunctionExpression(this.originalName, this.arguments, this.span,
-      {this.namespace});
+      {this.namespace})
+      : name = originalName.replaceAll('_', '-');
 
   T accept<T>(ExpressionVisitor<T> visitor) =>
       visitor.visitFunctionExpression(this);

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

@@ -18,6 +18,9 @@ abstract base class CallableDeclaration
   /// The name of this callable, with underscores converted to hyphens.
   final String name;
 
+  /// The callable's original name, without underscores converted to hyphens.
+  final String originalName;
+
   /// The comment immediately preceding this declaration.
   final SilentComment? comment;
 
@@ -26,8 +29,9 @@ abstract base class CallableDeclaration
 
   final FileSpan span;
 
-  CallableDeclaration(
-      this.name, this.arguments, Iterable<Statement> children, this.span,
+  CallableDeclaration(this.originalName, this.arguments,
+      Iterable<Statement> children, this.span,
       {this.comment})
-      : super(List.unmodifiable(children));
+      : name = originalName.replaceAll('_', '-'),
+        super(List.unmodifiable(children));
 }

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

@@ -25,6 +25,10 @@ final class IncludeRule
   /// hyphens.
   final String name;
 
+  /// The original name of the mixin being invoked, without underscores
+  /// converted to hyphens.
+  final String originalName;
+
   /// The arguments to pass to the mixin.
   final ArgumentInvocation arguments;
 
@@ -55,8 +59,9 @@ final class IncludeRule
     return startSpan.initialIdentifier();
   }
 
-  IncludeRule(this.name, this.arguments, this.span,
-      {this.namespace, this.content});
+  IncludeRule(this.originalName, this.arguments, this.span,
+      {this.namespace, this.content})
+      : name = originalName.replaceAll('_', '-');
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitIncludeRule(this);
 

lib/src/parse/stylesheet.dart

@@ -847,7 +847,7 @@ abstract class StylesheetParser extends Parser {
     var precedingComment = lastSilentComment;
     lastSilentComment = null;
     var beforeName = scanner.state;
-    var name = identifier(normalize: true);
+    var name = identifier();
 
     if (name.startsWith('--')) {
       logger.warnForDeprecation(
@@ -1221,8 +1221,6 @@ abstract class StylesheetParser extends Parser {
     if (scanner.scanChar($dot)) {
       namespace = name;
       name = _publicIdentifier();
-    } else {
-      name = name.replaceAll("_", "-");
     }
 
     whitespace();
@@ -1274,7 +1272,7 @@ abstract class StylesheetParser extends Parser {
     var precedingComment = lastSilentComment;
     lastSilentComment = null;
     var beforeName = scanner.state;
-    var name = identifier(normalize: true);
+    var name = identifier();
 
     if (name.startsWith('--')) {
       logger.warnForDeprecation(
@@ -3457,7 +3455,7 @@ abstract class StylesheetParser extends Parser {
   /// Like [identifier], but rejects identifiers that begin with `_` or `-`.
   String _publicIdentifier() {
     var start = scanner.state;
-    var result = identifier(normalize: true);
+    var result = identifier();
     _assertPublic(result, () => scanner.spanFrom(start));
     return result;
   }

lib/src/visitor/async_evaluate.dart

@@ -1853,6 +1853,18 @@ final class _EvaluateVisitor
   Future<Value?> visitIncludeRule(IncludeRule node) async {
     var mixin = _addExceptionSpan(node,
         () => _environment.getMixin(node.name, namespace: node.namespace));
+    if (node.originalName.startsWith('--') &&
+        mixin is UserDefinedCallable &&
+        !mixin.declaration.originalName.startsWith('--')) {
+      _warn(
+          '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',
+          node.nameSpan,
+          Deprecation.cssFunctionMixin);
+    }
+
     var contentCallable = node.content.andThen((content) => UserDefinedCallable(
         content, _environment.closure(),
         inDependency: _inDependency));
@@ -2510,6 +2522,19 @@ final class _EvaluateVisitor
           PlainCssCallable(node.originalName);
     }
 
+    if (node.originalName.startsWith('--') &&
+        function is UserDefinedCallable &&
+        !function.declaration.originalName.startsWith('--')) {
+      _warn(
+        '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',
+        node.nameSpan,
+        Deprecation.cssFunctionMixin,
+      );
+    }
+
     var oldInFunction = _inFunction;
     _inFunction = true;
     var result = await _addErrorSpan(

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: 7788c21fd8c721992490ac01d0ef4783dddf3f1f
+// Checksum: 116b8079719577ac6e4dad4aebe403282136e611
 //
 // ignore_for_file: unused_import
 
@@ -1845,6 +1845,18 @@ final class _EvaluateVisitor
   Value? visitIncludeRule(IncludeRule node) {
     var mixin = _addExceptionSpan(node,
         () => _environment.getMixin(node.name, namespace: node.namespace));
+    if (node.originalName.startsWith('--') &&
+        mixin is UserDefinedCallable &&
+        !mixin.declaration.originalName.startsWith('--')) {
+      _warn(
+          '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',
+          node.nameSpan,
+          Deprecation.cssFunctionMixin);
+    }
+
     var contentCallable = node.content.andThen((content) => UserDefinedCallable(
         content, _environment.closure(),
         inDependency: _inDependency));
@@ -2486,6 +2498,19 @@ final class _EvaluateVisitor
           PlainCssCallable(node.originalName);
     }
 
+    if (node.originalName.startsWith('--') &&
+        function is UserDefinedCallable &&
+        !function.declaration.originalName.startsWith('--')) {
+      _warn(
+        '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',
+        node.nameSpan,
+        Deprecation.cssFunctionMixin,
+      );
+    }
+
     var oldInFunction = _inFunction;
     _inFunction = true;
     var result = _addErrorSpan(