Make misplaced rest arguments a syntax error

CHANGELOG.md

@@ -7,6 +7,12 @@
   `meta.inspect()`, which is to provide full information about the structure of
   a Sass value.
 
+* Passing a rest argument (`$arg...`) before a positional or named argument when
+  calling a function or mixin is now deprecated. This was always outside the
+  specified syntax, but it was historically treated the same as passing the rest
+  argument at the end of the argument list whether or not that matched the
+  visual order of the arguments.
+
 ## 1.90.0
 
 * Allow a `@forward`ed module to be loaded with a configuration when that module

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: c57ab2eb07ab1df48581b8484ef9bdbad0ddceaa
+  // Checksum: 1c1efc2604729aea755453fefd1e8f56e100698e
 
   /// Deprecation for passing a string directly to meta.call().
   callString('call-string',
@@ -131,6 +131,11 @@ enum Deprecation {
       deprecatedIn: '1.88.0',
       description: 'Passing a relative url to compileString().'),
 
+  /// Deprecation for a rest parameter before a positional or named parameter.
+  misplacedRest('misplaced-rest',
+      deprecatedIn: '1.91.0',
+      description: 'A rest parameter before a positional or named parameter.'),
+
   // END AUTOGENERATED CODE
 
   /// Used for deprecations coming from user-authored code.

lib/src/parse/stylesheet.dart

@@ -15,6 +15,7 @@ import '../exception.dart';
 import '../interpolation_buffer.dart';
 import '../util/character.dart';
 import '../utils.dart';
+import '../util/multi_span.dart';
 import '../util/nullable.dart';
 import '../value.dart';
 import 'parser.dart';
@@ -1832,6 +1833,7 @@ abstract class StylesheetParser extends Parser {
     var named = <String, Expression>{};
     Expression? rest;
     Expression? keywordRest;
+    var emittedRestDeprecation = false;
     while (_lookingAtExpression()) {
       var expression = expressionUntilComma(singleEquals: !mixin);
       whitespace(consumeNewlines: true);
@@ -1842,6 +1844,19 @@ abstract class StylesheetParser extends Parser {
           error("Duplicate argument.", expression.span);
         }
         named[expression.name] = expressionUntilComma(singleEquals: !mixin);
+
+        if (rest != null && !emittedRestDeprecation) {
+          emittedRestDeprecation = true;
+          warnings.add((
+            deprecation: Deprecation.misplacedRest,
+            message: 'Named arguments must come before rest arguments.\n'
+                'This will be an error in Dart Sass 2.0.0.',
+            span: MultiSpan(
+                scanner.spanFromPosition(expression.span.start.offset),
+                'named argument',
+                {rest.span: 'rest argument'})
+          ));
+        }
       } else if (scanner.scanChar($dot)) {
         scanner.expectChar($dot);
         scanner.expectChar($dot);
@@ -1860,6 +1875,17 @@ abstract class StylesheetParser extends Parser {
         );
       } else {
         positional.add(expression);
+
+        if (rest != null && !emittedRestDeprecation) {
+          emittedRestDeprecation = true;
+          warnings.add((
+            deprecation: Deprecation.misplacedRest,
+            message: 'Positional arguments must come before rest arguments.\n'
+                'This will be an error in Dart Sass 2.0.0.',
+            span: MultiSpan(expression.span, 'positional argument',
+                {rest.span: 'rest argument'})
+          ));
+        }
       }
 
       whitespace(consumeNewlines: true);