[analysis_server] Pass experiments through to formatter

This passes any experiments that are both enabled, and marked as "future" through to the formatter.

I extracted most uses of the formatter to use the same shared `createFormatter()` helper that sets the appropriate values. Those that didn't already have a Result to pass I added TODOs to (though one is g3 so probably would require some internal migration).

See https://github.com/dart-lang/sdk/issues/55125

Change-Id: I8f4ef4242614dc240e217cdaf99105f2a5b49dc9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/486840
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

pkg/analysis_server/lib/src/handler/legacy/edit_format.dart

@@ -7,9 +7,9 @@ import 'dart:async';
 import 'package:analysis_server/protocol/protocol.dart';
 import 'package:analysis_server/protocol/protocol_generated.dart';
 import 'package:analysis_server/src/handler/legacy/legacy_handler.dart';
-import 'package:analysis_server/src/utilities/extensions/formatter_options.dart';
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart';
+import 'package:analyzer_plugin/src/utilities/formatter.dart';
 import 'package:dart_style/dart_style.dart' hide TrailingCommas;
 
 /// The handler for the `edit.format` request.
@@ -47,25 +47,17 @@ class EditFormatHandler extends LegacyHandler {
       length = null;
     }
 
+    var formatter = createFormatter(unit, defaultPageWidth: params.lineLength);
     var unformattedCode = unit.content;
-    var code = SourceCode(
-      unformattedCode,
-      selectionStart: start,
-      selectionLength: length,
-    );
-
-    var formatterOptions = unit.analysisOptions.formatterOptions;
-    var effectivePageWidth = formatterOptions.pageWidth ?? params.lineLength;
-    var effectiveTrailingCommas = formatterOptions.dartStyleTrailingCommas;
-    var effectiveLanguageVersion = unit.unit.languageVersion.effective;
-    var formatter = DartFormatter(
-      pageWidth: effectivePageWidth,
-      trailingCommas: effectiveTrailingCommas,
-      languageVersion: effectiveLanguageVersion,
-    );
     SourceCode formattedResult;
     try {
-      formattedResult = formatter.formatSource(code);
+      formattedResult = formatter.formatSource(
+        SourceCode(
+          unformattedCode,
+          selectionStart: start,
+          selectionLength: length,
+        ),
+      );
     } on FormatterException {
       sendResponse(Response.formatWithErrors(request));
       return;

pkg/analysis_server/lib/src/handler/legacy/edit_format_if_enabled.dart

@@ -36,6 +36,9 @@ class EditFormatIfEnabledHandler extends LegacyHandler {
     var originalContent = file.readAsStringSync();
     var code = SourceCode(originalContent);
 
+    // TODO(dantup): Consider using createFormatter() which takes a result and
+    //  correctly applies settings like page_width, trailing_commas, and enabled
+    //  experiments.
     var formatter = DartFormatter(
       languageVersion: languageVersion ?? DartFormatter.latestLanguageVersion,
     );

pkg/analysis_server/lib/src/lsp/source_edits.dart

@@ -8,7 +8,6 @@ import 'package:analysis_server/src/lsp/mapping.dart';
 import 'package:analysis_server/src/protocol_server.dart'
     as server
     show SourceEdit;
-import 'package:analysis_server/src/utilities/extensions/formatter_options.dart';
 import 'package:analyzer/dart/analysis/features.dart';
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ -17,6 +16,7 @@ import 'package:analyzer/source/line_info.dart';
 import 'package:analyzer/source/source.dart';
 import 'package:analyzer/src/dart/scanner/scanner.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart' as plugin;
+import 'package:analyzer_plugin/src/utilities/formatter.dart';
 import 'package:dart_style/dart_style.dart' hide TrailingCommas;
 
 /// Checks whether a string contains only characters that are allowed to differ
@@ -130,25 +130,10 @@ ErrorOr<List<TextEdit>?> generateEditsForFormatting(
 }) {
   var unformattedSource = result.content;
 
-  var formatterOptions = result.analysisOptions.formatterOptions;
-  // The analysis options page width always takes priority over the default from
-  // the LSP configuration.
-  var effectivePageWidth = formatterOptions.pageWidth ?? defaultPageWidth;
-  var effectiveTrailingCommas = formatterOptions.dartStyleTrailingCommas;
-  var effectiveLanguageVersion = result.unit.languageVersion.effective;
-
-  var code = SourceCode(unformattedSource);
-  SourceCode formattedResult;
+  var formatter = createFormatter(result, defaultPageWidth: defaultPageWidth);
+  String formattedSource;
   try {
-    // Create a new formatter on every request because it may contain state that
-    // affects repeated formats.
-    // https://github.com/dart-lang/dart_style/issues/1337
-    var formatter = DartFormatter(
-      pageWidth: effectivePageWidth,
-      trailingCommas: effectiveTrailingCommas,
-      languageVersion: effectiveLanguageVersion,
-    );
-    formattedResult = formatter.formatSource(code);
+    formattedSource = formatter.format(unformattedSource);
   } on FormatterException {
     // If the document fails to parse, just return no edits to avoid the
     // use seeing edits on every save with invalid code (if LSP gains the
@@ -156,7 +141,6 @@ ErrorOr<List<TextEdit>?> generateEditsForFormatting(
     // we may wish to change this to return an error for that case).
     return success(null);
   }
-  var formattedSource = formattedResult.text;
 
   if (formattedSource == unformattedSource) {
     return success(null);

pkg/analyzer_plugin/lib/src/utilities/change_builder/change_builder_dart.dart

@@ -23,6 +23,7 @@ import 'package:analyzer_plugin/src/utilities/change_builder/change_builder_core
 import 'package:analyzer_plugin/src/utilities/charcodes.dart';
 import 'package:analyzer_plugin/src/utilities/directive_sort.dart';
 import 'package:analyzer_plugin/src/utilities/extensions/resolved_unit_result.dart';
+import 'package:analyzer_plugin/src/utilities/formatter.dart';
 import 'package:analyzer_plugin/src/utilities/library.dart';
 import 'package:analyzer_plugin/src/utilities/string_utilities.dart';
 import 'package:analyzer_plugin/utilities/change_builder/change_builder_core.dart';
@@ -1895,16 +1896,14 @@ class DartFileEditBuilderImpl extends FileEditBuilderImpl
       }
     }
 
-    var languageVersion = resolvedUnit.libraryElement.languageVersion.effective;
-    var formattedResult = DartFormatter(languageVersion: languageVersion)
-        .formatSource(
-          SourceCode(
-            newContent,
-            isCompilationUnit: true,
-            selectionStart: newRangeOffset,
-            selectionLength: newRangeLength,
-          ),
-        );
+    var formatter = createFormatter(resolvedUnit);
+    var formattedResult = formatter.formatSource(
+      SourceCode(
+        newContent,
+        selectionStart: newRangeOffset,
+        selectionLength: newRangeLength,
+      ),
+    );
 
     replaceEdits(
       range,

pkg/analyzer_plugin/lib/src/utilities/formatter.dart

@@ -0,0 +1,42 @@
+// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:analyzer/dart/analysis/features.dart';
+import 'package:analyzer/dart/analysis/results.dart';
+import 'package:analyzer/src/dart/analysis/experiments.dart'
+    show ExperimentStatus;
+import 'package:analyzer_plugin/src/utilities/extensions/formatter_options.dart';
+import 'package:dart_style/dart_style.dart';
+
+/// A list of all features that are currently enabled by an experiment flag.
+final _allowedExperiments = ExperimentStatus.knownFeatures.values
+    .where((feature) => feature.status == FeatureStatus.future)
+    .toList();
+
+/// Creates a formatter with the appropriate settings for [result].
+DartFormatter createFormatter(
+  ParsedUnitResult result, {
+  int? defaultPageWidth,
+}) {
+  var featureSet = result.unit.featureSet;
+  var formatterOptions = result.analysisOptions.formatterOptions;
+  var effectivePageWidth = formatterOptions.pageWidth ?? defaultPageWidth;
+  var effectiveTrailingCommas = formatterOptions.dartStyleTrailingCommas;
+  var effectiveLanguageVersion = result.unit.languageVersion.effective;
+  return DartFormatter(
+    pageWidth: effectivePageWidth,
+    trailingCommas: effectiveTrailingCommas,
+    languageVersion: effectiveLanguageVersion,
+    experimentFlags: _getExperiments(featureSet),
+  );
+}
+
+/// Gets the list of experiment strings enabled by [featureSet] that are
+/// required for future features.
+List<String> _getExperiments(FeatureSet featureSet) {
+  return _allowedExperiments
+      .where(featureSet.isEnabled)
+      .map((feature) => feature.enableString)
+      .toList();
+}

pkg/analyzer_plugin/test/src/utilities/formatter_test.dart

@@ -0,0 +1,59 @@
+// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:analyzer_plugin/src/utilities/formatter.dart';
+import 'package:analyzer_testing/experiments/experiments.dart';
+import 'package:dart_style/dart_style.dart';
+import 'package:pub_semver/pub_semver.dart';
+import 'package:test/test.dart';
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+import '../../support/abstract_single_unit.dart';
+
+void main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(FormatterTest);
+  });
+}
+
+@reflectiveTest
+class FormatterTest extends AbstractSingleUnitTest {
+  Future<void> test_experiments() async {
+    await resolveTestCode('');
+    var formatter = createFormatter(result);
+    expect(formatter.experimentFlags, experimentsForTests);
+  }
+
+  Future<void> test_languageVersion_default() async {
+    await resolveTestCode('');
+    var formatter = createFormatter(result);
+    expect(formatter.languageVersion, DartFormatter.latestLanguageVersion);
+  }
+
+  Future<void> test_languageVersion_override() async {
+    await resolveTestCode('// @dart=2.12');
+    var formatter = createFormatter(result);
+    expect(formatter.languageVersion, Version(2, 12, 0));
+  }
+
+  Future<void> test_pageWidth() async {
+    newFile(convertPath('$testPackageRootPath/analysis_options.yaml'), '''
+formatter:
+  page_width: 123
+''');
+    await resolveTestCode('');
+    var formatter = createFormatter(result);
+    expect(formatter.pageWidth, 123);
+  }
+
+  Future<void> test_trailingCommas() async {
+    newFile(convertPath('$testPackageRootPath/analysis_options.yaml'), '''
+formatter:
+  trailing_commas: preserve
+''');
+    await resolveTestCode('');
+    var formatter = createFormatter(result);
+    expect(formatter.trailingCommas, TrailingCommas.preserve);
+  }
+}

pkg/analyzer_plugin/test/src/utilities/test_all.dart

@@ -7,6 +7,7 @@ import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'change_builder/test_all.dart' as change_builder;
 import 'client_uri_converter_test.dart' as client_uri_converter;
 import 'completion/test_all.dart' as completion;
+import 'formatter_test.dart' as formatter;
 import 'navigation/test_all.dart' as navigation;
 import 'string_utilities_test.dart' as string_utilities;
 import 'visitors/test_all.dart' as visitors;
@@ -16,6 +17,7 @@ void main() {
     change_builder.main();
     client_uri_converter.main();
     completion.main();
+    formatter.main();
     navigation.main();
     string_utilities.main();
     visitors.main();