`dart format` causes lines to exceed 80 character limit in Dart 3.7

[felangel]

Description

Running dart format with Dart 3.7 reformats code such that some lines exceed the 80 character limit and trigger the lines_longer_than_80_chars lint. I would expect the new formatter to adhere to the max line length configured (by default 80 characters).

Reproduction Steps

pubspec.yaml

name: example
publish_to: none

environment:
  sdk: ^3.7.0

dev_dependencies:
  mocktail: ^1.0.0
  test: ^1.0.0

analysis_options.yaml

linter:
  rules:
    - lines_longer_than_80_chars
formatter:
    page_width: 80

main.dart

import 'dart:convert';
import 'dart:io';

import 'package:mocktail/mocktail.dart';
import 'package:test/test.dart';

class BuildProcess {
  Stream<List<int>> get stdout => throw UnimplementedError();
}

class MockBuildProcess extends Mock implements BuildProcess {}

void main() {
  group('Root Group', () {
    group('Nested Group', () {
      test('Test', () {
        final buildProcess = MockBuildProcess();
        when(() => buildProcess.stdout).thenAnswer(
          (_) => Stream.fromIterable(
            [
              '[  +10 ms] Generating /Users/bryanoltman/Documents/sandbox/notification_extension/android/app/src/main/java/io/flutter/plugins/GeneratedPluginRegistrant.java',
              '[  +50 ms] executing: [/Users/bryanoltman/Documents/sandbox/notification_extension/ios/] /usr/bin/arch -arm64e xcrun xcodebuild -list',
              '[+32333 ms] Command line invocation:',
              '[   +6 ms] Exit code 0 from: mkfifo /var/folders/64/dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/pipe_to_stdout',
              '[+62601 ms] Running Xcode build... (completed in 62.6s)',
              '[        ]  └─Compiling, linking and signing...',
              '[+5925 ms] Command line invocation:',
              '/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild -configuration Release VERBOSE_SCRIPT_LOGGING=YES -workspace Runner.xcworkspace -scheme Runner -sdk iphoneos -destination generic/platform=iOS SCRIPT_OUTPUT_STREAM_FILE=/var/folders/64/dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/pipe_to_stdout -resultBundlePath /var/folders/64/dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/temporary_xcresult_bundle -resultBundleVersion 3 FLUTTER_SUPPRESS_ANALYTICS=true COMPILER_INDEX_STORE_ENABLE=NO -archivePath /Users/bryanoltman/Documents/sandbox/notification_extension/build/ios/archive/Runner archive',
            ]
            .map((line) => 'prefix$line${Platform.lineTerminator}')
            .map(utf8.encode),
          ),
        );
      });
    });
  });
}

1. Run dart format

$ dart format .
Formatted main.dart
Formatted 1 file (1 changed) in 0.12 seconds.

1. Observe the reformatted code now exceeds 80 characters

$ dart analyze .           
Analyzing ....                         0.5s

   info • main.dart:29:81 • The line length exceeds the 80-character limit. Try breaking the line across multiple lines. • lines_longer_than_80_chars

1 issue found.

The diff from running dart format:

import 'dart:convert';
import 'dart:io';

import 'package:mocktail/mocktail.dart';
import 'package:test/test.dart';

class BuildProcess {
  Stream<List<int>> get stdout => throw UnimplementedError();
}

class MockBuildProcess extends Mock implements BuildProcess {}

void main() {
  group('Root Group', () {
    group('Nested Group', () {
      test('Test', () {
        final buildProcess = MockBuildProcess();
        when(() => buildProcess.stdout).thenAnswer(
          (_) => Stream.fromIterable(
            [
              '[  +10 ms] Generating /Users/bryanoltman/Documents/sandbox/notification_extension/android/app/src/main/java/io/flutter/plugins/GeneratedPluginRegistrant.java',
              '[  +50 ms] executing: [/Users/bryanoltman/Documents/sandbox/notification_extension/ios/] /usr/bin/arch -arm64e xcrun xcodebuild -list',
              '[+32333 ms] Command line invocation:',
              '[   +6 ms] Exit code 0 from: mkfifo /var/folders/64/dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/pipe_to_stdout',
              '[+62601 ms] Running Xcode build... (completed in 62.6s)',
              '[        ]  └─Compiling, linking and signing...',
              '[+5925 ms] Command line invocation:',
              '/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild -configuration Release VERBOSE_SCRIPT_LOGGING=YES -workspace Runner.xcworkspace -scheme Runner -sdk iphoneos -destination generic/platform=iOS SCRIPT_OUTPUT_STREAM_FILE=/var/folders/64/dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/pipe_to_stdout -resultBundlePath /var/folders/64/dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/temporary_xcresult_bundle -resultBundleVersion 3 FLUTTER_SUPPRESS_ANALYTICS=true COMPILER_INDEX_STORE_ENABLE=NO -archivePath /Users/bryanoltman/Documents/sandbox/notification_extension/build/ios/archive/Runner archive',
-            ]
-            .map((line) => 'prefix$line${Platform.lineTerminator}')
-            .map(utf8.encode),
+            ].map((line) => 'prefix$line${Platform.lineTerminator}').map(utf8.encode),
          ),
        );
      });
    });
  });
}
}

$ dart --version
Dart SDK version: 3.7.0 (stable) (Wed Feb 5 04:53:58 2025 -0800) on "macos_arm64"

[felangel]

Tagging @munificent since I believe you were heavily involved in the new Dart formatter changes.

[ChunhThanhDe]

I had the same issue and reported it on Flutter, hope it will be fixed soon

flutter/flutter#163827

[munificent]

This is working as expected, or at least as I expect it. But I agree the output isn't great.

With the new formatting style, a collection literal as a method target gets its body indented if the method chain splits. So where the old style would give you:

[
  stuff,
  more,
]
    .map(fn)
    .map(another);

The new formatter gives you:

[
      stuff,
      more,
    ]
    .map(fn)
    .map(another);

Neither of these look great. I'd like to do what @loic-sharma suggests in #1583 and have it produce:

[
  stuff,
  more,
]
.map(fn)
.map(another);

But I haven't figured out how to do that.

In the meantime, in the example here, the formatter is trying to give you:

void main() {
  group('Root Group', () {
    group('Nested Group', () {
      test('Test', () {
        final buildProcess = MockBuildProcess();
        when(() => buildProcess.stdout).thenAnswer(
          (_) => Stream.fromIterable(
            [
                  '[  +10 ms] Generating /Users/bryanoltman/Documents/sandbox/notification_extension/android/app/src/main/java/io/flutter/plugins/GeneratedPluginRegistrant.java',
                  '[  +50 ms] executing: [/Users/bryanoltman/Documents/sandbox/notification_extension/ios/] /usr/bin/arch -arm64e xcrun xcodebuild -list',
                  '[+32333 ms] Command line invocation:',
                  '[   +6 ms] Exit code 0 from: mkfifo /var/folders/64/dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/pipe_to_stdout',
                  '[+62601 ms] Running Xcode build... (completed in 62.6s)',
                  '[        ]  └─Compiling, linking and signing...',
                  '[+5925 ms] Command line invocation:',
                  '/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild -configuration Release VERBOSE_SCRIPT_LOGGING=YES -workspace Runner.xcworkspace -scheme Runner -sdk iphoneos -destination generic/platform=iOS SCRIPT_OUTPUT_STREAM_FILE=/var/folders/64/dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/pipe_to_stdout -resultBundlePath /var/folders/64/dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/temporary_xcresult_bundle -resultBundleVersion 3 FLUTTER_SUPPRESS_ANALYTICS=true COMPILER_INDEX_STORE_ENABLE=NO -archivePath /Users/bryanoltman/Documents/sandbox/notification_extension/build/ios/archive/Runner archive',
                ]
                .map((line) => 'prefix$line${Platform.lineTerminator}')
                .map(utf8.encode),
          ),
        );
      });
    });
  });
}

But because of all of the long string literals inside the collection, that produces a lot of overflow characters. The formatter always tries to minimize the amount of text that exceeds the page width. So instead of the above, it gives you:

void main() {
  group('Root Group', () {
    group('Nested Group', () {
      test('Test', () {
        final buildProcess = MockBuildProcess();
        when(() => buildProcess.stdout).thenAnswer(
          (_) => Stream.fromIterable(
            [
              '[  +10 ms] Generating /Users/bryanoltman/Documents/sandbox/notification_extension/android/app/src/main/java/io/flutter/plugins/GeneratedPluginRegistrant.java',
              '[  +50 ms] executing: [/Users/bryanoltman/Documents/sandbox/notification_extension/ios/] /usr/bin/arch -arm64e xcrun xcodebuild -list',
              '[+32333 ms] Command line invocation:',
              '[   +6 ms] Exit code 0 from: mkfifo /var/folders/64/dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/pipe_to_stdout',
              '[+62601 ms] Running Xcode build... (completed in 62.6s)',
              '[        ]  └─Compiling, linking and signing...',
              '[+5925 ms] Command line invocation:',
              '/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild -configuration Release VERBOSE_SCRIPT_LOGGING=YES -workspace Runner.xcworkspace -scheme Runner -sdk iphoneos -destination generic/platform=iOS SCRIPT_OUTPUT_STREAM_FILE=/var/folders/64/dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/pipe_to_stdout -resultBundlePath /var/folders/64/dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/temporary_xcresult_bundle -resultBundleVersion 3 FLUTTER_SUPPRESS_ANALYTICS=true COMPILER_INDEX_STORE_ENABLE=NO -archivePath /Users/bryanoltman/Documents/sandbox/notification_extension/build/ios/archive/Runner archive',
            ].map((line) => 'prefix$line${Platform.lineTerminator}').map(utf8.encode),
          ),
        );
      });
    });
  });
}

That still overflows, but the total number of overflow characters is less because the strings aren't indented as far. So the formatter considers that the better solution.

I'm going to go ahead and close this issue because the formatter is working as intended here. If I'm able to fix #1583, then you'll get the output you want.

In the meantime, you could split string literals, like:

void main() {
  group('Root Group', () {
    group('Nested Group', () {
      test('Test', () {
        final buildProcess = MockBuildProcess();
        when(() => buildProcess.stdout).thenAnswer(
          (_) => Stream.fromIterable(
            [
                  '[  +10 ms] Generating /Users/bryanoltman/Documents/sandbox/'
                      'notification_extension/android/app/src/main/java/io/'
                      'flutter/plugins/GeneratedPluginRegistrant.java',
                  '[  +50 ms] executing: [/Users/bryanoltman/Documents/'
                      'sandbox/'
                      'notification_extension/ios/] /usr/bin/arch -arm64e '
                      'xcrun xcodebuild -list',
                  '[+32333 ms] Command line invocation:',
                  '[   +6 ms] Exit code 0 from: mkfifo /var/folders/64/'
                      'dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/'
                      'flutter_ios_build_temp_dirUAyStV/pipe_to_stdout',
                  '[+62601 ms] Running Xcode build... (completed in 62.6s)',
                  '[        ]  └─Compiling, linking and signing...',
                  '[+5925 ms] Command line invocation:',
                  '/Applications/Xcode.app/Contents/Developer/usr/bin/'
                      'xcodebuild -configuration Release '
                      'VERBOSE_SCRIPT_LOGGING=YES -workspace '
                      'Runner.xcworkspace -scheme Runner -sdk iphoneos '
                      '-destination generic/platform=iOS '
                      'SCRIPT_OUTPUT_STREAM_FILE=/var/folders/64/'
                      'dj6krpq1093dmx08dy4r1cwh0000gn/T/'
                      'flutter_tools.WDvaE9/flutter_ios_build_temp_dirUAyStV/'
                      'pipe_to_stdout -resultBundlePath /var/folders/64/'
                      'dj6krpq1093dmx08dy4r1cwh0000gn/T/flutter_tools.WDvaE9/'
                      'flutter_ios_build_temp_dirUAyStV/'
                      'temporary_xcresult_bundle -resultBundleVersion 3 '
                      'FLUTTER_SUPPRESS_ANALYTICS=true '
                      'COMPILER_INDEX_STORE_ENABLE=NO -archivePath '
                      '/Users/bryanoltman/Documents/sandbox/'
                      'notification_extension/build/ios/archive/Runner archive',
                ]
                .map((line) => 'prefix$line${Platform.lineTerminator}')
                .map(utf8.encode),
          ),
        );
      });
    });
  });
}

Though in this case, I admit that it's a chore to split those strings.