[Bug] Compared to Dart 3.6.2, IntelliJ "format selected text" gives different output than "format file"

[Mike278]

I have a code base that hasn't fully adopted the official dart formatter. To aid the incremental adoption, I often use the feature in IntelliJ that lets you apply formatting to just a selected block of code. This feature seemed deterministic in the sense that you'd get the same formatting for that block if you instead chose to format the entire file.

After upgrading from Dart 3.6.2 to 3.8.0, formatting a selected block of code no longer gives the same results as formatting the entire file. The format of the selected text seems to be influenced by the trailing_commas config, but not in an expected way.

IntelliJ version: 2024.2.5

Given the following:

void foo({required String aaa, required int bb, required int ccccccccccccccccccccccc,}) {
  final a = (aaaaaaaaaaaa: 1, bbbbbbbbbbb: 2, cccccccccccccc: 3, ddddddddddddddddd: 4);
  final b = (aaaaaaaaa: 1, bbbbbbbbbbb: 2, cccccccccccccccc: 3, dddddddddddddddd: 4, );
}

Expected:

1. Select all of line 1

2. Code > Reformat File > check off "Selected text" > Run

3. Each parameter should be split to a new line

4. Select all of line 2

5. Code > Reformat File > check off "Selected text" > Run

6. Each record field should be split to a new line

7. Select all of line 3

8. Code > Reformat File > check off "Selected text" > Run

9. Each record field should be split to a new line

This works in Dart 3.6.2.

In Dart 3.8.0:

void foo(
    {required String aaa, required int bb, required int ccccccccccccccccccccccc,}) {
  final a = (aaaaaaaaaaaa: 1, bbbbbbbbbbb: 2, cccccccccccccc: 3, ddddddddddddddddd: 4);
  final b = (aaaaaaaaa: 1, bbbbbbbbbbb: 2, cccccccccccccccc: 3, dddddddddddddddd: 4, );
}

A couple interesting notes:

• Add a comma to the end of line 2:

void foo({required String aaa, required int bb, required int ccccccccccccccccccccccc,}) {
  final a = (aaaaaaaaaaaa: 1, bbbbbbbbbbb: 2, cccccccccccccc: 3, ddddddddddddddddd: 4,);
  final b = (aaaaaaaaa: 1, bbbbbbbbbbb: 2, cccccccccccccccc: 3, dddddddddddddddd: 4, );
}

• Select all of line 3
• Code > Reformat File > check off "Selected text" > Run
• Each record field is split to a new line as expected:

void foo({required String aaa, required int bb, required int ccccccccccccccccccccccc,}) {
  final a = (aaaaaaaaaaaa: 1, bbbbbbbbbbb: 2, cccccccccccccc: 3, ddddddddddddddddd: 4,);
  final b = (
    aaaaaaaaa: 1,
    bbbbbbbbbbb: 2,
    cccccccccccccccc: 3,
    dddddddddddddddd: 4,
  );
}

• Code > Reformat File > check off "Whole file" > Run yields the expected results:

void foo({
  required String aaa,
  required int bb,
  required int ccccccccccccccccccccccc,
}) {
  final a = (
    aaaaaaaaaaaa: 1,
    bbbbbbbbbbb: 2,
    cccccccccccccc: 3,
    ddddddddddddddddd: 4,
  );
  final b = (
    aaaaaaaaa: 1,
    bbbbbbbbbbb: 2,
    cccccccccccccccc: 3,
    dddddddddddddddd: 4,
  );
}

[munificent]

Thank you for the detailed repro steps!

I believe this is an IntelliJ issue. IntelliJ has its own internal formatter that does... well, I don't really know what it does. But it's not using dart_style. As far as I can tell, when you use "Code > Reformat Code" and "Code > Reformat File...", you're getting IntelliJ's own internal formatter and not using dart_style / dart format. The dart_style package doesn't actually support reformatting only a selection (#92) because I never figured out a clean way to implement it.

I'm not sure why this behavior would have changed between Dart 3.6 and 3.8, but maybe there's something else going on. @pq or @jwren, do you have any additional info? Is "Code > Reformat File..." actually using dart_style under the hood?

[Mike278]

Thanks for the reply :)

It certainly feels like it's using dart_style under the hood - it knows Dart-isms like how to split on trailing commas, it stops working when I change the file extension to something other than .dart, and it even responds to changes in the trailing_commas config, but not in any kind of predictable or expected way. It's almost like it's making a virtual file with the selected content and handing that off to a dart_style instance (from where, who knows), and it was only by happenstance that it ever worked.

[pq]

Yep. I think you're right, I haven't dug too deeply but I think the heavy lifting is happening here:

https://github.com/JetBrains/intellij-plugins/blob/master/Dart/src/com/jetbrains/lang/dart/ide/formatter/DartIndentProcessor.java

Looking ahead, I wonder if it wouldn't be better to remove selection formatting in favor of use the dart formatter exclusively? The current state of things is confusing!

[munificent]

If it's possible to remove access to IntelliJ's built-in formatter, that would be amazing. There have been many bugs over the years where users inadvertently use IntelliJ's formatter and then file an issue because it doesn't format correctly.

[jwren]

The formatter was removed years ago, much to the dismay of power users.

By design, we were told to not call directly out to dartfmt as using a warmed up DAS is faster, the Dart IJ plugin uses the API declared here for dart formatting: https://htmlpreview.github.io/?https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server/doc/api.html#request_edit.format

Formatting cannot happen on each keystroke, but where we want code to be readable as the user is typing, extensions such as the DartIndentProcessor can be used. (This is not a formatter, and does not call dart format)

Many times users want to reformat a subset of Dart code instead of having the requirement of having an entire file reformatter. JetBrains added the functionality to emulate this by calling dart format, and only applying the changes within the selected text, leaving the code outside of the selection as-is. Unfortunately, it seems that with language changes, or the formatting with these versions of the Dart SDK, this has broken. The simplest UX here is probably to just remove the "Reformat Selected Text" ability in the Dart IJ plugin.

[jwren]

Some links: https://youtrack.jetbrains.com/issue/IDEA-328539/Dart-formatter-use-dartfmt-tool-when-reformatting-file-parts-not-only-the-whole-file, https://youtrack.jetbrains.com/issue/IDEA-328303/Dart-formatter-use-dartfmt-tool-always-remove-legacy-formatting-settings

And the change we need to revert (not automatically): JetBrains/intellij-plugins@59ecda7

[munificent]

Thanks for all the context! I'm going to go ahead and close this out because there's nothing to do in dart_style for this, except for maybe directly supporting formatting selections, which is tracked by #92.