Mark `IconData` `final` and `@mustBeConst` by dcharkes · Pull Request #181345 · flutter/flutter

dcharkes · 2026-01-22T20:38:26Z

2026-03-18 update, no push-back on the breaking changes. Migrations done downstream:

Applying breaking changes:

See the issue descriptions for the motivation.

It seems we have a non-const use in the Remote Flutter Widgets:

https://github.com/flutter/packages/blob/d0c7d1fad77331724882614345137221c77d5758/packages/rfw/lib/src/flutter/argument_decoders.dart#L801-L811

The package itself makes no mention of it not work with the Flutter Icon Tree Shaker. If it's a know issue that the icon tree shaker is not supported with package:rfw we can add // ignore: non_const_argument_for_const_parameter. (flutter/packages#10858)

flutter-dashboard · 2026-01-22T20:38:30Z

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gemini-code-assist

Code Review

This pull request makes IconData a final class and adds @mustBeConst to its constructor parameters, which is a great improvement for enforcing immutability and enabling compiler optimizations. My review includes a suggestion to apply the @mustBeConst annotation to all constructor parameters for completeness and consistency.

packages/flutter/lib/src/widgets/icon_data.dart

flutter-dashboard · 2026-01-22T21:07:02Z

This pull request has been changed to a draft. The currently pending flutter-gold status will not be able to resolve until a new commit is pushed or the change is marked ready for review again.

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Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

A patch for: * flutter/flutter#181345

justinmc

Seems like a win to me. I want to be sure that Material and Cupertino don't have any breaking usages of IconData though (doesn't look like it to me if I grep through the codebase).

I haven't worked on the rfw codebase, but taking a look it seems like it won't be possible for it to support icon tree shaking. That makes sense for a package that can execute code at runtime that is unknown at compile time, so I expect users of rfw will understand.

loic-sharma · 2026-01-22T22:44:02Z

FYI, some folks use the enum Foo implements IconData pattern, which I believe will be broken if we make IconData final. Some examples:

This comment explains why folks use this pattern: #126261 (comment)

One of the major if not the biggest value from keeping such values as an enum for me is the values property provided by the language itself. I use values in widgetbook; keeping such list manually is error-prone

However, it seems that this improvement is nice enough to warrant breaking breaking this pattern.

dcharkes · 2026-01-23T06:39:37Z

One of the major if not the biggest value from keeping such values as an enum for me is the values property provided by the language itself. I use values in widgetbook; keeping such list manually is error-prone

@Albert221 You rely on subtyping IconData to get an automated .values. Could you elaborate on how .values is used in the code? My understanding of the tree-shaking mechanism is if .values is used, then all the codePoints are reachable, which means you'll never get any code points tree-shaken from the fonts.

Albert221 · 2026-01-23T14:51:25Z

@dcharkes Hey! That is correct, they're not tree-shaken at all if I use .values. But I use it in my Widgetbook instance, where I want my developers and designers to have access to the whole library of available icons. The widgetbook is a separate package, so it doesn't hurt me. In the mobile app itself, I don't use .values at all for icons to have the tree-shaking :)

Setup we use for many projects and that many clients use in their codebases usually is:

app_package (depends on app_design_system)
app_design_system my custom icon datas
design_system_widgetbook (depends on app_design_system) where devs/designers can preview all available components, colors, icons, etc.

dcharkes · 2026-01-23T15:29:07Z

Thanks for the reply @Albert221 🙏

Okay, then the tree-shaking is WAI in your use case. I suppose asking you to migrate away from enum ... implements IconData is asking you to break all your users? Would it be possible to migrate to the same structure as the other icon sets, an unrelated class around it with static const members and a code-genned(?) values const.

Albert221 · 2026-01-23T15:54:12Z

Not ideal but possible, yes. If that's for the sake of making Flutter better for everyone, go for it

dcharkes · 2026-01-23T16:03:09Z

Not ideal but possible, yes. If that's for the sake of making Flutter better for everyone, go for it

🙏 👍 !

Okay, when we do this as a breaking change, then older versions of WidgetBook will stop working in a newer Flutter version. Please let me know when you have migrated, and if the migration didn't caused breaking change issues on your side (looking at the code I don't think so, but I might be wrong). I'll wait with landing anything on master for a while (and need to file a breaking change announcement as well) to ensure no one breaks.

We are introducing the `@mustBeConst` annotation on `IconData` parameters that must be const in order for the icon tree shaker to work: * flutter/flutter#181344 * flutter/flutter#181345 RFW does not support tree-shaking. This PR adds the lint ignores with an explicit comment that the lint ignore is intentional.

Roll in flutter/packages#11216. To make flutter/flutter#181345 go green.

Roll in flutter/devtools#9718. To make flutter/flutter#181345 go green.

gemini-code-assist

Code Review

This pull request enhances IconData by making it a final class and annotating its constructor parameters with @mustBeConst. These changes improve const-correctness, which is beneficial for optimizations like icon tree shaking. The implementation is sound, and I have one suggestion to apply the @mustBeConst annotation more consistently across all relevant parameters.

gemini-code-assist · 2026-03-18T20:37:03Z

packages/flutter/lib/src/widgets/icon_data.dart

    this.matchTextDirection = false,
    this.fontFamilyFallback,


For consistency and to ensure all properties used by the icon tree shaker are constant, consider also annotating matchTextDirection and fontFamilyFallback with @mustBeConst. All fields of IconData are marked with @_retainForIconTreeShaker, which suggests they are all relevant for tree shaking.

// ignore: experimental_member_use @mustBeConst this.matchTextDirection = false, // ignore: experimental_member_use @mustBeConst this.fontFamilyFallback,

Nope they are not read:

final Object? package = iconDataMap['fontPackage']; final Object? fontFamily = iconDataMap['fontFamily']; final Object? codePoint = iconDataMap['codePoint'];

packages/flutter_tools/lib/src/build_system/targets/icon_tree_shaker.dart

Piinks · 2026-03-19T17:34:19Z

Do we have a migration guide staged in flutter/website for this? There is a template we used for explaining the context of the breakage and how to migrate, which ends up listed here: https://docs.flutter.dev/release/breaking-changes

dcharkes · 2026-03-19T17:58:19Z

Do we have a migration guide staged in flutter/website for this? There is a template we used for explaining the context of the breakage and how to migrate, which ends up listed here: https://docs.flutter.dev/release/breaking-changes

We haven't landed that yet, do we do that first?

Migration guide for IconData website#13005

Piinks · 2026-03-19T18:00:13Z

We haven't landed that yet, do we do that first?

Nah, usually right after this lands. We just make sure they're both ready so we don't forget to add the guide later. :)

Piinks

LGTM

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dcharkes marked this pull request as draft January 22, 2026 21:06

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[rfw] Workaround IconData parameters must be const

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A patch for: * flutter/flutter#181345

dcharkes mentioned this pull request Jan 22, 2026

[rfw] Workaround IconData parameters must be const flutter/packages#10858

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Migration guide for IconData flutter/website#13005

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This was referenced Mar 10, 2026

[rfw] Opt out of icon tree shaking flutter/packages#11216

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Icon tree shaking ignores IconData implementation getters #126261

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dcharkes force-pushed the icon-data-class-modifier branch from 6c1c25c to 63b8163 Compare March 10, 2026 08:43

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Update git hash in flutter_packages.test flutter/tests#482

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Update git hash in flutter_packages.test (#482)

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Roll in flutter/packages#11216. To make flutter/flutter#181345 go green.

dcharkes force-pushed the icon-data-class-modifier branch from 8569697 to f9750dd Compare March 16, 2026 18:21

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Explicitly opt out of Flutter Icon Tree Shaking flutter/devtools#9718

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Update git hash for flutter/devtools flutter/tests#483

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Update git hash for flutter/devtools (#483)

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Roll in flutter/devtools#9718. To make flutter/flutter#181345 go green.

Mark IconData final and @mustBeConst

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