feat: add #[frb(sync)] to RustState::new for synchronous initialization

[guoyu07]

Fix “tearoff_of_generative_constructor_of_abstract_class” in rust_ui_counter

Changes

Fixes error in frb_example/rust_ui_counter/ui/lib/main.dart

void main() => runRustApp(body: body, state: RustState.new);

Details about the 'tearoff_of_generative_constructor_of_abstract_class' diagnostic produced by the Dart analyzer.

A generative constructor of an abstract class can't be torn off.
Try tearing off a constructor of a concrete class, or a non-generative constructor.

abstract class RustState implements RustOpaqueInterface

To resolve this, when added this #[frb(sync)] attribute to pub fn new() -> Self, the generated wrapper class in Dart, allows us to use its constructor tear‑off safely.

Version

• flutter_rust_bridge: 2.11.1
• rust: 1.93.1
• dart: Dart SDK version: 3.11.0 (stable)
• flutter: 3.41.2 (channel stable)
• os: MacOS Tahoe 26.0.1

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a Dart analyzer diagnostic that occurred when attempting to tear off a generative constructor of an abstract class, specifically RustState.new, which is generated by flutter_rust_bridge. By introducing a #[frb(sync)] attribute to the Rust RustState::new function, the generated Dart code now correctly supports constructor tear-offs, allowing for a more idiomatic and error-free initialization pattern in the Dart UI.

Highlights

• Dart Analyzer Error Resolution: Resolved the "tearoff_of_generative_constructor_of_abstract_class" diagnostic in Dart by modifying the Rust code, which previously prevented safe constructor tear-offs for the generated RustState class.
• Synchronous Initialization Attribute: Added the #[frb(sync)] attribute to the RustState::new function in frb_example/rust_ui_counter/src/app.rs. This change enables the generated Dart wrapper class to safely use its constructor tear-off, aligning with Dart's requirements for abstract class constructors.

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Changelog

• frb_example/rust_ui_counter/src/app.rs
  • Added #[frb(sync)] attribute to pub fn new() -> Self.

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[gemini-code-assist]

Code Review

This pull request introduces the #[frb(sync)] attribute to the RustState::new function. This change correctly configures the code generator to produce a synchronous Dart function for state initialization, resolving a tearoff_of_generative_constructor_of_abstract_class error in the Dart UI code. The change is appropriate and effectively addresses the issue.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (172230f) to head (b60d2c3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3003   +/-   ##
=======================================
  Coverage   98.57%   98.57%           
=======================================
  Files         464      464           
  Lines       19202    19202           
=======================================
  Hits        18928    18928           
  Misses        274      274           

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[fzyzcjy]

Great job! But a bit confused:

flutter_rust_bridge/frb_example/rust_ui_counter/ui/flutter_rust_bridge.yaml

Line 5 in 172230f

default_dart_async: false

it seems it is already sync by default. could you please check a bit about your config yaml

[guoyu07]

default_dart_async: false

My config is the default flutter_rust_bridge.yaml:

rust_input: crate::api
rust_root: rust/
dart_output: lib/src/rust

I've tried to add this option with true and false, this error still exits.

[fzyzcjy]

(replied in #3004)