feat(turbopack): support useDefineForClassFields

[fireairforce]

issue: utooland/utoo#2450

[gemini-code-assist]

Summary of Changes

Hello @fireairforce, 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 significantly enhances Turbopack's ECMAScript transformation pipeline by implementing full support for the useDefineForClassFields TypeScript compiler option. This ensures that class field declarations are handled correctly according to the specified configuration, particularly addressing the removal of uninitialized class fields when operating in the legacy TypeScript mode. The change improves compatibility and correctness for projects relying on this specific TypeScript behavior.

Highlights

• Support for useDefineForClassFields: This pull request introduces explicit support for the useDefineForClassFields compiler option within Turbopack's ECMAScript transformations, specifically for TypeScript and Decorator processing.
• New Class Field Stripping Logic: A new visitor, StripUninitializedClassFields, has been added. This visitor is responsible for removing class properties and private properties that lack an initializer, which is the expected behavior when useDefineForClassFields is set to false (legacy TypeScript behavior).
• Integration into Transforms: The useDefineForClassFields parameter is now properly consumed by both the TypeScript and Decorators transformation variants. For TypeScript, the new stripping logic is applied conditionally, and for Decorators, the option is passed directly to the underlying SWC transform configuration.

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[gemini-code-assist]

Code Review

This pull request adds support for the useDefineForClassFields option in Turbopack's ecmascript transform pipeline. A new SWC visitor, StripUninitializedClassFields, is introduced to handle the TypeScript legacy behavior of removing uninitialized class fields when useDefineForClassFields is false. While the implementation for the plain TypeScript transform path is correct, there's an inconsistency in the decorators transform path that leads to different behavior. My review includes a suggestion to fix this inconsistency.

[gemini-code-assist]

There's an inconsistency in how useDefineForClassFields: false is handled between the TypeScript and Decorators transform arms.

In the TypeScript arm, uninitialized class fields are correctly stripped to match TypeScript's legacy behavior. However, in this Decorators arm, the use_define_for_class_fields: false option is passed to SWC's decorators transform, which will likely transpile uninitialized fields to this.field = undefined; instead of stripping them.

To ensure consistent behavior, uninitialized fields should also be stripped in this path before applying the decorators transform when useDefineForClassFields is false.

                use swc_core::ecma::transforms::proposal::decorators::{Config, decorators};

                // When useDefineForClassFields is false (TypeScript legacy behavior),
                // class field declarations without initializers should be stripped
                // as they are type-only declarations.
                if !*use_define_for_class_fields {
                    program.visit_mut_with(&mut StripUninitializedClassFields);
                }

                let config = Config {
                    legacy: *is_legacy,
                    emit_metadata: *emit_decorators_metadata,
                    use_define_for_class_fields: *use_define_for_class_fields,
                    ..Default::default()
                };
                apply_transform(program, helpers, decorators(config))