feat(fmt): use SWC Next parser

[SoonIter]

Motivation

Use @swc-next/parser@0.2.2 as the default JavaScript, JSX and TypeScript parser in rs fmt.

Changes

• Reuse the existing Prettier adapter and ESTree printer, with SWC Next's flat comments and parser diagnostics.
• Preserve explicit Prettier parser selection and project plugin precedence.
• Replace Yuku dependencies and update Renovate, release-age exclusions, documentation and attribution.

Validation

Check	Result
Full test suite	388 passed, one existing skipped test
Builds, lint, types, formatting and spelling	Passed
Packed package	Resolves @swc-next/parser to 0.2.2
Rsbuild JS/TS corpus	2,321 files match both main's Yuku adapter and native Prettier; all results are idempotent

The corpus check used Rsbuild f69eb5a, Yuku 0.10.2 and Prettier 3.9.7, honoring its existing JS/TS fixture exclusions. It did not modify repository files or measure performance.

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[SoonIter]

Known compatibility difference in swc-next 0.2.0, found by the existing rs fmt tests for .d.ts, .d.mts, and .d.cts:

export function value() { return 1; }

This is valid in a normal .ts file, but declaration files should contain a signature such as export function value(): number;. swc-next 0.2.0 currently returns no diagnostics for the implementation even with lang: "dts" explicitly selected, so this is not a filename-to-language mapping issue.

The previous Yuku adapter reported this with semanticErrors: false, and Babel reports it with its TypeScript dts: true option. TypeScript reports TS1183 during semantic checking when declaration checking is enabled (skipLibCheck: false); its syntactic diagnostics alone do not report it.

Upstream draft fix: https://github.com/swc-project/swc-next/pull/631. It proposes extending the existing ambient-implementation diagnostic to inherited ambient contexts, with regression tests and four additional passing conformance cases.

This PoC remains pinned to 0.2.0, and its tests explicitly record the current acceptance difference. The draft fix is not included in that published version; the rejection expectation can be restored after upgrading to a release containing it.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-22T07:58:20.674886Z	6269c42	Manual request

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[Copilot]

Copilot review overview

🟡 Changes recommended

The broad @swc-next/* release-age exemption must be scoped to the pinned packages and version.

Get a fresh assessment by requesting another Copilot review.

Review effort: Lite
Findings: 1

Open (1)

• Scope @​swc-next release-age exemption to pinned packages and versions · New

What changed in this PR

Migrates rs fmt to use @swc-next/parser@0.2.2 by default while retaining Prettier compatibility and replacing Yuku.

Changes:

• Added SWC Next parser integration and tests.
• Updated dependencies, lockfile, Renovate rules, and attribution.
• Updated English and Chinese formatting documentation.

File	Reviewed changes
website/​docs/​zh/​guide/​formatting.mdx	Updated Chinese formatting documentation.
website/​docs/​en/​guide/​formatting.mdx	Updated English formatting documentation.
pnpm-workspace.yaml	Updated dependency and release-age configuration. Critical: the @swc-next/* exemption is broader than the pinned dependency and should be scoped.
pnpm-lock.yaml	Replaced Yuku packages with SWC Next dependencies.
packages/​rstack/​THIRD_PARTY_NOTICES.md	Updated third-party attribution.
packages/​rstack/​tests/​fmt/​swcNextPlugin.test.ts	Added and updated parser integration coverage.
packages/​rstack/​src/​fmt/​swcNextPlugin.ts	Implemented the SWC Next parser adapter.
packages/​rstack/​src/​fmt/​prettierPlugins.ts	Registered SWC Next as the default parser plugin.
packages/​rstack/​package.json	Updated runtime dependencies.
.github/​renovate.json	Updated dependency maintenance rules.

Files not reviewed (1)

• pnpm-lock.yaml: Generated file

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[SoonIter]

The upstream fixes included in @swc-next/parser@0.2.2 improve rs fmt as follows:

Fix	Actual improvement for rs fmt
swc-next#635 — Typed arrows in nested conditionals	Compared with Yuku 0.10.2: formats valid expressions that previously failed, with output matching native Prettier TypeScript.
swc-next#636 — Bare call signatures	Compared with Yuku 0.10.2: reports an error for standalone TS instead of returning empty output; preserves MDX code blocks instead of clearing them.
swc-next#631 — Ambient function bodies	Compared with SWC Next 0.2.0: correctly rejects function implementations in declaration files. Recovery also preserves surrounding scopes; relative to Yuku, both formatters already reject the invalid input.

Verified inputs, outputs, and implementation details

Re-tested the three upstream fixes included in @swc-next/parser@0.2.2 against Yuku 0.10.2, using this PR at 7195815 and the adapter from base e8210f0, with Prettier 3.9.7. Parser comparisons use module mode, the matching language mode, and Yuku's semanticErrors: false.

These fixes are implemented upstream; this PR brings them into rs fmt.

1. Nested conditional + typed arrow — swc-next#635

Input (input.ts):

cond ? inner ? items.map((item): Item => item) : (a) : b => b;

Implementation	Actual result
Yuku / main	Expected ')' after function arguments, but found ':' (1:32); formatting fails.
SWC Next / this PR	No diagnostics; preserves the typed callback and the outer alternate b => b.

Formatted output with this PR, also identical to native Prettier TypeScript:

cond ? (inner ? items.map((item): Item => item) : a) : (b) => b;

The simpler cond ? items.map((item): Item => item) : undefined already works in Yuku 0.10.2. The nested case above demonstrates the additional improvement. Yuku's conditional retry applies the return-type restriction to nested expressions; #635 restores permission at independent expression boundaries such as call arguments.

2. Bare call signature: prevent silent content loss — swc-next#636

Input (input.ts):

(name: string): void;

Implementation	Parser output	Formatter output
Yuku / main	diagnostics: [], program.body: []	"" — the signature disappears.
SWC Next / this PR	Expected '=>' in arrow function, but found ';'	Throws at (1:21) instead of producing empty output.

The same content inside an MDX code fence:

```ts
(name: string): void;
```

Main produces an empty fence:

```ts

```

This PR preserves the original fence and signature unchanged. Prettier retains embedded source when its parser rejects it. Yuku's missing-arrow branch returns no node without a diagnostic; #636 adds the diagnostic for a committed arrow head.

3. Ambient function bodies: preserve scope during recovery — swc-next#631

Compared with SWC Next 0.2.0, #631 adds the missing diagnostic for inputs such as export function value() { return 1; } in input.d.ts, so rs fmt rejects the invalid declaration file. Yuku already reports that error; the example below demonstrates the additional AST recovery improvement relative to Yuku.

Input (input.d.ts):

// Keep the namespace on one line to exercise closing-brace recovery.
declare namespace N { function value() { return 1; } const after: number; }
declare const outside: number;

Implementation	Diagnostics	Recovered AST structure
Yuku 0.10.2	Ambient implementation error plus Unexpected token '}'	N.body is empty; after is incorrectly placed at the top level next to outside.
SWC Next 0.2.2	Only the ambient implementation error	N.body contains value as TSDeclareFunction with body: null, followed by after; outside stays at the top level.

Yuku returns immediately at the invalid body's opening brace. #631 consumes that body before recovering a bodyless signature, so its } is not mistaken for the namespace's closing brace.

This is a parser recovery improvement: both formatter adapters still reject this invalid declaration file. The function body is deliberately omitted from SWC Next's recovered AST; it is not formatted as valid code.

[SoonIter]

Benchmarked the SWC Next and Yuku adapters bundled in rs fmt against the same Rsbuild checkout.

Corpus	Files	Yuku 0.10.2 median	SWC Next 0.2.2 median	SWC Next paired time change¹
Whole repository	3,349	1.495 s	1.538 s	+2.9%
JS/TS only	2,320	0.663 s	0.678 s	+4.0%

SWC Next was slightly slower in this run. It was faster in 6/20 whole-repository pairs and 5/20 JS/TS pairs. These results describe this machine and corpus, not parser performance in general.

Setup

• Apple M5 Max, macOS arm64, Node 24.20.0.
• Both variants use Prettier 3.9.7 and the same native Rstack binding.
• SWC Next: this PR at 6269c42; Yuku: the common base e8210f0.
• Rsbuild: f69eb5a, using its existing configuration and exclusions. packages/core/src/inspectConfig.ts is included.
• 3 warmup rounds and 20 measured rounds per variant and corpus. Fresh CLI processes ran serially, with balanced, shuffled variant order. Node's compile cache was disabled.

node <variant>/packages/rstack/bin/rs.js fmt --check --no-cache --parallel-workers 8 .
node <variant>/packages/rstack/bin/rs.js fmt --check --no-cache --parallel-workers 8 '**/*.{js,jsx,mjs,cjs,ts,tsx,mts,cts}'

All 92 runs, including warmups, passed. Both variants discovered identical file manifests and produced output identical to the source files; source hashes were unchanged afterward. Times include CLI startup, discovery, worker startup, parsing and Prettier printing, but exclude file write-back.

¹ Median of the 20 per-round percentage changes; it is not the percentage difference between the two independently calculated medians. Positive values mean SWC Next took longer.

[SoonIter]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

Reviewed commit: 6269c427d8

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[chenjiahan]

LGTM 👍