[Feature]: coverage provider API for rstest

[travzhang]

What problem does this feature solve?

Background & Motivation

Test coverage is a critical metric in JavaScript/TypeScript projects. Currently, rstest lacks built-in coverage collection and reporting. This RFC proposes adding coverage support via a pluggable provider system, initially supporting Istanbul instrumentation with future plans for V8 and custom providers.

---

Goals

• Add coverage config in rstest.config.ts
• Support code instrumentation and runtime coverage collection
• Merge coverage data from multiple workers
• Generate coverage reports (HTML and JSON)
• Provide a CoverageProvider interface for custom implementations

What does the proposed API look like?

Configuration Example

export default defineConfig({
  coverage: {
    enabled: true,
    provider: 'istanbul', // future: 'v8'
  },
});

---

CoverageProvider Interface

export interface CoverageProvider {
  init(): void;
  collect(): CoverageMap | null;
  generateReports(coverageMap: CoverageMap, options: CoverageOptions): Promise<void>;
  cleanup(): void;
}

---

Instrumentation Implementation Notes

We explored three approaches for Istanbul instrumentation:

Strategy	Description	Status	Notes
rsbuild-plugin-istanbul	rsbuild Babel plugin instrumentation	Failed	Rspack transforms code earlier; source maps unavailable
Manual Babel transform in rspack	Babel transform applied in rspack plugin	Working	Functional but performance untested
SWC plugin swc-plugin-coverage-instrument	Native SWC instrumentation	Unusable	swc-core version mismatch with rspack

Currently, the manual Babel transform in rspack is used.

---

Future Plans

• Add V8 coverage support
• Support configurable reporters
• Benchmark instrumentation strategies

---

Appendix: Implementation Notes

Originally tried rsbuild-plugin-istanbul but rspack’s pre-transform breaks source map chain. Switched to manual Babel transform inside rspack plugin. Also attempted SWC plugin but incompatible versions blocked usage.

[9aoy]

Based on past experience, the performance of babel transform in Rspack is not very good. It seems that swc-plugin-coverage-instrument has caught up with the latest version of swc, perhaps we can try using the swc plugin.

https://github.com/kwonoj/swc-plugin-coverage-instrument/releases

[travzhang]

That's amazing — just updated earlier this morning! I'll give it a try sometime this week.

[travzhang]

Based on past experience, the performance of babel transform in Rspack is not very good. It seems that swc-plugin-coverage-instrument has caught up with the latest version of swc, perhaps we can try using the swc plugin.

https://github.com/kwonoj/swc-plugin-coverage-instrument/releases

I gave it a try — it works! commit

if (coverage.enabled && coverage.provider === 'istanbul') {
  config.module.rules ??= [];
  config.module.rules.push({
    test: /\.(js|ts)$/,
    exclude: [/node_modules/],
    loader: 'builtin:swc-loader',
    options: {
      jsc: {
        parser: {
          syntax: 'typescript',
        },
        experimental: {
          plugins: [
            [
              require.resolve('swc-plugin-coverage-instrument'),
              {},
            ],
          ],
        },
      },
    },
    type: 'javascript/auto',
  });
}

[9aoy]

Supported in #399 🎉