Description

This PR adds a new coverage.changed option that allows running all tests while only computing coverage for changed files. This is useful for CI pipelines that need to:

1. Run the full test suite to ensure all tests pass
2. Compute coverage only for files changed in a pull request

Previously, this required running two separate commands, which doubled the test execution time.

Resolves #8747

Usage

export default defineConfig({
  test: {
    coverage: {
      changed: 'HEAD', // or any git ref like 'main', 'origin/main'
    },
  },
})

The changed option accepts:

• 'HEAD' - Compare against the last commit
• 'main' - Compare against the main branch
• 'origin/main' - Compare against remote main branch
• Any valid git reference

Testing Instructions

1. Navigate to the test directory:

   cd test/coverage-test

2. Create a demo config file vitest.demo.config.ts:

   import { defineConfig } from 'vitest/config'
   
   export default defineConfig({
     test: {
       include: ['**/file-to-change.test.ts', '**/math.test.ts'],
       coverage: {
         enabled: true,
         provider: 'istanbul',
         reporter: ['text'],
         reportsDirectory: './coverage-demo',
       },
     },
   })

3. Run tests without the changed option:

   pnpm vitest run --config=vitest.demo.config.ts

   Result: Coverage includes all files

   File               | % Stmts | % Branch | % Funcs | % Lines |
   -------------------|---------|----------|---------|---------|
   All files          |   33.33 |      100 |   33.33 |   33.33 |
    file-to-change.ts |      50 |      100 |      50 |      50 |
    math.ts           |      25 |      100 |      25 |      25 |
   

4. Modify fixtures/src/file-to-change.ts (make any change)

5. Update config to add changed: 'HEAD':

   coverage: {
     enabled: true,
     provider: 'istanbul',
     reporter: ['text'],
     reportsDirectory: './coverage-demo',
     changed: 'HEAD', // ← Add this line
   },

6. Run tests again:

   pnpm vitest run --config=vitest.demo.config.ts

   Result: Coverage includes only changed files

   File               | % Stmts | % Branch | % Funcs | % Lines |
   -------------------|---------|----------|---------|---------|
   All files          |      50 |      100 |      50 |      50 |
    file-to-change.ts |      50 |      100 |      50 |      50 |
   

Note: Both test runs execute all tests (file-to-change.test.ts and math.test.ts), but the second run only reports coverage for the modified file.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.
• Please check Allow edits by maintainers to make review process faster. Note that this option is not available for repositories that are owned by Github organizations.

Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.