Ensure Dart Sass file:// sources use relative paths

This is consistent with our Node Sass output

shared/tasks/assets.mjs

@@ -1,5 +1,5 @@
 import { mkdir, writeFile } from 'fs/promises'
-import { dirname } from 'path'
+import { dirname, relative } from 'path'
 
 /**
  * Write asset helper
@@ -22,7 +22,18 @@ export async function write (filePath, result) {
   writeTasks.push(writeFile(filePath, code))
 
   // 2. Write source map (example.js.map)
-  if (map) {
+  if (map && 'sources' in map) {
+    map.sources = map.sources
+
+      /**
+       * Make source file:// paths relative (e.g. for Dart Sass)
+       * {@link https://sass-lang.com/documentation/js-api/interfaces/CompileResult#sourceMap}
+       */
+      .map((path) => path.startsWith('file://')
+        ? relative(dirname(filePath), new URL(path).pathname)
+        : path
+      )
+
     writeTasks.push(writeFile(`${filePath}.map`, JSON.stringify(map)))
   }
 

shared/tasks/build/dist.test.mjs

@@ -46,6 +46,21 @@ describe('dist/', () => {
     })
   })
 
+  describe('govuk-frontend-[version].min.css.map', () => {
+    let filename
+    let sourcemap
+
+    beforeAll(async () => {
+      filename = `govuk-frontend-${pkg.version}.min.css.map`
+      sourcemap = JSON.parse(await readFile(join(paths.dist, filename), 'utf8'))
+    })
+
+    it('should contain relative paths to sources', () => {
+      expect(sourcemap.sources).toContain('../src/govuk/all.scss')
+      expect(sourcemap.sources).toContain('../src/govuk/core/_govuk-frontend-version.scss')
+    })
+  })
+
   describe('govuk-frontend-ie8-[version].min.css', () => {
     let filename
     let stylesheet
@@ -64,6 +79,21 @@ describe('dist/', () => {
     })
   })
 
+  describe('govuk-frontend-ie8-[version].min.css.map', () => {
+    let filename
+    let sourcemap
+
+    beforeAll(async () => {
+      filename = `govuk-frontend-ie8-${pkg.version}.min.css.map`
+      sourcemap = JSON.parse(await readFile(join(paths.dist, filename), 'utf8'))
+    })
+
+    it('should contain relative paths to sources', () => {
+      expect(sourcemap.sources).toContain('../src/govuk/all-ie8.scss')
+      expect(sourcemap.sources).toContain('../src/govuk/core/_govuk-frontend-version.scss')
+    })
+  })
+
   describe('govuk-frontend-[version].min.js', () => {
     let filename
     let javascript
@@ -86,6 +116,21 @@ describe('dist/', () => {
     })
   })
 
+  describe('govuk-frontend-[version].min.js.map', () => {
+    let filename
+    let sourcemap
+
+    beforeAll(async () => {
+      filename = `govuk-frontend-${pkg.version}.min.js.map`
+      sourcemap = JSON.parse(await readFile(join(paths.dist, filename), 'utf8'))
+    })
+
+    it('should contain relative paths to sources', () => {
+      expect(sourcemap.sources).toContain('../src/govuk/all.mjs')
+      expect(sourcemap.sources).toContain('../src/govuk/common/govuk-frontend-version.mjs')
+    })
+  })
+
   describe('VERSION.txt', () => {
     let filename
     let version