fix(heft-sass-plugin): set sourceMapUrl in loadAsync to prevent data:…

common/changes/@rushstack/heft-sass-plugin/fix-sourcemap-linux-crash_2026-06-01-00-00.json

@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "comment": "Fix sourceMap: true crashing on Linux/macOS when compiled .scss files use @use or @import",
+      "type": "patch",
+      "packageName": "@rushstack/heft-sass-plugin"
+    }
+  ],
+  "packageName": "@rushstack/heft-sass-plugin",
+  "email": "cmalonzo@microsoft.com"
+}

heft-plugins/heft-sass-plugin/src/SassProcessor.ts

@@ -242,7 +242,10 @@ export class SassProcessor {
 
       return {
         contents: record.content,
-        syntax: determineSyntaxFromFilePath(absolutePath)
+        syntax: determineSyntaxFromFilePath(absolutePath),
+        // Without sourceMapUrl, sass-embedded falls back to a data: URL for this file in the
+        // source map. data: URLs crash heftUrlToPath on Linux/macOS (non-empty URL host).
+        sourceMapUrl: url
       };
     };
 
@@ -860,6 +863,7 @@ export class SassProcessor {
           // Rewrite heft: URL sources to paths relative to the map file's directory
           // so that source-map-loader can resolve them back to the original .scss.
           const rewrittenSources: string[] = result.sourceMap.sources.map((source) => {
+            if (!source.startsWith('heft:')) return source;
             const absoluteSourcePath: string = heftUrlToPath(source);
             return Path.convertToSlashes(path.relative(mapDir, absoluteSourcePath));
           });

heft-plugins/heft-sass-plugin/src/test/SassProcessor.test.ts

@@ -759,6 +759,24 @@ describe(SassProcessor.name, () => {
       expect(parsedMap.sources[0]).toMatch(/classes-and-exports\.module\.scss$/);
     });
 
+    it('emits a valid .css.map when the entry file @uses a partial (Linux/macOS regression)', async () => {
+      // use-with-partial.module.scss uses @use 'partial', which goes through loadAsync.
+      // Without sourceMapUrl on the ImporterResult, sass-embedded falls back to a data: URL
+      // for the partial in the source map; heftUrlToPath then crashes on Linux/macOS.
+      const { processor } = createProcessor(terminalProvider, { sourceMap: true });
+      await compileFixtureAsync(processor, 'use-with-partial.module.scss');
+
+      const mapPaths: string[] = getAllWrittenPathsMatching('.css.map');
+      expect(mapPaths).toHaveLength(1);
+
+      const mapJson: string = getWrittenFile('use-with-partial.module.css.map');
+      const parsedMap: { version: number; mappings: string; sources: string[] } = JSON.parse(mapJson);
+      expect(parsedMap.version).toBe(3);
+      expect(parsedMap.mappings).toBeTruthy();
+      // Both the entry file and the partial must resolve to real paths, not data: URLs
+      expect(parsedMap.sources.every((s: string) => !s.startsWith('data:'))).toBe(true);
+    });
+
     it('does not emit .css.map or sourceMappingURL comment by default', async () => {
       const { processor } = createProcessor(terminalProvider);
       await compileFixtureAsync(processor, 'classes-and-exports.module.scss');

heft-plugins/heft-sass-plugin/src/test/__snapshots__/SassProcessor.test.ts.snap

@@ -1331,6 +1331,35 @@ export default styles;",
 }
 `;
 
+exports[`SassProcessor sourceMap option emits a valid .css.map when the entry file @uses a partial (Linux/macOS regression): terminal-output 1`] = `
+Array [
+  "[verbose] Checking for changes to 1 files...[n]",
+  "[    log] Compiling 1 files...[n]",
+]
+`;
+
+exports[`SassProcessor sourceMap option emits a valid .css.map when the entry file @uses a partial (Linux/macOS regression): written-files 1`] = `
+Map {
+  "/fake/output/dts/use-with-partial.module.scss.d.ts" => "declare interface IStyles {
+  container: string;
+  header: string;
+}
+declare const styles: IStyles;
+export default styles;",
+  "/fake/output/css/use-with-partial.module.css" => ".container {
+  color: #0078d4;
+  padding: 8px;
+}
+
+.header {
+  border-bottom: 1px solid #0078d4;
+}
+/*# sourceMappingURL=use-with-partial.module.css.map */
+",
+  "/fake/output/css/use-with-partial.module.css.map" => "{\\"version\\":3,\\"sourceRoot\\":\\"\\",\\"sources\\":[\\"fixtures/use-with-partial.module.scss\\",\\"fixtures/_partial.scss\\"],\\"names\\":[],\\"mappings\\":\\"AAIA;EACE,OCHY;EDIZ;;;AAGF;EACE\\",\\"sourcesContent\\":[\\"// Uses the modern @use syntax to import variables from a local partial.\\\\n// Verifies that SassProcessor resolves _partial.scss when @use 'partial' is written.\\\\n@use 'partial' as tokens;\\\\n\\\\n.container {\\\\n  color: tokens.$brand-color;\\\\n  padding: tokens.$spacing-unit * 2;\\\\n}\\\\n\\\\n.header {\\\\n  border-bottom: 1px solid tokens.$brand-color;\\\\n}\\\\n\\",\\"// Sass partial exposing shared design tokens.\\\\n// Imported via @use 'partial' in use-with-partial.module.scss.\\\\n$brand-color: #0078d4;\\\\n$spacing-unit: 4px;\\\\n\\"],\\"file\\":\\"use-with-partial.module.css\\"}",
+}
+`;
+
 exports[`SassProcessor sourceMap option uses the correct map filename when doNotTrimOriginalFileExtension is true: terminal-output 1`] = `
 Array [
   "[verbose] Checking for changes to 1 files...[n]",