fix(css): always use css module content (#8936)

Author: sapphi-red

packages/vite/src/node/plugins/css.ts

@@ -426,7 +426,11 @@ export function cssPostPlugin(config: ResolvedConfig): Plugin {
           code = `export default ${JSON.stringify(content)}`
         }
       } else {
-        code = `export default ''`
+        // if moduleCode exists return it **even if** it does not have `?used`
+        // this will disable tree-shake to work with `import './foo.module.css'` but this usually does not happen
+        // this is a limitation of the current approach by `?used` to make tree-shake work
+        // See #8936 for more details
+        code = modulesCode || `export default ''`
       }
 
       return {

playground/css/__tests__/css.spec.ts

@@ -349,8 +349,7 @@ test('PostCSS dir-dependency', async () => {
   }
 })
 
-// skip because #8278 is reverted
-test.skip('import dependency includes css import', async () => {
+test('import dependency includes css import', async () => {
   expect(await getColor('.css-js-dep')).toBe('green')
   expect(await getColor('.css-js-dep-module')).toBe('green')
 })
@@ -437,9 +436,9 @@ test('PostCSS source.input.from includes query', async () => {
   )
 })
 
-// skip because #8278 is reverted
-test.skip('aliased css has content', async () => {
+test('aliased css has content', async () => {
   expect(await getColor('.aliased')).toBe('blue')
-  expect(await page.textContent('.aliased-content')).toMatch('.aliased')
+  // skipped: currently not supported see #8936
+  // expect(await page.textContent('.aliased-content')).toMatch('.aliased')
   expect(await getColor('.aliased-module')).toBe('blue')
 })