perf(css): only run postcss when needed

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

@@ -1272,7 +1272,6 @@ async function compileCSS(
 ): Promise<{
   code: string
   map?: SourceMapInput
-  ast?: PostCSS.Result
   modules?: Record<string, string>
   deps?: Set<string>
 }> {
@@ -1281,51 +1280,49 @@ async function compileCSS(
     return compileLightningCSS(id, code, environment, urlReplacer)
   }
 
+  const lang = CSS_LANGS_RE.exec(id)?.[1] as CssLang | undefined
+  const deps = new Set<string>()
+
+  // pre-processors: sass etc.
+  let preprocessorMap: ExistingRawSourceMap | undefined
+  if (isPreProcessor(lang)) {
+    const preprocessorResult = await compileCSSPreprocessors(
+      environment,
+      id,
+      lang,
+      code,
+      workerController,
+    )
+    code = preprocessorResult.code
+    preprocessorMap = preprocessorResult.map
+    preprocessorResult.deps?.forEach((dep) => deps.add(dep))
+  }
+
   const { modules: modulesOptions, devSourcemap } = config.css
   const isModule = modulesOptions !== false && cssModuleRE.test(id)
   // although at serve time it can work without processing, we do need to
   // crawl them in order to register watch dependencies.
   const needInlineImport = code.includes('@import')
   const hasUrl = cssUrlRE.test(code) || cssImageSetRE.test(code)
-  const lang = CSS_LANGS_RE.exec(id)?.[1] as CssLang | undefined
   const postcssConfig = await resolvePostcssConfig(
     environment.getTopLevelConfig(),
   )
 
-  // 1. plain css that needs no processing
+  // postcss processing is not needed
   if (
-    lang === 'css' &&
+    lang !== 'sss' &&
     !postcssConfig &&
     !isModule &&
     !needInlineImport &&
     !hasUrl
   ) {
-    return { code, map: null }
-  }
-
-  let modules: Record<string, string> | undefined
-  const deps = new Set<string>()
-
-  // 2. pre-processors: sass etc.
-  let preprocessorMap: ExistingRawSourceMap | undefined
-  if (isPreProcessor(lang)) {
-    const preprocessorResult = await compileCSSPreprocessors(
-      environment,
-      id,
-      lang,
-      code,
-      workerController,
-    )
-    code = preprocessorResult.code
-    preprocessorMap = preprocessorResult.map
-    preprocessorResult.deps?.forEach((dep) => deps.add(dep))
+    return { code, map: preprocessorMap ?? null, deps }
   }
 
-  // 3. postcss
+  // postcss
   const atImportResolvers = getAtImportResolvers(
     environment.getTopLevelConfig(),
   )
-  const postcssOptions = postcssConfig?.options ?? {}
   const postcssPlugins = postcssConfig?.plugins.slice() ?? []
 
   if (needInlineImport) {
@@ -1387,7 +1384,13 @@ async function compileCSS(
     )
   }
 
-  if (urlReplacer) {
+  if (
+    urlReplacer &&
+    // if there's an @import, we need to add this plugin
+    // regradless of whether it contains url() or image-set(),
+    // because we don't know the content referenced by @import
+    (needInlineImport || hasUrl)
+  ) {
     postcssPlugins.push(
       UrlRewritePostcssPlugin({
         replacer: urlReplacer,
@@ -1396,6 +1399,8 @@ async function compileCSS(
     )
   }
 
+  let modules: Record<string, string> | undefined
+
   if (isModule) {
     postcssPlugins.unshift(
       (await importPostcssModules()).default({
@@ -1429,7 +1434,11 @@ async function compileCSS(
     )
   }
 
-  if (!postcssPlugins.length) {
+  const postcssOptions = postcssConfig?.options ?? {}
+  const postcssParser =
+    lang === 'sss' ? loadSss(config.root) : postcssOptions.parser
+
+  if (!postcssPlugins.length && !postcssParser) {
     return {
       code,
       map: preprocessorMap,
@@ -1441,10 +1450,11 @@ async function compileCSS(
   try {
     const source = removeDirectQuery(id)
     const postcss = await importPostcss()
+
     // postcss is an unbundled dep and should be lazy imported
     postcssResult = await postcss.default(postcssPlugins).process(code, {
       ...postcssOptions,
-      parser: lang === 'sss' ? loadSss(config.root) : postcssOptions.parser,
+      parser: postcssParser,
       to: source,
       from: source,
       ...(devSourcemap
@@ -1510,7 +1520,6 @@ async function compileCSS(
 
   if (!devSourcemap) {
     return {
-      ast: postcssResult,
       code: postcssResult.css,
       map: { mappings: '' },
       modules,
@@ -1528,7 +1537,6 @@ async function compileCSS(
   )
 
   return {
-    ast: postcssResult,
     code: postcssResult.css,
     map: combineSourcemapsIfExists(cleanUrl(id), postcssMap, preprocessorMap),
     modules,
@@ -2150,8 +2158,8 @@ function loadSassPackage(root: string): {
   }
 }
 
-let cachedSss: any
-function loadSss(root: string) {
+let cachedSss: PostCSS.Syntax
+function loadSss(root: string): PostCSS.Syntax {
   if (cachedSss) return cachedSss
 
   const sssPath = loadPreprocessorPath(PostCssDialectLang.sss, root)

playground/css-sourcemap/__tests__/css-sourcemap.spec.ts

@@ -138,8 +138,8 @@ describe.runIf(isServe)('serve', () => {
     const map = extractSourcemap(css)
     expect(formatSourcemapForSnapshot(map)).toMatchInlineSnapshot(`
       {
-        "ignoreList": [],
-        "mappings": "AAGE;EACE,UCJM",
+        "mappings": "AAGE;EACE,OCJM",
+        "sourceRoot": "",
         "sources": [
           "/root/imported.sass",
           "/root/imported-nested.sass",
@@ -186,7 +186,7 @@ describe.runIf(isServe)('serve', () => {
     expect(formatSourcemapForSnapshot(map)).toMatchInlineSnapshot(`
       {
         "ignoreList": [],
-        "mappings": "AACE;EACE",
+        "mappings": "AACE,SAAC;EACC",
         "sources": [
           "/root/imported.less",
         ],
@@ -209,7 +209,7 @@ describe.runIf(isServe)('serve', () => {
     expect(formatSourcemapForSnapshot(map)).toMatchInlineSnapshot(`
       {
         "ignoreList": [],
-        "mappings": "AACE;EACE,cAAM",
+        "mappings": "AACE;EACE,OAAM,QAAN",
         "sources": [
           "/root/imported.styl",
         ],