fix(plugin-legacy): respect custom filenames formats, fix #2356 (#2641)

Co-authored-by: Shinigami <chrissi92@hotmail.de>

packages/playground/legacy/package.json

@@ -5,6 +5,7 @@
   "scripts": {
     "dev": "vite",
     "build": "vite build --debug legacy",
+    "build:custom-filename": "vite --config ./vite.config-custom-filename.js build  --debug legacy",
     "debug": "node --inspect-brk ../../vite/bin/vite",
     "serve": "vite preview"
   },

packages/playground/legacy/vite.config-custom-filename.js

@@ -0,0 +1,15 @@
+const legacy = require('@vitejs/plugin-legacy').default
+
+module.exports = {
+  plugins: [legacy()],
+  build: {
+    manifest: true,
+    minify: false,
+    rollupOptions: {
+      output: {
+        entryFileNames: `assets/[name].js`,
+        chunkFileNames: `assets/[name].js`
+      }
+    }
+  }
+}

packages/plugin-legacy/index.js

@@ -165,6 +165,35 @@ function viteLegacyPlugin(options = {}) {
         return
       }
 
+      /**
+       * @param {string|((chunkInfo: import('rollup').PreRenderedChunk)=>string)} fileNames
+       * @param {string?} defaultFileName
+       */
+      const getLegacyOutputFileName = (
+        fileNames,
+        defaultFileName = '[name]-legacy.[hash].js'
+      ) => {
+        if (!fileNames) {
+          return path.posix.join(config.build.assetsDir, defaultFileName)
+        }
+
+        // does not support custom functions.
+        if (typeof fileNames === 'function') {
+          throw new Error(
+            `@vitejs/plugin-legacy rollupOptions.output.entryFileNames and rollupOptions.output.chunkFileNames` +
+              ` does not support the function format.`
+          )
+        }
+
+        let fileName = defaultFileName
+        // Custom string file return format.
+        if (fileNames && typeof fileNames === 'string') {
+          fileName = fileNames.replace(/\[name\]/, '[name]-legacy')
+        }
+
+        return fileName
+      }
+
       /**
        * @param {import('rollup').OutputOptions} options
        * @returns {import('rollup').OutputOptions}
@@ -173,14 +202,8 @@ function viteLegacyPlugin(options = {}) {
         return {
           ...options,
           format: 'system',
-          entryFileNames: path.posix.join(
-            config.build.assetsDir,
-            `[name]-legacy.[hash].js`
-          ),
-          chunkFileNames: path.posix.join(
-            config.build.assetsDir,
-            `[name]-legacy.[hash].js`
-          )
+          entryFileNames: getLegacyOutputFileName(options.entryFileNames),
+          chunkFileNames: getLegacyOutputFileName(options.chunkFileNames)
         }
       }