perf: avoid encoding intermediate source maps

packages/vite/src/node/__tests__/utils.spec.ts

@@ -2,6 +2,8 @@ import crypto from 'node:crypto'
 import fs from 'node:fs'
 import os, { type NetworkInterfaceInfoIPv4 } from 'node:os'
 import path from 'node:path'
+import type { DecodedSourceMap, RawSourceMap } from '@jridgewell/remapping'
+import MagicString from 'magic-string'
 import { fileURLToPath } from 'mlly'
 import { describe, expect, test, vi, onTestFinished } from 'vitest'
 import type { CommonServerOptions, ResolvedServerUrls } from '..'
@@ -902,6 +904,28 @@ describe('combineSourcemaps', () => {
     return normalizePath(path.resolve(_dirname, file))
   }
 
+  test('composes decoded intermediate mappings identically', () => {
+    const filename = 'assets/index.js'
+    const s = new MagicString('export const value = 1')
+    const originalMap = s.generateMap({
+      source: filename,
+      hires: true,
+      includeContent: true,
+    })
+    const options = { source: filename, hires: 'boundary' as const }
+    const encoded = combineSourcemaps(filename, [
+      s.generateMap(options) as RawSourceMap,
+      originalMap as RawSourceMap,
+    ])
+    const decoded = combineSourcemaps(filename, [
+      s.generateDecodedMap(options) as DecodedSourceMap,
+      originalMap as RawSourceMap,
+    ])
+
+    expect(decoded).toStrictEqual(encoded)
+    expect(decoded.sourcesContent).toStrictEqual(originalMap.sourcesContent)
+  })
+
   test('should combine sourcemaps with single sources', () => {
     const sourcemaps = [
       // processed with magic-string

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

@@ -1,5 +1,5 @@
 import path from 'node:path'
-import type { RawSourceMap } from '@jridgewell/remapping'
+import type { DecodedSourceMap, RawSourceMap } from '@jridgewell/remapping'
 import convertSourceMap from 'convert-source-map'
 import type { ImportSpecifier } from 'es-module-lexer'
 import { init, parse as parseImports } from 'es-module-lexer'
@@ -604,13 +604,13 @@ export function buildImportAnalysisPlugin(config: ResolvedConfig): Plugin[] {
           if (s.hasChanged()) {
             chunk.code = s.toString()
             if (buildSourcemap && chunk.map) {
-              const nextMap = s.generateMap({
+              const nextMap = s.generateDecodedMap({
                 source: chunk.fileName,
                 hires: 'boundary',
               })
               const originalFile = chunk.map.file
               const map = combineSourcemaps(chunk.fileName, [
-                nextMap as RawSourceMap,
+                nextMap as DecodedSourceMap,
                 chunk.map as RawSourceMap,
               ]) as SourceMap
               map.toUrl = () => genSourceMapUrl(map)

packages/vite/src/node/ssr/ssrTransform.ts

@@ -1,5 +1,5 @@
 import path from 'node:path'
-import type { RawSourceMap } from '@jridgewell/remapping'
+import type { DecodedSourceMap, RawSourceMap } from '@jridgewell/remapping'
 import { walk as eswalk } from 'estree-walker'
 import MagicString from 'magic-string'
 import { extract_names as extractNames } from 'periscopic'
@@ -414,21 +414,27 @@ async function ssrTransformScript(
   if (inMap?.mappings === '') {
     map = inMap
   } else {
-    map = s.generateMap({ hires: 'boundary' }) as SourceMap
-    map.sources = [path.basename(url)]
-    // needs to use originalCode instead of code
-    // because code might be already transformed even if map is null
-    map.sourcesContent = [originalCode]
-    if (
+    const shouldCombine = !!(
       inMap &&
       inMap.mappings &&
       'sources' in inMap &&
       inMap.sources.length > 0
-    ) {
+    )
+    const mapOptions = { hires: 'boundary' as const }
+    const nextMap = shouldCombine
+      ? s.generateDecodedMap(mapOptions)
+      : s.generateMap(mapOptions)
+    nextMap.sources = [path.basename(url)]
+    // needs to use originalCode instead of code
+    // because code might be already transformed even if map is null
+    nextMap.sourcesContent = [originalCode]
+    if (shouldCombine) {
       map = combineSourcemaps(url, [
-        map as RawSourceMap,
+        nextMap as DecodedSourceMap,
         inMap as RawSourceMap,
       ]) as SourceMap
+    } else {
+      map = nextMap as SourceMap
     }
   }