fix(ssr): combine empty source mappings

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

@@ -5,6 +5,7 @@ import type { SourceMap } from 'rollup'
 import { TraceMap, originalPositionFor } from '@jridgewell/trace-mapping'
 import { transformWithEsbuild } from '../../plugins/esbuild'
 import { ssrTransform } from '../ssrTransform'
+import { createServer } from '../..'
 
 const ssrTransformSimple = async (code: string, url = '') =>
   ssrTransform(code, null, url, code)
@@ -1385,3 +1386,76 @@ const c = () => {
   `,
   )
 })
+
+test('combine mappings', async () => {
+  const server = await createServer({
+    configFile: false,
+    envFile: false,
+    logLevel: 'error',
+    plugins: [
+      {
+        name: 'test-mappings',
+        resolveId(source) {
+          if (source.startsWith('virtual:test-mappings')) {
+            return '\0' + source
+          }
+        },
+        load(id) {
+          if (id.startsWith('\0virtual:test-mappings')) {
+            const code = `export default "test";\n`
+            if (id === '\0virtual:test-mappings:empty') {
+              return { code, map: { mappings: '' } }
+            }
+            if (id === '\0virtual:test-mappings:null') {
+              return { code, map: null }
+            }
+          }
+        },
+      },
+    ],
+  })
+
+  {
+    const result = await server.environments.ssr.transformRequest(
+      'virtual:test-mappings:empty',
+    )
+    expect(result?.map).toMatchInlineSnapshot(`
+      {
+        "mappings": "",
+      }
+    `)
+    const mod = await server.ssrLoadModule('virtual:test-mappings:empty')
+    expect(mod).toMatchInlineSnapshot(`
+      {
+        "default": "test",
+      }
+    `)
+  }
+
+  {
+    const result = await server.environments.ssr.transformRequest(
+      'virtual:test-mappings:null',
+    )
+    expect(result?.map).toMatchInlineSnapshot(`
+      SourceMap {
+        "file": undefined,
+        "mappings": "AAAA,8BAAc,CAAC,CAAC,IAAI,CAAC;",
+        "names": [],
+        "sources": [
+          "virtual:test-mappings:null",
+        ],
+        "sourcesContent": [
+          "export default "test";
+      ",
+        ],
+        "version": 3,
+      }
+    `)
+    const mod = await server.ssrLoadModule('virtual:test-mappings:null')
+    expect(mod).toMatchInlineSnapshot(`
+      {
+        "default": "test",
+      }
+    `)
+  }
+})

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

@@ -419,21 +419,26 @@ async function ssrTransformScript(
     },
   })
 
-  let map = s.generateMap({ hires: 'boundary' })
-  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 (
-    inMap &&
-    inMap.mappings &&
-    'sources' in inMap &&
-    inMap.sources.length > 0
-  ) {
-    map = combineSourcemaps(url, [
-      map as RawSourceMap,
-      inMap as RawSourceMap,
-    ]) as SourceMap
+  let map: TransformResult['map']
+  if (inMap?.mappings === '') {
+    map = inMap
+  } else {
+    map = s.generateMap({ hires: 'boundary' })
+    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 (
+      inMap &&
+      inMap.mappings &&
+      'sources' in inMap &&
+      inMap.sources.length > 0
+    ) {
+      map = combineSourcemaps(url, [
+        map as RawSourceMap,
+        inMap as RawSourceMap,
+      ]) as SourceMap
+    }
   }
 
   return {