fix(dev): declare an empty source map for route chunk placeholders when an export can't be split

integration/split-route-modules-test.ts

@@ -11,6 +11,7 @@ import {
   reactRouterServe,
   viteConfig,
   reactRouterConfig,
+  viteMajorTemplates,
 } from "./helpers/vite.js";
 
 const js = String.raw;
@@ -272,7 +273,9 @@ test.describe("Split route modules", async () => {
 
     let { status, stderr } = build({ cwd });
     expect(status).toBe(0);
-    expect(stderr.toString()).not.toContain("SOURCEMAP_BROKEN");
+    expect(stderr.toString()).not.toContain(
+      "Sourcemap is likely to be incorrect",
+    );
 
     let chunkPath = globSync(path.join(cwd, "build/client/assets/*.js")).find(
       (file) => readFileSync(file, "utf8").includes(marker),
@@ -309,6 +312,45 @@ test.describe("Split route modules", async () => {
     expect(entry.originalLine).toBe(originalLine);
   });
 
+  viteMajorTemplates.forEach(({ templateName, templateDisplayName }) => {
+    test(`${templateDisplayName} / declares source maps for placeholder chunks of exports that can't be split`, async () => {
+      let cwd = await createProject(
+        {
+          "react-router.config.ts": reactRouterConfig({
+            splitRouteModules: true,
+          }),
+          "vite.config.js": await viteConfig.basic({ sourcemap: true }),
+          "app/routes/_index.tsx": js`
+            let shared = 0;
+
+            export async function clientLoader() {
+              shared++;
+              return shared;
+            }
+
+            export default function Index() {
+              return <h1>{shared}</h1>;
+            }
+          `,
+        },
+        templateName,
+      );
+
+      let { status, stderr } = build({ cwd });
+      expect(status).toBe(0);
+      expect(stderr.toString()).not.toContain(
+        "Sourcemap is likely to be incorrect",
+      );
+
+      let placeholder = globSync(
+        path.join(cwd, "build/client/assets/*.js"),
+      ).find((file) =>
+        readFileSync(file, "utf8").includes("No clientLoader chunk"),
+      );
+      expect(placeholder).toBeDefined();
+    });
+  });
+
   test.describe("enabled", () => {
     let port: number;
     let cwd: string;

packages/react-router-dev/.changes/patch.route-chunk-placeholder-sourcemap.md

@@ -0,0 +1 @@
+Declare an empty source map for the placeholder chunk of a route export that can't be split, so Rollup no longer warns that `react-router:split-route-modules` dropped the source map

packages/react-router-dev/vite/plugin.ts

@@ -2145,8 +2145,17 @@ export const reactRouterVitePlugin: ReactRouterVitePlugin = () => {
           code,
         );
 
-        let preventEmptyChunkSnippet = ({ reason }: { reason: string }) =>
-          `Math.random()<0&&console.log(${JSON.stringify(reason)});`;
+        let preventEmptyChunkSnippet = ({
+          reason,
+        }: {
+          reason: string;
+        }): Vite.Rollup.SourceDescription => ({
+          code: `Math.random()<0&&console.log(${JSON.stringify(reason)});`,
+          // The placeholder is generated, so there's no original code to map
+          // back to. An empty map says so rather than leaving Rollup to warn
+          // that the transform dropped the source map.
+          map: { mappings: "" },
+        });
 
         if (chunk === null) {
           return preventEmptyChunkSnippet({