Wrap resource route raw object return in json for v2 back compat

.changeset/rich-spoons-draw.md

@@ -0,0 +1,5 @@
+---
+"@remix-run/server-runtime": patch
+---
+
+Automatically wrap resource route naked object returns in `json()` for back-compat in v2 (and log deprecation warning)

docs/guides/single-fetch.md

@@ -270,6 +270,30 @@ And then when `c` calls `serverLoader`, it'll make it's own call for just the `c
 GET /a/b/c.data?_routes=routes/c
 ```
 
+### Resource Routes
+
+Because of the new streaming format used by Single Fetch, raw JavaScript objects returned from `loader` and `action` functions are no longer automatically converted to `Response` instances via the `json()` utility.
+
+However, you may have been relying on this wrapping in [resource routes][resource-routes] previously, so in v2 we will continue this automatic conversion. When Remix v2 detects a raw object returned from a resource route, it will log a deprecation warning and wrap the value in `json()` for easier opt-into the Single Fetch feature. At your convenience, you can add the `json()` call to your resource route handlers. Once you've addressed all of the deprecation warnings in your application's resource routes, you will be better prepared for the eventual Remix v3 upgrade.
+
+```tsx filename=app/routes/resource.tsx bad
+export function loader() {
+  return {
+    message: "My resource route",
+  };
+}
+```
+
+```tsx filename=app/routes/resource.tsx good
+import { json } from "@remix-run/react";
+
+export function loader() {
+  return json({
+    message: "My resource route",
+  });
+}
+```
+
 [future-flags]: ../file-conventions/remix-config#future
 [should-revalidate]: ../route/should-revalidate
 [entry-server]: ../file-conventions/entry.server
@@ -284,3 +308,4 @@ GET /a/b/c.data?_routes=routes/c
 [starttransition]: https://react.dev/reference/react/startTransition
 [headers]: ../route/headers
 [mdn-headers]: https://developer.mozilla.org/en-US/docs/Web/API/Headers
+[resource-routes]: ../guides/resource-routes

integration/single-fetch-test.ts

@@ -1500,6 +1500,45 @@ test.describe("single-fetch", () => {
     expect(await app.getHtml("#target")).toContain("Target");
   });
 
+  test("wraps resource route naked object returns in json with a deprecation warning", async () => {
+    let oldConsoleWarn = console.warn;
+    let warnLogs: unknown[] = [];
+    console.warn = (...args) => warnLogs.push(...args);
+
+    let fixture = await createFixture(
+      {
+        config: {
+          future: {
+            unstable_singleFetch: true,
+          },
+        },
+        files: {
+          ...files,
+          "app/routes/resource.tsx": js`
+            export function loader() {
+              return { message: "RESOURCE" };
+            }
+          `,
+        },
+      },
+      ServerMode.Development
+    );
+    let res = await fixture.requestResource("/resource");
+    expect(await res.json()).toEqual({
+      message: "RESOURCE",
+    });
+
+    expect(warnLogs).toEqual([
+      "⚠️ REMIX FUTURE CHANGE: Resource routes will no longer be able to return " +
+        "raw JavaScript objects in v3 when Single Fetch becomes the default. You " +
+        "can prepare for this change at your convenience by wrapping the data " +
+        "returned from your `loader` function in the `routes/resource` route with " +
+        "`json()`.  For instructions on making this change see " +
+        "https://remix.run/docs/en/v2.9.2/guides/single-fetch#resource-routes",
+    ]);
+    console.warn = oldConsoleWarn;
+  });
+
   test.describe("client loaders", () => {
     test("when no routes have client loaders", async ({ page }) => {
       let fixture = await createFixture(

packages/remix-server-runtime/deprecations.ts

@@ -0,0 +1,13 @@
+export function resourceRouteJsonWarning(
+  type: "loader" | "action",
+  routeId: string
+) {
+  return (
+    "⚠️ REMIX FUTURE CHANGE: Resource routes will no longer be able to " +
+    "return raw JavaScript objects in v3 when Single Fetch becomes the default. " +
+    "You can prepare for this change at your convenience by wrapping the data " +
+    `returned from your \`${type}\` function in the \`${routeId}\` route with ` +
+    "`json()`.  For instructions on making this change see " +
+    "https://remix.run/docs/en/v2.9.2/guides/single-fetch#resource-routes"
+  );
+}

packages/remix-server-runtime/server.ts

@@ -29,6 +29,7 @@ import {
   isRedirectResponse,
   isRedirectStatusCode,
   isResponse,
+  json,
 } from "./responses";
 import { createServerHandoffString } from "./serverHandoff";
 import { getDevServerHooks } from "./dev";
@@ -43,6 +44,7 @@ import {
   singleFetchLoaders,
   SingleFetchRedirectSymbol,
 } from "./single-fetch";
+import { resourceRouteJsonWarning } from "./deprecations";
 
 export type RequestHandler = (
   request: Request,
@@ -226,6 +228,7 @@ export const createRequestHandler: CreateRequestHandlerFunction = (
     ) {
       response = await handleResourceRequest(
         serverMode,
+        _build,
         staticHandler,
         matches.slice(-1)[0].route.id,
         request,
@@ -559,6 +562,7 @@ async function handleDocumentRequest(
 
 async function handleResourceRequest(
   serverMode: ServerMode,
+  build: ServerBuild,
   staticHandler: StaticHandler,
   routeId: string,
   request: Request,
@@ -573,13 +577,25 @@ async function handleResourceRequest(
       routeId,
       requestContext: loadContext,
     });
+
     if (typeof response === "object") {
       invariant(
         !(DEFERRED_SYMBOL in response),
         `You cannot return a \`defer()\` response from a Resource Route.  Did you ` +
           `forget to export a default UI component from the "${routeId}" route?`
       );
     }
+
+    if (build.future.unstable_singleFetch && !isResponse(response)) {
+      console.warn(
+        resourceRouteJsonWarning(
+          request.method === "GET" ? "loader" : "action",
+          routeId
+        )
+      );
+      response = json(response);
+    }
+
     // callRouteLoader/callRouteAction always return responses (w/o single fetch).
     // With single fetch, users should always be Responses from resource routes
     invariant(