## How to reproduce
1. create a next.js app with a middleware (or an edge route) that imports a node.js module:
```js
// middleware.js
import { NextResponse } from 'next/server'
import { basename } from 'path'
export default async function middleware() {
basename()
return NextResponse.next()
}
```
2. deploy it to vercel with `vc`
3. go to the your function logs in Vercel Front (https://vercel.com/$user/$project/$deployment/functions)
4. in another tab, query your application
> it results in a 500 page:
<img width="517" alt="image" src="https://user-images.githubusercontent.com/186268/179557102-72568ca9-bcfd-49e2-9b9c-c51c3064f2d7.png">
> in the logs you should see:
<img width="1220" alt="image" src="https://user-images.githubusercontent.com/186268/179557266-498f3290-b7df-46ac-8816-7bb396821245.png">
## Expected behavior
The route should fail indeed in a 500, because Edge runtime **does not support node.js modules**. However the error in logs should be completely different:
```shell
error - Error: The edge runtime does not support Node.js 'path' module.
Learn More: https://nextjs.org/docs/messages/node-module-in-edge-runtime
```
## Notes to reviewers
I introduced this issue in #38234.
Prior to the PR above, the app would not even build, as we were checking imported node.js module during the build with AST analysis.
Since #38234, the app would build and should fail at runtime, with an appropriate error.
The mistake was to declare `__import_unsupported` function in the sandbox's context, that is only used in `next dev` and `next start`, but not shipped to Vercel platform.
By loading it inside webpack loaders (both middleware and edge route), we ensure it will be defined on Vercel as well.
The existing test suite (`pnpm testheadless --testPathPattern=runtime-module-error`) covers them.
* [edge] allow importing blob assets
* Fix test
* extract to a new file, to make it easier to read and review
* Use webpack asset discovery and transform with a loader
* fix tests
* don't prefix assets
* use emitFile
* rename assets to blobs to be more specific
* rename blobs to assets and use webpack's hashing algo
* Dedupe correctly
* Add a Node.js dep test
* Update packages/next/server/next-server.ts
Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>
* [code review] test remote URL fetches
* [code review] use `import type` for type-only imports
* Update packages/next/server/next-server.ts
Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>
* Apply suggestions from code review
Co-authored-by: JJ Kasper <jj@jjsweb.site>
Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>
Co-authored-by: JJ Kasper <jj@jjsweb.site>
## Bug
x-ref: #38691
* Previous configured loader.issue results into a single function and will bail during next build, tune the rule set conditions to make it work for both pages and app dir
* prefer to use `mod.resourceResolveData?.path` instead of `mod.userRequest` since userRquest contains the applied loaders info
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
### Changes
#### Add missing virtual client entry matcher for css modules
client entry module has no file path, add the missing file matcher for client css modules loader
#### Use relative paths for imports in client entry to avoid module resolving failure.
if it's `next/head` or other 3rd party installed path, just import it instead of the resolved file path
@delbaoliveira caught an issue where if you edit a client component multiple times it would cause a hydration error when reloading. Root cause of this was that the `require.cache` entry for `.__sc_client__` did not get removed, which this PR fixes.
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
In certain cases, mini-css-extractor will create independent `CSSModule` to the module graph, which doesn't have a user request or resource. Hence we can't match them in the flight manifest plugin like before. This PR adds a work around to use `mod.type` and `mod._identifier` to get such CSS assets.
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
Improve CSS modules support in server components.
## Bug
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
Co-authored-by: Jiachi Liu <4800338+huozhi@users.noreply.github.com>
* Enable css in server components
* inject server css into flight
* refactor and fix test
* fix lint
* resolve css from module deps
* fix dev & prod inconsistentce, collect client css
* simplify
* dedupe duplicated css chunks
* remove ssr link injection and css flight
Co-authored-by: Shu Ding <g@shud.in>
Follow-up of #38439.
Found a small issue with booting `next start` that is now resolved.
Also added optional catchall routes support.
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
## What's in there?
The Edge runtime [does not support Node.js modules](https://edge-runtime.vercel.app/features/available-apis#unsupported-apis).
When building Next.js application, we currently fail the build when detecting node.js module imported from middleware.
This is an blocker for using code that is conditionally loading node.js modules (based on platform/env detection), as @cramforce reported.
This PR implements a new strategy where:
- we can build such middleware/Edge API route code **with a warning**
- we fail at run time, with graceful errors in dev (console & react-dev-overlay error)
- we fail at run time, with console errors in production
## How to test?
All cases are covered with integration tests.
To try them live, create a simple app with a page, a `middleware.js` file and a `pages/api/route.js`file.
Here are iconic examples:
### node.js modules
```js
// middleware.js
import { NextResponse } from 'next/server'
// static
import { basename } from 'path'
export default async function middleware() {
// dynamic
const { basename } = await import('path')
basename()
return NextResponse.next()
}
export const config = { matcher: '/' }
```
```js
// pags/api/route.js
// static
import { isAbsolute } from 'path'
export default async function handle() {
// dynamic
const { isAbsolute } = await import('path')
return Response.json({ useNodeModule: isAbsolute('/test') })
}
export const config = { runtime: 'experimental-edge' }
```
Desired error (+ source code highlight in dev):
> The edge runtime does not support Node.js 'path' module
Learn More: https://nextjs.org/docs/messages/node-module-in-edge-runtime
Desired warning at build time:
> A Node.js module is loaded ('path' at line 2) which is not supported in the Edge Runtime.
Learn More: https://nextjs.org/docs/messages/node-module-in-edge-runtime
- [x] in dev middleware, static, shows desired error on stderr
- [x] in dev route, static, shows desired error on stderr
- [x] in dev middleware, dynamic, shows desired error on stderr
- [x] in dev route, dynamic, shows desired error on stderr
- [x] in dev middleware, static, shows desired error on react error overlay
- [x] in dev route, static, shows desired error on react error overlay
- [x] in dev middleware, dynamic, shows desired error on react error overlay
- [x] in dev route, dynamic, shows desired error on react error overlay
- [x] builds middleware successfully, shows build warning, shows desired error on stderr on call
- [x] builds route successfully, shows build warning, shows desired error on stderr on call
### 3rd party modules not found
```js
// middleware.js
import { NextResponse } from 'next/server'
// static
import Unknown from 'unknown'
export default async function middleware() {
// dynamic
const Unknown = await import('unknown')
new Unknown()
return NextResponse.next()
}
```
export const config = { matcher: '/' }
```
```js
// pags/api/route.js
// static
import Unknown from 'unknown'
export default async function handle() {
// dynamic
const Unknown = await import('unknown')
return Response.json({ use3rdPartyModule: Unknown() })
}
export const config = { runtime: 'experimental-edge' }
```
Desired error (+ source code highlight in dev):
> Module not found: Can't resolve 'does-not-exist'
Learn More: https://nextjs.org/docs/messages/module-not-found
- [x] in dev middleware, static, shows desired error on stderr
- [x] in dev route, static, shows desired error on stderr
- [x] in dev middleware, dynamic, shows desired error on stderr
- [x] in dev route, dynamic, shows desired error on stderr
- [x] in dev middleware, static, shows desired error on react error overlay
- [x] in dev route, static, shows desired error on react error overlay
- [x] in dev middleware, dynamic, shows desired error on react error overlay
- [x] in dev route, dynamic, shows desired error on react error overlay
- [x] fails to build middleware, with desired error on stderr
- [x] fails to build route, with desired error on stderr
### unused node.js modules
```js
// middleware.js
import { NextResponse } from 'next/server'
export default async function middleware() {
if (process.exit) {
const { basename } = await import('path')
basename()
}
return NextResponse.next()
}
```
```js
// pags/api/route.js
export default async function handle() {
if (process.exit) {
const { basename } = await import('path')
basename()
}
return Response.json({ useNodeModule: false })
}
export const config = { runtime: 'experimental-edge' }
```
Desired warning at build time:
> A Node.js module is loaded ('path' at line 2) which is not supported in the Edge Runtime.
Learn More: https://nextjs.org/docs/messages/node-module-in-edge-runtime
- [x] invoke middleware in dev with no error
- [x] invoke route in dev with no error
- [x] builds successfully, shows build warning, invoke middleware with no error
- [x] builds successfully, shows build warning, invoke api-route with no error
## Notes to reviewers
The strategy to implement this feature is to leverages webpack [externals](https://webpack.js.org/configuration/externals/#externals) and run a global `__unsupported_module()` function when using a node.js module from edge function's code.
For the record, I tried using [webpack resolve.fallback](https://webpack.js.org/configuration/resolve/#resolvefallback) and [Webpack.IgnorePlugin](https://webpack.js.org/plugins/ignore-plugin/) but they do not allow throwing proper errors at runtime that would contain the loaded module name for reporting.
`__unsupported_module()` is defined in `EdgeRuntime`, and returns a proxy that's throw on use (whether it's property access, function call, new operator... synchronous & promise-based styles).
However there's an issue with error reporting: webpack does not includes the import lines in the generated sourcemaps, preventing from displaying useful errors.
I extended our middleware-plugin to supplement the sourcemaps (when analyzing edge function code, it saves which module is imported from which file, together with line/column/source)
The react-dev-overlay was adapted to look for this additional information when the caught error relates to modules, instead of looking at sourcemaps.
I removed the previous mechanism (built by @nkzawa ) which caught webpack errors at built time to change the displayed error message (files `next/build/index.js`, `next/build/utils.ts` and `wellknown-errors-plugin`)
since we no longer have a single compiler for browser & edge, we can enable
source map generation by default for all edge functions. This would make it
much easier to debug and understand what's happening when deploying to prod
as the log statements will show us the actual code location instead of post
bundling and minified location.
This also removes the experimental flag as it's not needed anymore.
Client components might result in a page chunk or a standalone splitted chunk marked in flight manifest, this PR filters out the page chunk for standalone chunk so that while loading a client chunk (like for `next/link`) it won't load the page chunk to break the hydration. `chunk.ids` is not enough for getting required chunks, so we get the chunks from chunk group then filter the required ones.
tests: Re-enable few previous rsc tests
chore: refactor few webpack api usage
## Bug
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
Continue the work in #38310, this PR includes CSS files as chunks in the manifest for each client component, and then make sure the flight client loads the CSS files correctly.
## Bug
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
The `chunkFilenameMap` should take priority over the webpack built-in one here.
## Bug
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
x-ref: #31506
This PR migrates existing SSR on edge from middleware to edge functions implmentation. So that we can get rid of limitation of middleware and resolve the conflicts between middleware and edge SSR routes.
* Adding edge functions matching route in middleware catch all route,keep the order as `middleware catch all` -> redirects/rewrites -> `edge catch all` -> others
* Dropping middleware related code for edge SSR: removing client info and preflight request handling
In Middlewares, dynamic code execution is not allowed. Currently, we warn if eval / new Function are invoked in dev but don't warn another dynamic code execution in WebAssembly.
This PR adds warnings for `WebAssembly.compile` and `WebAssembly.instantiate` with a buffer parameter (note that `WebAssembly.instantiate` with a **module** parameter is legit) invocations. Note that other methods that compile WASM dynamically such as `WebAssembly.compileStreaming` are not exposed to users so we don't need to cover them.
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [x] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>
* Ensure custom middleware matcher is used correctly in client manifest
* lint-fix
* patch e2e case
* fix rsc case
* update test
* add missing normalize
* Re-introduce Edge API Endpoints
This reverts commit 210fa39961, and
re-introduces Edge API endpoints as a possible runtime selection in API
endpoints.
This is done by exporting a `config` object:
```ts
export config = { runtime: 'edge' }
```
Note: `'edge'` will probably change into `'experimental-edge'` to show
that this is experimental and the API might change in the future.
* Support `experimental-edge`, but allow `edge` too
Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>
This commit enables the following patterns in Middleware:
```ts
// with a dot notation
const { ENV_VAR, "ENV-VAR": myEnvVar } = process.env;
// or with an object access
const { ENV_VAR2, "ENV-VAR2": myEnvVar2 } = process["env"];
```
### Related
- @cramforce asked this fixed here: https://github.com/vercel/next.js/pull/37514#discussion_r892437257
## Feature
- [x] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Bug
fixes#37106
Please note that, as for `pages/` the `src/middleware` file is ignored when `/middleware` is present.
## How to test
1. Rebuild next.js `pnpm build`
2. Run dedicated tests: `pnpm testheadless --testPathPattern middleware-src/`
It would be nice to have this attribute to compare runs with and without the `swcMinify` options.
Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>
This PR will allow Middleware to set its matcher through `export const config = { matching: ... }`
## Related
* This PR is rebased off #37121
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `yarn lint`
Adopt the new `moduleMap` option added in https://github.com/facebook/react/pull/24629, which helps us getting rid of our hacky implementation injected to `globalThis.__next_require__`. The map will be attached to the flight manifest as `__ssr_module_mapping__`.
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
## Feature
This PR introduces the ability to provide `runtime: "edge"` in API endpoints, the same as the experimental RSC runtime configurations.
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `yarn lint`
Another strategy to do achieve the goal of #36995. With this PR the module IDs will be:
- (1) RSC: named (_this one doesn't matter_)
- (2) SSR: named
- (3) Client: deterministic
And we include the client module IDs in the flight manifest, as well as an extra mapping of 3) → 2) so during SSR it can require the correct module still.
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `yarn lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)
Follow-up PR for #37134, this PR adds back the hash in each chunk's filename. This `chunks` in the flight manifest will now be `chunkId:chunkFilename` with this PR, and `globalThis.__next_chunk_load__` then handles the chunk registration (uses chunkId) and chunk loading (uses chunkFilename). (Q: how can we port this to React?)
We can't use `[contenthash]` but only `[chunkhash]` because at the stage of generating the flight manifest, the content isn't finalized and the hash changes after that.
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `yarn lint`
This PR makes sure that chunks of client components can be loaded via `__webpack_chunk_load__`, and hydrated correctly inside `viewsDir`.
Side note: we have to get rid of `[contenthash]` from the chunk filename because of a conflict currently which can be resolved later.
## Bug
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `yarn lint`
We added custom _app as server component support in #33149, but we found it's pretty confusing on usage like support it both server component pages and regular pages at the same time for having similar layout purpose.
When using the _app.server and _app at the same time, applying them into proper places become more confusing.
In that case, we decide to make _app.js can't be a server component, and you can still keep all the existing thing there. And also you don't need to think of the corresponding APIs of custom _app in RSC
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Docs updated
_Hello Next.js team! First PR here, I hope I've followed the right practices._
### What's in there?
It has been decided to only support the following uses cases in Next.js' middleware:
- rewrite the URL (`x-middleware-rewrite` response header)
- redirect to another URL (`Location` response header)
- pass on to the next piece in the request pipeline (`x-middleware-next` response header)
1. during development, a warning on console tells developers when they are returning a response (either with `Response` or `NextResponse`).
2. at build time, this warning becomes an error.
3. at run time, returning a response body will trigger a 500 HTTP error with a JSON payload containing the detailed error.
All returned/thrown errors contain a link to the documentation.
This is a breaking feature compared to the _beta_ middleware implementation, and also removes `NextResponse.json()` which makes no sense any more.
### How to try it?
- runtime behavior: `HEADLESS=true yarn jest test/integration/middleware/core`
- build behavior : `yarn jest test/integration/middleware/build-errors`
- development behavior: `HEADLESS=true yarn jest test/development/middleware-warnings`
### Notes to reviewers
The limitation happens in next's web adapter. ~The initial implementation was to check `response.body` existence, but it turns out [`Response.redirect()`](https://github.com/vercel/next.js/blob/canary/packages/next/server/web/spec-compliant/response.ts#L42-L53) may set the response body (https://github.com/vercel/next.js/pull/31886). Hence why the proposed implementation specifically looks at response headers.~
`Response.redirect()` and `NextResponse.redirect()` do not need to include the final location in their body: it is handled by next server https://github.com/vercel/next.js/blob/canary/packages/next/server/next-server.ts#L1142
Because this is a breaking change, I had to adjust several tests cases, previously returning JSON/stream/text bodies. When relevant, these middlewares are returning data using response headers.
About DevEx: relying on AST analysis to detect forbidden use cases is not as good as running the code.
Such cases are easy to detect:
```js
new Response('a text value')
new Response(JSON.stringify({ /* whatever */ })
```
But these are false-positive cases:
```js
function returnNull() { return null }
new Response(returnNull())
function doesNothing() {}
new Response(doesNothing())
```
However, I see no good reasons to let users ship middleware such as the one above, hence why the build will fail, even if _technically speaking_, they are not setting the response body.
## Feature
- [x] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [x] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [x] Make sure the linting passes by running `yarn lint`
This PR deprecates declaring a middleware under `pages` in favour of the project root naming it after `middleware` instead of `_middleware`. This is in the context of having a simpler execution model for middleware and also ships some refactor work. There is a ton of a code to be simplified after this deprecation but I think it is best to do it progressively.
With this PR, when in development, we will **fail** whenever we find a nested middleware but we do **not** include it in the compiler so if the project is using it, it will no longer work. For production we will **fail** too so it will not be possible to build and deploy a deprecated middleware. The error points to a page that should also be reviewed as part of **documentation**.
Aside from the deprecation, this migrates all middleware tests to work with a single middleware. It also splits tests into multiple folders to make them easier to isolate and work with. Finally it ships some small code refactor and simplifications.
