You'll probably want to disable whitespace in the diff
## Description
This allows for better editor support by using `describe` or functions called `describe` with the same syntax instead of custom names.
Changes:
- `nextTestSetup` can be used instead of `createNextDescribe` keeping the same behaviour but being called inside a `describe` "block" (not applied everywhere)
- `getSnapshotTestDescribe` replaced with a custom `describe.each`
- `sandbox` helper function for `acceptance`/`acceptance-app` merged into a single shared one
- `outdent` to remove the indent from inline files in tests which helps with consistent snapshots
This PR updates the Edge Runtime to use a new version that loads dependencies differently. This addresses https://github.com/vercel/next.js/pull/38766 so `instanceof` works as expected.
It involved a few code changes, mostly regarding to types. The most important change is that the `Runner` function in the sandbox doesn't take a `ReadableStream` as `body` anymore since this implies creating the instance on "node land" and makes the runtime `fetch` function not to be able to compare with `ReadableStream` using `instanceof`. Instead we introduce a "clonable body" abstraction that allows to create the `ReadableStream` from `Readable` by using the edge runtime primitive which would hold the correct prototype.
Also, this PR changes the way we pre-compile the Edge Runtime to adapt it to the new version.
_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`
