_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`
Fixes: https://github.com/vercel/next.js/issues/15278
> Bug report
> When using next dev with emacs, as you develop, emacs creates symbolic link files starting with .# as lock files. Next.js seems to attempt to load these but fails, spewing out errors constantly.
Prevents dev server from crashing when emacs creates lockfiles
tested with:
- GNU Emacs 27.1
- OSX 11.1
- Node v15.4.0
Also added `.now`, this should make sure that if the Vercel CLI is used inside the examples to deploy a project, the project folder is not added to the list of changes.
* Added the RouteUrl type and improved router types
* Added more tests for router types
* Add build test for typescript types
* Add next-env.d.ts to the typescript test
* Removed next-env.d.ts
* Added next-env.d.ts to gitignore
* Remove route url re-exports
* renamed PublicRouterInstance to be NextRouter
* export the Url type
* Replaced BaseRouter with NextRouter in server/utils
* Don't export the Url type
* Update tsconfig.json
Update tests to setup webdriver stuff in `jest-environment` and re-use one browser session instead of spawning one for each webdriver call to prevent creating too many BrowserStack sessions.
Minor changes to examples. Updating major semver updates with only `package.json` changes.
I've done my best to make sure that these packages.json files all have `latest` for the `nextjs` package, `cross-env` for those with `server.js` files, etc.
I also added a `package.json` to `with-dynamic-app-layout` (it was missing one previously)
Made sure to test all of these packages post-upgrade to ensure maintained functionality
* Update references to `.next`
* Remove console logs and extraneous semi colons
* Remove lint errors
* Update references to .next and update docs
* Update options from nested to flat with `distDir`
* Add integration tests, and update `.gitignore`
* Rename integration folder to dist-dir to match standards
