Check `pagesDir` to bypass empty pages folder when appDir is enabled
* Output empty loadable manifest for now if there's no `pagesDir`
* No custom aliases with all page extensions for `/_app`, `_document` if pagesDir is empty, only keep the built-in ones
* Check pagesDir in build/dev-server/eslint
* Type safe: change arguments of some APIs from optional to required, so that we won't mess up with default arguments
This PR re-includes ESLint with some notable changes, namely a guided setup similar to how TypeScript is instantiated in a Next.js application.
To add ESLint to a project, developers will have to create an `.eslintrc` file in the root of their project or add an empty `eslintConfig` object to their `package.json` file.
```js
touch .eslintrc
```
Then running `next build` will show instructions to install the required packages needed:
<img width="862" alt="Screen Shot 2021-04-19 at 7 38 27 PM" src="https://user-images.githubusercontent.com/12476932/115316182-dfd51b00-a146-11eb-830c-90bad20ed151.png">
Once installed and `next build` is run again, `.eslintrc` will be automatically configured to include the default config:
```json
{
"extends": "next"
}
```
In addition to this change:
- The feature is now under the experimental flag and requires opt-in. After testing and feedback, it will be switched to the top-level namespace and turned on by default.
- A new ESLint shareable configuration package is included that can be extended in any application with `{ extends: 'next' }`
- This default config extends recommended rule sets from [`eslint-plugin-react`](https://www.npmjs.com/package/eslint-plugin-react), [`eslint-plugin-react-hooks`](https://www.npmjs.com/package/eslint-plugin-react-hooks), and [`eslint-plugin-next`](https://www.npmjs.com/package/@next/eslint-plugin-next)
- All rules in [`eslint-plugin-next`](https://www.npmjs.com/package/@next/eslint-plugin-next) have been modified to include actionable links that show more information to help resolve each issue
This pull request **temporarily** removes ESLint, as it was not landed in accordance with our standard experimental policies. We are fully committed to landing this change again.
This is being reverted because:
- Next.js has very strict goals for its install size. This feature resulted in adding over 17MB, or a 43.6% increase.
- The feature was not first landed under the `experimental` key in `next.config.js`, rather, it was added under the stable namespace (top-level)
- Using the feature doesn't do a "guided setup" like TypeScript, it should ask you to "bring your own" dependencies for ESLint
- It uses a undesirable ESLint plugin name: `plugin:@next/next/recommended`. This should read out as strictly `next`, or as short as we can get it.
- Does not provide actionable warnings (missing link to resolve issue)
- Does not follow appropriate console output styling. We need to revisit how these are presented.
To re-land this, we need to ensure the following minimums are met:
- Very minor change in install size
- Fully experimental (i.e. flagged) with warnings
- Finalized package name and configuration shape, preferably so we can do ` { extends: 'next' } `.
For #22228
This PR:
- Adds ESLint to toolchain
- Included by default for builds (`next build`)
- Can be enabled for development (`next dev`)
- Custom formatter built for output
- Adds appropriate tests
- Adds two documentation pages
