Adds `data-next-font` data attribute to the preload tag if added by
`@next/font`.
```js
// Using `size-adjust` fallback font.
<link data-next-font="size-adjust" rel="preload" href="..." as="font" type="font/woff2" crossorigin="anonymous">
// Not using `size-adjust` fallback font.
<link data-next-font="" rel="preload" href="..." as="font" type="font/woff2" crossorigin="anonymous">
```
If no fonts are preloaded, the tag is added on the preconnect tag.
Fixes NEXT-350
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/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`
- [ ]
[e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs)
tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)
Currently there's a bug when selecting Chinese, Japanese or Korean (CJK)
as subsets.
```js
const notoSans = Noto_Sans_JP({
subsets: ['japanese'],
})
```
It actually doesn't work, nothing preloads. This PR solves this by
removing CJK languages as candidates for preloading. The reason is that
they contain so many glyphs that each font-family is split up in 100+
font files. It doesn't make sense to preload all of them.
So CJK users will have to disable preloading.
```js
const notoSansJapanese = Noto_Sans_JP({
weight: '400',
preload: false,
})
```
In case you do manually disable preloading like above, the default
`font-display` is changed to `swap`.
This PR also improves the validation errors of subsets.
1. Providing unknown subset
```
`@next/font` error:
Unknown subset `japanese` for font `Inter`.
Available subsets: `cyrillic`, `cyrillic-ext`, `greek`, `greek-ext`, `latin`, `latin-ext`, `vietnamese`
```
2. Missing specified subset. The error has a link with further
instructions.
```
`@next/font` error:
Missing selected subsets for font `Inter`. Please specify subsets in the function call or in your `next.config.js`. Read more: https://nextjs.org/docs/messages/google-fonts-missing-subsets
```
fixes NEXT-336
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/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`
- [ ]
[e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs)
tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)
## Documentation / Examples
- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)
