`svelte-kit package` errors due to not removing `lang="ts"`

[probablykasper]

Describe the bug

When running svelte-kit package, code is preprocessed but the lang="ts" attributes are not removed. This is giving me errors when trying to use the generated components. I wanted to open this issue in this repo to make sure it's either resolved before SvelteKit 1.0, or it's mentioned in the docs that preprocessing is broken.

Same issue in svelte-preprocess: sveltejs/svelte-preprocess#260
Related SvelteKit issue: #1843

Reproduction

1. npm init svelte@next my-app with TypeScript enabled
2. cd my-app
3. npm install sass svelte2tsx && npm install
4. Add src/lib/Example.svelte:

   <script lang="ts">
     const x: number = 8
   </script>
   
   <p>{x}</p>
   
   <style lang="sass">
     p
       color: red
   </style>

5. svelte-kit package

Logs

Error from using a component generated by svelte-kit package:

5:22:33 PM [vite] Internal server error: Expected newline.
  ╷
2 │ .date-time-picker {
  │                   ^
  ╵
  ../svelte-date-picker/package/DatePicker.svelte 2:19  root stylesheet
  Plugin: vite-plugin-svelte
  File: /Users/kasper/dev/git/svelte-date-picker/package/DatePicker.svelte
  1  |  <script lang="ts">
  2  |    import { getMonthLength, getWeeks } from './utils/date-utils'
     |                     ^
  3  |    export let value = new Date()
  4  |    export let visible = true
      at Object._newRenderError (/Users/kasper/dev/git/kryp/node_modules/sass/sass.dart.js:2203:19)
      at Object._wrapException (/Users/kasper/dev/git/kryp/node_modules/sass/sass.dart.js:2023:16)
      at _render_closure1.call$2 (/Users/kasper/dev/git/kryp/node_modules/sass/sass.dart.js:82163:21)
      at _RootZone.runBinary$3$3 (/Users/kasper/dev/git/kryp/node_modules/sass/sass.dart.js:27566:18)
      at _FutureListener.handleError$1 (/Users/kasper/dev/git/kryp/node_modules/sass/sass.dart.js:26096:21)
      at _Future__propagateToListeners_handleError.call$0 (/Users/kasper/dev/git/kryp/node_modules/sass/sass.dart.js:26395:49)
      at Object._Future__propagateToListeners (/Users/kasper/dev/git/kryp/node_modules/sass/sass.dart.js:12483:77)
      at _Future._completeError$2 (/Users/kasper/dev/git/kryp/node_modules/sass/sass.dart.js:26241:9)
      at _AsyncAwaitCompleter.completeError$2 (/Users/kasper/dev/git/kryp/node_modules/sass/sass.dart.js:25910:12)
      at Object._asyncRethrow (/Users/kasper/dev/git/kryp/node_modules/sass/sass.dart.js:12286:17)

System Info

System:
    OS: macOS 10.15.7
    CPU: (8) x64 Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz
    Memory: 677.92 MB / 32.00 GB
    Shell: 3.2.2 - /usr/local/bin/fish
  Binaries:
    Node: 14.16.0 - /var/folders/44/3sxbyc2d6wg90mj1s7s59bnm0000gn/T/fnm_multishells/19056_1631295056028/bin/node
    Yarn: 1.22.10 - /var/folders/44/3sxbyc2d6wg90mj1s7s59bnm0000gn/T/fnm_multishells/19056_1631295056028/bin/yarn
    npm: 6.14.11 - /var/folders/44/3sxbyc2d6wg90mj1s7s59bnm0000gn/T/fnm_multishells/19056_1631295056028/bin/npm
  Browsers:
    Brave Browser: 93.1.29.77
    Chrome: 93.0.4577.82
    Firefox: 89.0.2
    Safari: 14.1.2

Severity

serious, but I can work around it

[bluwy]

Even though this is actually a real issue, I don't think the fix should be in SvelteKit nor svelte-preprocess. There are 3rd party preprocessor that needs a simple way to remove the lang as well, so it should be supported by Svelte instead, which is tracked here: sveltejs/svelte#5900

[dummdidumm]

I tend to add a quick fix for this into SvelteKit until we have a way forward. I'm not sure if Svelte core should be responsible of this, or if it should be the preprocessors. If the preprocessor rework RFC is implemented in some form, it means that every preprocessor has the whole source content at its hand and then should be responsible (IMO) for removing the lang tag.

[Valexr]

https://github.com/sveltejs/svelte-preprocess/blob/main/docs/preprocessing.md#replace

[bluwy]

I tend to add a quick fix for this into SvelteKit until we have a way forward. I'm not sure if Svelte core should be responsible of this, or if it should be the preprocessors. If the preprocessor rework RFC is implemented in some form, it means that every preprocessor has the whole source content at its hand and then should be responsible (IMO) for removing the lang tag.

A quick fix would be nice in the meantime since the svelte fix (sveltejs/svelte#6611) doesn't seem it would be merged soon. Though maybe we can extract the attributes feature out and leave the custom parser aside?

I do think Svelte core should be responsible for it though since any script/style preprocessor outside of svelte-preprocess would need a common way of removing attributes, and also to prevent the need to manually sequence the preprocessors. The RFC would definitely be a welcome change once the API has been agreed upon.

https://github.com/sveltejs/svelte-preprocess/blob/main/docs/preprocessing.md#replace

Interesting, we could use that for svelte-kit package perhaps.

[probablykasper]

Even though this is actually a real issue, I don't think the fix should be in SvelteKit nor svelte-preprocess. There are 3rd party preprocessor that needs a simple way to remove the lang as well, so it should be supported by Svelte instead, which is tracked here: sveltejs/svelte#5900

Absolutely, main reason for opening this issue is just so something is done about it before 1.0