export enum in Typescript ends up producing a Svelte compiler warning

[rgossiaux]

Describe the bug

I'm not at all confident that this issue belongs in this repo, since it could plausibly go into several different ones, but I thought I'd file it here to begin with since it's ultimately the Svelte compiler which is generating the warning and there is no way to ignore it (adding the ability to silence individual compiler warnings like this in JS, similar to the <!-- svelte-ignore --> template comment, would be one way of resolving this).

Anyway, the issue: Typescript code with export enum from inside <script context="module" lang="ts"> block ends up generating an unused-export-let warning; see sveltejs/language-tools#573 While the language-tools project added special handling to deal with this as an editor warning, it still generates warnings in other parts of the ecosystem. Users of my library may see this from vite-plugin-svelte, for example: rgossiaux/svelte-headlessui#25

Besides the ability to silence the warning, alternate resolutions could be:

• Modifying preprocessing so that the generated code is different and does not produce this warning;
• Modifying the check for this warning (if it's possible to do this safely)

Reproduction

It's not a minimal repo, but you can repo this by installing https://github.com/rgossiaux/svelte-headlessui and running npm run dev. But I think something very basic should work too, along the lines of

<script lang="ts" context="module">
  export enum MyEnum {
    One,
    Two,
  }
</script>

Logs

No response

System Info

System:
    OS: macOS 12.1
    CPU: (10) arm64 Apple M1 Max
    Memory: 26.12 GB / 64.00 GB
    Shell: 3.3.1 - /opt/homebrew/bin/fish
  Binaries:
    Node: 16.8.0 - /var/folders/rn/2kdgb0jj25q2v656j3qc_ywr0000gn/T/fnm_multishells/23662_1640400023652/bin/node
    Yarn: 1.22.17 - /opt/homebrew/bin/yarn
    npm: 7.21.0 - /var/folders/rn/2kdgb0jj25q2v656j3qc_ywr0000gn/T/fnm_multishells/23662_1640400023652/bin/npm
  Browsers:
    Chrome: 96.0.4664.110
    Safari: 15.2
  npmPackages:
    svelte: ^3.44.0 => 3.44.3

Severity

annoyance

[jtlapp]

Here's how I'm dealing with this in rollup.config.js. Unfortunately, having to list the individual enums makes this an undesirable hack. I didn't see a way to reliably detect the presence of an enum in the frame.

  plugins: [
    svelte({
      /* ... */
      onwarn: (warning, handler) => {
        const exportedEnums = ['InteractiveTreeFlags'];
        const { code, frame } = warning;
        const mentionsEnum = () =>
            exportedEnums.reduce((found, name) => (found || frame.includes(name)), false);
        if (code == "unused-export-let" && mentionsEnum()) {
          return;
        }
        handler(warning);
      }
    }),

[Conduitry]

This should be fixed in 3.46.5.

[jtlapp]

Thank you! I was able to delete my onwarn enum handling code using Svelte 3.47.0.