fix(plugin-legacy): fix legacy plugin not working in worker context

[sapenlei]

closes #19073

[sapphi-red]

I don't think this is correct. The worker entry chunks are not legacy chunks.

[sapenlei]

I don't think this is correct. The worker entry chunks are not legacy chunks.

Okay, I'll change it.

[sapphi-red]

I guess this change will make the built application error in runtime because the legacy plugin does not transpile the worker content. I think we need to

1. show an warning if a worker is used when legacy chunks are generated
2. [optional] avoid injecting __vite_legacy_guard to worker chunks so that workers work in that case

The ideal fix is to generate legacy chunks for workers. But that's hard to do.

[sapenlei]

It's because when iife format,esbuild doesn't convert, I think I changed it correctly and tested it locally with no problems! 😭 @sapphi-red

[sapenlei]

So when it's a worker and it's in iife format, esbuild converts the iife

[sapphi-red]

I guess this PR would make esbuild to transform export function __vite_legacy_guard() into a different syntax. But that code is there to avoid running the modern chunk in legacy browsers. If that code is transformed to a different code, the modern chunk (which is not transformed for legacy browsers and contains modern syntaxes) will be loaded in legacy browsers and will error.

I haven't tested, but this reproduction probably won't work on Chrome 45 because dist/assets/worker-C_luD0Fa.js contains spread syntax as-is.
https://stackblitz.com/edit/vitejs-vite-5f1azdhv?file=vite.config.js&terminal=build

[sapphi-red]

Oh, I noticed that the reproduction of #19073 is passing plugin-legacy to worker.plugins.
I think we should output a warning here by checking config.isWorker in this case.

vite/packages/plugin-legacy/src/index.ts

Line 255 in 282496d

configResolved(config) {