Bun adapter: `load` hook cannot override the inferred `Loader`

[81reap]

Environment

unplugin@3.0.0 + bun@1.3.13

Reproduction

import { createBunPlugin } from 'unplugin'

await Bun.write(
  './entry.ts',
  "import Hello from 'virtual:component'; console.log(<Hello />);\n",
)

const plugin = createBunPlugin(() => ({
  name: 'jsx-virtual',
  resolveId(id) {
    return id === 'virtual:component' ? id : null
  },
  load(id) {
    if (id === 'virtual:component') {
      return 'export default () => <h1>hi</h1>'
    }
    return null
  },
}))()

await Bun.build({
  entrypoints: ['./entry.ts'],
  plugins: [plugin],
})

Describe the bug

In the Bun adapter, the Loader for a load hook's returned contents is inferred from path.extname(id) via guessLoader (src/bun/utils.ts). The result is passed as the loader field on Bun's onLoad return.

This is fine for 'file' namespace loads where the id is a real file path, but for virtual modules the id has no meaningful extension (or carries a query suffix that confuses extname). Bun then falls back to js, which fails when the plugin's returned contents are JSX/TSX/CSS/etc.

Plugins have no way to tell unplugin "this content is tsx" — the existing LoadResult type only carries code and map. esbuild's own onLoad accepts { contents, loader }; unplugin's Bun adapter should let plugins pass the same hint.

Additional context

No response

Logs

$ bun run 02-loader.ts 
1 | export default () => <h1>hi</h1>
                         ^
error: Unexpected <
    at virtual:component:1:22

[81reap]

raised this fix, happy to iterate and bar raise :) :: #601

[sxzz]

unplugin/src/types.ts

Lines 139 to 146 in 11bea2a

	esbuild?: {
	// using regexp in esbuild improves performance
	onResolveFilter?: RegExp | undefined
	onLoadFilter?: RegExp | undefined
	loader?: Loader | ((code: string, id: string) => Loader) | undefined
	setup?: ((build: PluginBuild) => void | Promise<void>) | undefined
	config?: ((options: BuildOptions) => void) | undefined
	} | undefined

I think for Bun we could also add a specific loader function, similar to how esbuild does it.

[81reap]

ope, missed this this morning. I can take a look after work and raise a PR for that. I think it would be easier to wait for the current fix to merge to avoid merge conflicts

[81reap]

@sxzz I have updated the PR with plugin.bun.loader for per-plugin loader resolution. lmk if there is any more feedback