[Bug]: import defer does not work as expected

[chenjiahan]

System Info

@rspack/core v1.6.1

Details

After upgrading to @rspack/core v1.6.1, import defer is not working as expected; all modules that are imported with defer are imported at the top level:

Reproduce link

web-infra-dev/rsbuild#6490

Reproduce Steps

1. git checkout import_defer_1103
2. pnpm i
3. cd packages/core
4. pnpm build

[ahabhgk]

No solution to solve this for now, the output of external module (with externalType: "module") need to:

1. synchronously load the external module by ESModule syntax
2. load but can't execute immediately

The only way that matches these conditions is defer import itself, so we can pass-through the syntax (just like typescript), but it's not supported in any js engine AFAIK

[chenjiahan]

Got it. Can we document this?