Summary

https://github.com/web-infra-dev/rspack/pull/8286/files#r1831012797

Cause lots of projects use object item of externals when not umd and it can actually work well. For example:

module.exports = {
  output: {
     library: {
       type: "commonjs"
     }
  },
  externalsType: "commonjs",
  externals: {
    // actually same as: `react: "REACT"` or `react: "commonjs REACT"`
    react: {
      commonjs: "REACT"
    }
  }
};

so if we just add a strict object validation to object item of externals, it will be a breaking change to these projects.

The object item of externals should be validate according to the values of output.library.type/output.libraryTarget and externalsType. Only if output.library.type/output.libraryTarget is set to umd and externalsType is not set or also set to umd, the object item in externals should be handle as an umd dependency.

We can add a new ZodType named ZodCrossFieldsOptions to validate the object item in externals. In its _parse method we can get the zod context, and then get its root context through its parent field. Then we can do some checking cross configuration options and make sure that the object item in externals should only be validated when it is really handle as an umd dependency.

But there is a trick in ZodUnion. The parent field is set to null here. Not sure if it is a bug or not. So we need to override the ZodUnion._parse to pass the parent context to children like other ZodType does.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).