How to generate files using dart source_gen to a different directory

This issue describes the concept https://github.com/dart-lang/source_gen/issues/272

To summarize:

• I am using source_gen to generate some dart code.

• I am using json_serializable on the generated dart code.

• I wish to output all of the results to a source directory adjacent or below my target source.

The desired directory structure

• src
  • feature_a
    • model.dart
    • gen
      • model.g.dart
      • model.g.g.dart
  • feature_b
    • ...

I have considered building to cache however it seems json_serializable doesn't support this and even if it did I don't know if its even possible to run a builder on files in the cache.

I've also considered an aggregated builder that is mentioned here.

Generate one file for a list of parsed files using source_gen in dart

But json_serializable is still an issue and the source_gen version in that post is super old and doesn't describe the solution well.

This is not possible with build_runner. The issue to follow is https://github.com/dart-lang/build/issues/1689

Note that this doesn't help much with builders that you don't author, and wouldn't work with things like SharedPartBuilder.