µPickle VS scodec

The upickle library has traditionally had great performance for handling json in Scala apps so is likely to be seen as a safe choice for someone starting a Scala project. It appears though that not just upickle, but other json library projects are having difficulties maintaining their old level of performance when they release using Scala 3's macros. uPickle currently has an open issue where you can see some of these issues: https://github.com/com-lihaoyi/upickle/issues/389 and here you can see the weePickle folks are also having the same performance problems. Looks like things changed up significantly enough between Scala 2 and Scala 3 so that in order to maintain the same functionality they have resorted to using runtime reflection for mapping to/from case classes.

See here for sample code size numbers (not picking on upickle specifically, it's just what I'm using myself. I've heard similar reports about e.g. circe)

A good starting point is the note in MacroImplicits.scala in upickle sources. "derives Writer" for a specific case class Foo simply adds a given Writer[Foo] to a companion object of the same specific class. However, this cannot be done automatically for a trait defining sum type - in this case trait Thing. The required given must be defined manually, and the ones automatically obtained for case classes can be used in it.

I know that libraries like Scodec and shapeless were rewritten practically from scratch for Scala 3, taking advantage of the next syntax and internals, as well as protoquill - a Scala 3 implementation of Quill.