feat: transientNull option support

[Dolfik1]

Introduces a new transientNull flag to CodecMakerConfig, allowing users to exclude fields with null values from JSON output. The flag is disabled by default to preserve existing behavior.

Also adds related logic in the macro and a test suite (JsonCodecMakerNullableSpec) to verify serialization with and without transientNull enabled.

[plokhotnyuk]

@Dolfik1 Thanks for the contribution!

Could you please share your use case?

Is it about migration from Circe to jsoniter-scala and that is why your propose transientNull = true in makeCirceLikeSnakeCased?

Have you considered to make similar improvements for Scala 2 macros?

[Dolfik1]

@plokhotnyuk First of all, thank you for this great library!

Could you please share your use case?

In our project, we frequently use nullable types YYY | Null instead of Option[YYY]. This also applies to types that are serialized into JSON using jsoniter-scala. Unfortunately, there was no way to configure the behavior so that fields with null values are excluded from the final JSON. That's why I added support for such types, similar to how transientNone works.

Is it about migration from Circe to jsoniter-scala and that is why your propose transientNull = true in makeCirceLikeSnakeCased?

No, it's not related to Circe. I just followed the approach used for transientNone.

Have you considered to make similar improvements for Scala 2 macros?

I'm not very familiar with Scala 2, but as far as I understand, union types are not supported there, so it may not be directly applicable.

[plokhotnyuk]

Am I understand right that the configuration option should be limited for Null types only?

If yes, then how about using the following implementation instead?

      def isNullable(tpe: TypeRepr): Boolean = tpe match {
        case OrType(left, right) => isNullable(right) || isNullable(left)
        case _ => tpe =:= TypeRepr.of[Null]
      }

It will fix remaining test failures and extend this feature for all union types including nested ones like String | Null | Boolean | Double.

Also, increasing the minor version (in the ./version.sbt) is the remaining one requirement to pass MiMa checks.

[Dolfik1]

Am I understand right that the configuration option should be limited for Null types only?

If yes, then how about using the following implementation instead?

      def isNullable(tpe: TypeRepr): Boolean = tpe match {
        case OrType(left, right) => isNullable(right) || isNullable(left)
        case _ => tpe =:= TypeRepr.of[Null]
      }

It will fix remaining test failures and extend this feature for all union types including nested ones like String | Null | Boolean | Double.

Also, increasing the minor version (in the ./version.sbt) is the remaining one requirement to pass MiMa checks.

Thank you for your feedback. I've fixed it.

[plokhotnyuk]

@Dolfik1 Please peek the latest release with your new compile-time option.

It is already available on the Maven Central.