Ensure backward compatibility of Hikari Config

[sideeffffect]

/cc @jatcwang

[jatcwang]

Thanks @sideeffffect I'm not too sure what motivated this change given that we're breaking back compat in 1.0?
When you're talking about backwards compatibility do you mean binary back compat?

[sideeffffect]

The point is to enable backward compatible evolution of Config. When we would potentially add a new field to Config, we don't want to break binary compatibility.

Binary compatibility with case classes is a tricky topic in Scala https://docs.scala-lang.org/overviews/core/binary-compatibility-for-library-authors.html#evolving-code-without-breaking-binary-compatibility

[jatcwang]

If you want to keep backwards binary compatibility you'll need to use package private I think. Using private makes it unavailable at the bytecode level.

[sideeffffect]

Are you referring to the access modifier of the constructor of Config?
Making it private doesn't make it private in JVM bytecode, it is actually still publicly accessible in JVM bytecode. But it makes it inaccessible from Scala code. And that is good enough for keeping backward compatibility with our Scala users.

[jatcwang]

I've added a demo of making method private breaking bin compat https://github.com/jatcwang/binary-compatibility-guide

[error] Test suite IncompatibilityShowcases failed with java.lang.IllegalAccessError: class com.example.LibA$ tried to access private method 'int all_incompat.MakeMethodPrivate$.willBePrivate()'

You can see that projects like cats use package private to maintain bin compat.

[sideeffffect]

making method private breaking bin compat

Of course it breaks 😃 There's probably a misunderstanding. I know the changes that I'm proposing in this PR are breaking bin compatibility. I'm introducing them so that once we get to 1.0.0 we can evolve Config in a bin compatible way.

[jatcwang]

Sorry not sure why we need this?

[sideeffffect]

Scala 3 defines fromProduct in companion object of case classes. If you add a new field to the case class, you need to add a new case to fromProduct to ensure compatibility with old code which counts with the old number of fields.
I've improved the comment on top of the case class.

[sideeffffect]

@jatcwang so what do you think? Can we merge this?

[jatcwang]

Will need some time @sideeffffect. I ran javap on a Scala 3 compiled case class but still couldn't find fromProduct so I'm a bit confused. I don't see it in the docs either (I think fromProduct is a method on Mirror?). Will get this in for RC3

[sideeffffect]

Did you look into the companion object?

[sideeffffect]

@jatcwang do you think we could merge this PR?

[sideeffffect]

@jatcwang ping 😸