Low-hanging perf improvements

[hubertp]

Pull Request Description

• avoid loading shapeless for the sole purpose of having a compile-time
  type inequality
• don't use sys.env to avoid some Scala conversions
• lazy initialization of fields

Important Notes

On a slow machine, so easier to spot.

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  TypeScript,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• Unit tests have been written where possible.

[JaroslavTulach]

Removing a library dependency is always a good sign.

[JaroslavTulach]

Not that copying this code into our repository would simplify the magic...

[JaroslavTulach]

I don't see any other usages of shapelessVersion. I guess:

val shapelessVersion        = "2.3.10"

should also be removed then.

[hubertp]

Yes, thanks

[JaroslavTulach]

Is this a Scala way of doing Kotlin's reified classes?

Wouldn't it be simpler to just delete the method implicit magic altogether and just pass in the argument explicitly?

Btw. Why is the method called unsafeAs then? Reified classes can be checked during runtime in Kotlin and I see some runtime check here as well.

[hubertp]

The sole purpose of this logic is to ensure at compile time that neither type inferencer nor explicit type argument result in T being Metadata rather than its subclass.

I don't think providing explicit argument to every usage site is better, plus it doesn't prevent incorrect usage this implicit prevents from.