Liftable derivation

[anatoliykmetyuk]

No description provided.

[anatoliykmetyuk]

Blocked by #7322 (since unable to use the derivation givens in stdlib – or any separate compilation for that matter).

[anatoliykmetyuk]

Rebased on top of #7333

[nicolasstucki]

To avoid duplication of most of this logic you can split it into

inline given [T <: Product: Type](given
    m : Mirror.ProductOf[T],
    em: Expr[Mirror.ProductOf[T]]): Liftable[T] =
      productLiftable(given m, em)(summonAll[Tuple.Map[m.MirroredElemTypes, Liftable]])

private def productLiftable[T <: Product: Type](given
    m : Mirror.ProductOf[T],
    em: Expr[Mirror.ProductOf[T]])(liftables: List[Any]): Liftable[T] = new Liftable[T] {
  def toExpr(x: T) =
    val genRepr = Tuple.fromProductTyped(x)
    val elemsWithLiftables = liftables.zip(genRepr.asInstanceOf[Product].productIterator.toList)
    val tupleOfExprs = elemsWithLiftables.map {
      case (l: Liftable[a], x) => l.toExpr(x.asInstanceOf[a])
    }
    val exprOfTuple = Expr.ofTuple(tupleOfExprs)
    '{$em.fromProduct($exprOfTuple.asInstanceOf[Product])}
}

[nicolasstucki]

To avoid inlining too much