add runtime.TupleMirror

Co-authored-by: Jamie Thompson <bishbashboshjt@gmail.com>
Co-authored-by: Nicolas Stucki <nicolas.stucki@gmail.com>

Author: bishabosha

library/src/scala/runtime/TupleMirror.scala

@@ -0,0 +1,16 @@
+package scala.runtime
+
+/** A concrete subclass of `scala.deriving.Mirror.Product`, enabling reduction of bytecode size.
+ *  as we do not need to synthesize an anonymous Mirror class at every callsite.
+ */
+final class TupleMirror(arity: Int) extends scala.deriving.Mirror.Product with Serializable:
+  assert(arity > 0) // EmptyTuple is not a valid `MirroredType` for TupleMirror
+
+  override type MirroredMonoType <: NonEmptyTuple
+
+  final def fromProduct(product: Product): MirroredMonoType =
+    if product.productArity != arity then
+      throw IllegalArgumentException(s"expected Product with $arity elements, got ${product.productArity}")
+    runtime.Tuples.fromProduct(product).asInstanceOf[MirroredMonoType]
+
+  override final def toString: String = s"<tuple-mirror@${Integer.toHexString(hashCode).nn.take(6)}>"