Fix #5941: implement GenLens macro

Author: liufengyun

tests/run-with-compiler/i5941/macro_1.scala

@@ -0,0 +1,43 @@
+abstract class Lens[S, T] {
+  def get(s: S): T
+  def set(t: T, s: S) :S
+}
+
+import scala.quoted._
+import scala.tasty._
+
+object Lens {
+  def apply[S, T](_get: S => T)(_set: T => S => S): Lens[S, T] = new Lens {
+    def get(s: S): T = _get(s)
+    def set(t: T, s: S): S = _set(t)(s)
+  }
+
+  /** case class Address(streetNumber: Int, streetName: String)
+   *
+   *  Lens.gen[Address, Int](_.streetNumber)   ~~>
+   *
+   *  Lens[Address, Int](_.streetNumber)(n => a => a.copy(streetNumber = n))
+   */
+  inline def gen[S, T](get: S => T): Lens[S, T] = ~impl('(get))
+
+  def impl[S: Type, T: Type](getter: Expr[S => T])(implicit refl: Reflection): Expr[Lens[S, T]] = {
+    import refl._
+    import util._
+    import quoted.Toolbox.Default._
+
+    // obj.copy(field = value)
+    def setterBody(obj: Expr[S], value: Expr[T], field: String): Expr[S] =
+      Term.Select.overloaded(obj.unseal, "copy", Nil, Term.NamedArg(field, value.unseal) :: Nil).seal[S]
+
+    getter.unseal.underlyingArgument match {
+      case Term.Block(
+        DefDef(_, Nil, (param :: Nil) :: Nil, _, Some(Term.Select(o, field))) :: Nil,
+        Term.Lambda(meth, _)
+      ) =>
+        '{
+          val setter = (t: T) => (s: S) => ~setterBody('(s), '(t), field)
+          apply(~getter)(setter)
+        }
+    }
+  }
+}

tests/run-with-compiler/i5941/usage_2.scala

@@ -0,0 +1,12 @@
+case class Address(streetNumber: Int, streetName: String)
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    // val len = Lens.gen[Address, Int](_.streetNumber)
+    val len = Lens.gen[Address, Int]( (a: Address) => a.streetNumber)
+    val address = Address(10, "High Street")
+    assert(len.get(address) == 10)
+    val addr2 = len.set(5, address)
+    assert(len.get(addr2) == 5)
+  }
+}