Add docs for Scala 3 enum derivation

#2233

Author: jatcwang

modules/core/src/main/scala-3/doobie/util/package.scala

@@ -15,8 +15,8 @@ private[util] inline final def summonLabels[T <: Tuple]: List[String] =
 private[util] inline final def summonSingletonCases[T <: Tuple, A](inline typeName: String): List[A] =
   inline erasedValue[T] match
     case _: EmptyTuple => Nil
-    case _: (h *: t) =>
+    case _: (h *: t)   =>
       inline summonInline[Mirror.Of[h]] match
         case m: Mirror.Singleton => m.fromProduct(EmptyTuple).asInstanceOf[A] :: summonSingletonCases[t, A](typeName)
-        case m: Mirror =>
+        case m: Mirror           =>
           error("Enum " + typeName + " contains non singleton case " + constValue[m.MirroredLabel])

modules/core/src/test/scala-3/doobie/util/GetSuitePlatform.scala

@@ -46,9 +46,14 @@ trait GetDBSuitePlatform { self: munit.CatsEffectSuite & TransactorProvider =>
   given Get[EnumWithOnlySingletonCases] = Get.deriveEnumString
 
   test("Get should properly read existing value of enum") {
-    sql"select 'One'".query[EnumWithOnlySingletonCases].unique.transact(xa).attempt.assertEquals(
+    sql"select 'One', 'Two', 'Three'".query[(
+        EnumWithOnlySingletonCases,
+        EnumWithOnlySingletonCases,
+        EnumWithOnlySingletonCases)].unique.transact(xa).attempt.assertEquals(
       Right(
-        EnumWithOnlySingletonCases.One
+        EnumWithOnlySingletonCases.One,
+        EnumWithOnlySingletonCases.Two,
+        EnumWithOnlySingletonCases.Three
       )
     )
   }

modules/docs/src/main/mdoc/docs/12-Custom-Mappings.md

@@ -177,6 +177,25 @@ implicit val jsonMeta: Meta[Json] =
 
 There are similar constructors for array types and other possibilities, but `other` is by far the most common in user code and we won't discuss the others here. See the Scaladoc for more information.
 
+## Derivation for Scala 3 enums
+
+You can derive `Get` and `Put` instances for Scala 3 enums using the `Get.deriveEnumString` or `Put.deriveEnumString`.
+
+```scala
+enum Color:
+  case Red, Green, Blue
+
+object Color:
+  given Get[Color] = Get.deriveEnumString[Color]
+  given Put[Color] = Put.deriveEnumString[Color]
+
+fr"SELECT 'Red'".query[Color].unique
+```
+
+As shown above, 
+
+- The instances need to be explicitly derived
+- The string values converts from/to are the literal name of the enum cases
 
 ## Column Vector Mappings