Make input tasks only require a JSON writer, not reader (com-lihaoyi#…

…1601)

Resolves com-lihaoyi#598. Input tasks results are re-evaluated every time and thus never read from the disk, and so a `upickle.default.Reader` is not required. Nevertheless, we still require that a `upickle.default.Writer` in order to generate the `*.json` files used for `./mill show` and other debugging purposes.

Targeting com-lihaoyi#1600 to avoid merge conflicts

Let's see what CI says...

TBH not sure if this is worth doing, or whether we should just specify that `Input` tasks require a `ReadWriter` just to keep things consistent. We don't currently have any use case for de-serializing the input task JSON (e.g. even `mill show` takes the JSON from disk and spits it out verbatim without de-serializing) but that doesn't mean we won't find such use cases in future. But if such use cases do re-appear, we can always add that functionality back. `Command`s are write-only, so making `Input`s write-only would also not be unprecedented

docs/antora/modules/ROOT/pages/Tasks.adoc

@@ -306,7 +306,7 @@ different Task types:
 | |Target |Command |Source/Input |Anonymous Task |Persistent Target |Worker 
 
 |Cached to Disk |X |X | | |X |
-|Must be JSON Writable |X |X | | |X | 
+|Must be JSON Writable |X |X |X| |X |
 |Must be JSON Readable |X | | | |X | 
 |Runnable from the Command Line |X |X | | |X | 
 |Can Take Arguments | |X | |X | | 

main/core/src/mill/define/Task.scala

@@ -185,11 +185,11 @@ object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Resul
       )
     )
   }
-  def input[T](value: Result[T])(implicit rw: RW[T], ctx: mill.define.Ctx): Input[T] =
+  def input[T](value: Result[T])(implicit w: upickle.default.Writer[T], ctx: mill.define.Ctx): Input[T] =
     macro inputImpl[T]
 
   def inputImpl[T: c.WeakTypeTag](c: Context)(value: c.Expr[T])(
-      rw: c.Expr[RW[T]],
+      w: c.Expr[upickle.default.Writer[T]],
       ctx: c.Expr[mill.define.Ctx]
   ): c.Expr[Input[T]] = {
     import c.universe._
@@ -199,7 +199,7 @@ object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Resul
         new Input[T](
           Applicative.impl[Task, T, mill.api.Ctx](c)(value).splice,
           ctx.splice,
-          rw.splice
+          w.splice
         )
       )
     )
@@ -328,20 +328,16 @@ class Persistent[+T](t: Task[T], ctx0: mill.define.Ctx, readWrite: RW[_])
   override def flushDest = false
 }
 
-class Input[T](t: Task[T], ctx0: mill.define.Ctx, val readWrite: RW[_])
-    extends NamedTaskImpl[T](ctx0, t)
-    with Target[T] {
+class Input[T](t: Task[T], ctx0: mill.define.Ctx, val writer: upickle.default.Writer[_])
+    extends NamedTaskImpl[T](ctx0, t) {
   override def sideHash = util.Random.nextInt()
 }
 
 class Sources(t: Task[Seq[PathRef]], ctx0: mill.define.Ctx)
     extends Input[Seq[PathRef]](
       t,
       ctx0,
-      RW.join(
-        upickle.default.SeqLikeReader[Seq, PathRef],
-        upickle.default.SeqLikeWriter[Seq, PathRef]
-      )
+      upickle.default.SeqLikeWriter[Seq, PathRef]
     )
 
 class Source(t: Task[PathRef], ctx0: mill.define.Ctx)

main/core/src/mill/eval/Evaluator.scala

@@ -18,13 +18,14 @@ import scala.jdk.CollectionConverters._
 import scala.reflect.ClassTag
 import scala.util.control.NonFatal
 
-case class Labelled[T](task: NamedTask[T], segments: Segments) {
+case class Labelled[T](task: NamedTask[T], segments: Segments){
   def format = task match {
     case t: Target[T] => Some(t.readWrite.asInstanceOf[upickle.default.ReadWriter[T]])
     case _ => None
   }
   def writer = task match {
     case t: mill.define.Command[T] => Some(t.writer.asInstanceOf[upickle.default.Writer[T]])
+    case t: mill.define.Input[T] => Some(t.writer.asInstanceOf[upickle.default.Writer[T]])
     case t: Target[T] => Some(t.readWrite.asInstanceOf[upickle.default.ReadWriter[T]])
     case _ => None
   }

main/src/mill/main/Resolve.scala

@@ -14,7 +14,7 @@ object ResolveMetadata extends Resolve[String] {
     val targets =
       obj
         .millInternal
-        .reflectAll[Target[_]]
+        .reflectAll[NamedTask[_]]
         .map(_.toString)
     val commands =
       for {
@@ -192,15 +192,15 @@ object ResolveTasks extends Resolve[NamedTask[Any]] {
       Right(
         obj.millInternal.modules
           .filter(_ != obj)
-          .flatMap(m => m.millInternal.reflectAll[Target[_]])
+          .flatMap(m => m.millInternal.reflectAll[NamedTask[_]])
       )
-    case "_" => Right(obj.millInternal.reflectAll[Target[_]])
+    case "_" => Right(obj.millInternal.reflectAll[NamedTask[_]])
 
     case _ =>
       val target =
         obj
           .millInternal
-          .reflectSingle[Target[_]](last)
+          .reflectSingle[NamedTask[_]](last)
           .map(Right(_))
 
       val command = Resolve.invokeCommand(