[2.x] Short-circuit x := expr.value (#9831)

**Problem**
Identity map of a task (x := expr.value) goes through the task macro
machinery requiring JsonFormat etc for cached task.
This is not necessary for dynamic task etc.

**Solution**
This implements a short-circuit for x := expr.value shaped tree.

main-settings/src/main/scala/sbt/std/TaskMacro.scala

@@ -128,7 +128,13 @@ object TaskMacro:
     val cached = ContextUtil.isTaskCacheByDefault && !isUncacheApplied && cl.nonEmpty
     t match
       case '{ if $cond then $thenp else $elsep } => taskIfImpl[A1](t, cached)
-      case _                                     =>
+      case '{ InputWrapper.`wrapInitTask_\u2603\u2603`[A1]($in) }
+          if in.isExprOf[Initialize[Task[A1]]] =>
+        if in.isExprOf[Scoped] then
+          val convert1 = new FullConvert(qctx, 0)
+          convert1.contMapN[A1, F, Id](t, convert1.appExpr, None)
+        else in.asExprOf[Initialize[Task[A1]]]
+      case _ =>
         val convert1 = new FullConvert(qctx, 0)
         validateTaskReferences(convert1, t)
         if cached then

sbt-app/src/sbt-test/cache/identity-map/build.sbt

@@ -0,0 +1,21 @@
+val example = taskKey[Uncacheable]("example")
+val alias = taskKey[Uncacheable]("alias")
+val both = taskKey[Unit]("both")
+val check = taskKey[Unit]("check")
+
+example := Def.taskDyn {
+  Def.task {
+    Uncacheable.counter.incrementAndGet()
+    new Uncacheable()
+  }
+}.value
+
+alias := example.value
+
+both := Def.uncached {
+  val a = example.value
+  val b = alias.value
+  assert(a eq b)
+}
+
+check := assert(Uncacheable.counter.get == 1, s"counter = ${Uncacheable.counter.get}")

sbt-app/src/sbt-test/cache/identity-map/project/Uncacheable.scala

@@ -0,0 +1,7 @@
+import java.util.concurrent.atomic.AtomicInteger
+
+class Uncacheable:
+  override def toString: String = "Uncacheable"
+
+object Uncacheable:
+  val counter = new AtomicInteger(0)

sbt-app/src/sbt-test/cache/identity-map/test

@@ -0,0 +1,2 @@
+> both
+> check