Implements registerStringCodec

Uses TypeTag to recover the full name of type parameter, which is calculated by StringTypeTag. This is sent along in ObjectEvent.
On the other end, we can lookup typeclass instances using the tag key.

internal/util-collection/src/main/scala/sbt/internal/util/INode.scala

@@ -95,7 +95,7 @@ abstract class EvaluateSettings[Scope] {
       keyString
 
     private[this] def keyString =
-      (static.toSeq.flatMap { case (key, value) => if (value eq this) init.showFullKey(key) :: Nil else Nil }).headOption getOrElse "non-static"
+      (static.toSeq.flatMap { case (key, value) => if (value eq this) init.showFullKey.show(key) :: Nil else Nil }).headOption getOrElse "non-static"
 
     final def get: T = synchronized {
       assert(value != null, toString + " not evaluated")

internal/util-collection/src/main/scala/sbt/internal/util/Settings.scala

@@ -6,6 +6,7 @@ package sbt.internal.util
 import scala.language.existentials
 
 import Types._
+import sbt.util.Show
 
 sealed trait Settings[Scope] {
   def data: Map[Scope, AttributeMap]
@@ -119,7 +120,7 @@ trait Init[Scope] {
   def mapScope(f: Scope => Scope): MapScoped = new MapScoped {
     def apply[T](k: ScopedKey[T]): ScopedKey[T] = k.copy(scope = f(k.scope))
   }
-  private final class InvalidReference(val key: ScopedKey[_]) extends RuntimeException("Internal settings error: invalid reference to " + showFullKey(key))
+  private final class InvalidReference(val key: ScopedKey[_]) extends RuntimeException("Internal settings error: invalid reference to " + showFullKey.show(key))
 
   private[this] def applyDefaults(ss: Seq[Setting[_]]): Seq[Setting[_]] =
     {
@@ -215,11 +216,11 @@ trait Init[Scope] {
     {
       val guessed = guessIntendedScope(validKeys, delegates, u.referencedKey)
       val derived = u.defining.isDerived
-      val refString = display(u.defining.key)
+      val refString = display.show(u.defining.key)
       val sourceString = if (derived) "" else parenPosString(u.defining)
-      val guessedString = if (derived) "" else guessed.map(g => "\n     Did you mean " + display(g) + " ?").toList.mkString
+      val guessedString = if (derived) "" else guessed.map(g => "\n     Did you mean " + display.show(g) + " ?").toList.mkString
       val derivedString = if (derived) ", which is a derived setting that needs this key to be defined in this scope." else ""
-      display(u.referencedKey) + " from " + refString + sourceString + derivedString + guessedString
+      display.show(u.referencedKey) + " from " + refString + sourceString + derivedString + guessedString
     }
   private[this] def parenPosString(s: Setting[_]): String =
     s.positionString match { case None => ""; case Some(s) => " (" + s + ")" }
@@ -255,7 +256,7 @@ trait Init[Scope] {
       new Uninitialized(keys, prefix + suffix + " to undefined setting" + suffix + ": " + keysString + "\n ")
     }
   final class Compiled[T](val key: ScopedKey[T], val dependencies: Iterable[ScopedKey[_]], val settings: Seq[Setting[T]]) {
-    override def toString = showFullKey(key)
+    override def toString = showFullKey.show(key)
   }
   final class Flattened(val key: ScopedKey[_], val dependencies: Iterable[ScopedKey[_]])
 

internal/util-collection/src/main/scala/sbt/internal/util/Show.scala

@@ -1,8 +0,0 @@
-package sbt.internal.util
-
-trait Show[T] {
-  def apply(t: T): String
-}
-object Show {
-  def apply[T](f: T => String): Show[T] = new Show[T] { def apply(t: T): String = f(t) }
-}

internal/util-collection/src/main/scala/sbt/util/Show.scala

@@ -0,0 +1,12 @@
+package sbt.util
+
+trait Show[A] {
+  def show(a: A): String
+}
+object Show {
+  def apply[A](f: A => String): Show[A] = new Show[A] { def show(a: A): String = f(a) }
+
+  def fromToString[A]: Show[A] = new Show[A] {
+    def show(a: A): String = a.toString
+  }
+}

internal/util-collection/src/main/scala/sbt/util/ShowLines.scala

@@ -1,4 +1,4 @@
-package sbt.internal.util
+package sbt.util
 
 trait ShowLines[A] {
   def showLines(a: A): Seq[String]

internal/util-collection/src/test/scala/SettingsExample.scala

@@ -1,5 +1,7 @@
 package sbt.internal.util
 
+import sbt.util.Show
+
 /** Define our settings system */
 
 // A basic scope indexed by an integer.
@@ -12,9 +14,10 @@ final case class Scope(nestIndex: Int, idAtIndex: Int = 0)
 //  That would be a general pain.)
 case class SettingsExample() extends Init[Scope] {
   // Provides a way of showing a Scope+AttributeKey[_]
-  val showFullKey: Show[ScopedKey[_]] = new Show[ScopedKey[_]] {
-    def apply(key: ScopedKey[_]) = s"${key.scope.nestIndex}(${key.scope.idAtIndex})/${key.key.label}"
-  }
+  val showFullKey: Show[ScopedKey[_]] = Show[ScopedKey[_]]((key: ScopedKey[_]) =>
+    {
+      s"${key.scope.nestIndex}(${key.scope.idAtIndex})/${key.key.label}"
+    })
 
   // A sample delegation function that delegates to a Scope with a lower index.
   val delegates: Scope => Seq[Scope] = {