SI-9206 REPL custom continuation prompt

Because who doesn't want to customize their continuation prompt?

`scala -Dscala.repl.continue="..."` looks especially nice
with `-Dscala.color`.

Somewhat works when pasting, but the test rig for running a
transcript does not seek to support custom secondary prompts.

Author: som-snytt

src/repl/scala/tools/nsc/interpreter/ILoop.scala

@@ -753,7 +753,8 @@ class ILoop(in0: Option[BufferedReader], protected val out: JPrintWriter)
 
   private object paste extends Pasted {
     import scala.util.matching.Regex.quote
-    val ContinueString = "     | "
+    val ContinuePrompt = replProps.continuePrompt
+    val ContinueString = replProps.continueText     // "     | "
     val PromptString   = prompt.lines.toList.last
     val anyPrompt = s"""\\s*(?:${quote(PromptString.trim)}|${quote(AltPromptString.trim)})\\s*""".r
 
@@ -797,7 +798,7 @@ class ILoop(in0: Option[BufferedReader], protected val out: JPrintWriter)
         echo("You typed two blank lines.  Starting a new command.")
         None
       case IR.Incomplete =>
-        in.readLine(paste.ContinueString) match {
+        in.readLine(paste.ContinuePrompt) match {
           case null =>
             // we know compilation is going to fail since we're at EOF and the
             // parser thinks the input is still incomplete, but since this is
@@ -940,8 +941,9 @@ object ILoop {
       Console.withOut(ostream) {
         val output = new JPrintWriter(new OutputStreamWriter(ostream), true) {
           // skip margin prefix for continuation lines, unless preserving session text for test
+          // should test for repl.paste.ContinueString or replProps.continueText.contains(ch)
           override def write(str: String) =
-            if (!inSession && (str forall (ch => ch.isWhitespace || ch == '|'))) ()  // repl.paste.ContinueString
+            if (!inSession && (str forall (ch => ch.isWhitespace || ch == '|'))) ()
             else super.write(str)
         }
         val input = new BufferedReader(new StringReader(code.trim + "\n")) {

src/repl/scala/tools/nsc/interpreter/ReplProps.scala

@@ -45,11 +45,17 @@ class ReplProps {
 
   // Handy system prop for shell prompt, or else pick it up from compiler.properties
   val promptString = Prop[String]("scala.repl.prompt").option getOrElse (if (info) "%nscala %s> " else shellPromptString)
-  def promptText   = enversion(promptString)
-  val prompt = {
-    import scala.io.AnsiColor.{ MAGENTA, RESET }
-    if (colorOk) s"$MAGENTA$promptText$RESET" else promptText
+  val promptText   = enversion(promptString)
+  val prompt       = encolor(promptText)
+
+  // Prompt for continued input, will be right-adjusted to width of the primary prompt
+  val continueString = Prop[String]("scala.repl.continue").option getOrElse "| "
+  val continueText   = {
+    val text   = enversion(continueString)
+    val margin = promptText.lines.toList.last.length - text.length
+    if (margin > 0) " " * margin + text else text
   }
+  val continuePrompt = encolor(continueText)
 
   //def welcome = enversion(Prop[String]("scala.repl.welcome") or shellWelcomeString)
   def welcome = enversion {