Properly report problems to sbt

Dotty has its own logic for displaying problems with the proper file
path, position, and caret, but if we store this information in
Problem#message we end up with duplicated information in the output
since Zinc will prepend/append similar things (see
sbt.internal.inc.ProblemStringFormats). So far, we worked around this in
Dotty by using an empty position in the sbt bridge reporter, but this
means that crucial semantic information that could be used by a Build
Server Protocol implementation and other tools is lost.

Thanks to sbt/zinc#588 we can now fully
customize how the message is displayed to the user using
Problem#rendered, so we can now store the position information without
any adverse effect.

Author: smarter

sbt-bridge/src/xsbt/DelegatingReporter.scala

@@ -29,7 +29,7 @@ final class DelegatingReporter(delegate: xsbti.Reporter) extends Reporter
       }
 
     val position =
-      if (false && cont.pos.exists) { // Disabled because it duplicates the information printed by Dotty
+      if (cont.pos.exists) {
         val pos = cont.pos
         val src = pos.source
         new Position {
@@ -46,13 +46,14 @@ final class DelegatingReporter(delegate: xsbti.Reporter) extends Reporter
       } else
         noPosition
 
-    val sb = new StringBuilder()
-    sb.append(messageAndPos(cont.contained(), cont.pos, diagnosticLevel(cont)))
-    if (ctx.shouldExplain(cont) && cont.contained().explanation.nonEmpty) {
-      sb.append(explanation(cont.contained()))
+    val message = cont.contained()
+    val rendered = new StringBuilder()
+    rendered.append(messageAndPos(message, cont.pos, diagnosticLevel(cont)))
+    if (ctx.shouldExplain(cont) && message.explanation.nonEmpty) {
+      rendered.append(explanation(message))
     }
 
-    delegate.log(Problem(position, sb.toString(), severity))
+    delegate.log(Problem(position, message.msg, severity, rendered.toString))
   }
 
   private[this] def maybe[T](opt: Option[T]): Optional[T] = opt match {

sbt-bridge/src/xsbt/Problem.scala

@@ -1,12 +1,12 @@
 package xsbt
 
+import java.util.Optional
 import xsbti.{Position, Severity}
 
 final case class Problem(override val position: Position,
                          override val message: String,
-                         override val severity: Severity) extends xsbti.Problem {
+                         override val severity: Severity,
+                         rendered0: String) extends xsbti.Problem {
   override val category = ""
-  override def toString = s"[$severity] $position: $message"
-
+  override val rendered = Optional.of(rendered0)
 }
-