Improve typer traces

1. Drop traceIndented from isSubType, which calls recur so it was just
   doubling the lines

2. Replace newlines with spaces, in doTrace and traceIndented, as
   they're meant to be single lines.  For example ErrorType and
   MatchType have toText that are multi-line and mess up the output.

3. Change ExplainingTypeComparer#addConstraint to use show like the
   other methods.

Author: dwijnand

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -210,7 +210,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
    *  types (as when we go from an abstract type to one of its bounds). In that case
    *  one should use `isSubType(_, _, a)` where `a` defines the kind of approximation.
    *
-   *  Note: Logicaly, `recur` could be nested in `isSubType`, which would avoid
+   *  Note: Logically, `recur` could be nested in `isSubType`, which would avoid
    *  the instance state consisting `approx` and `leftRoot`. But then the implemented
    *  code would have two extra parameters for each of the many calls that go from
    *  one sub-part of isSubType to another.
@@ -2955,27 +2955,24 @@ class ExplainingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
     if (skipped) op
     else {
       indent += 2
-      b.append("\n").append(" " * indent).append("==> ").append(str)
+      val str1 = str.replace('\n', ' ')
+      b.append("\n").append(" " * indent).append("==> ").append(str1)
       val res = op
-      b.append("\n").append(" " * indent).append("<== ").append(str).append(" = ").append(show(res))
+      b.append("\n").append(" " * indent).append("<== ").append(str1).append(" = ").append(show(res))
       indent -= 2
       res
     }
 
   private def frozenNotice: String =
     if frozenConstraint then " in frozen constraint" else ""
 
-  override def isSubType(tp1: Type, tp2: Type, approx: ApproxState): Boolean =
+  override def recur(tp1: Type, tp2: Type): Boolean =
     def moreInfo =
       if Config.verboseExplainSubtype || ctx.settings.verbose.value
       then s" ${tp1.getClass} ${tp2.getClass}"
       else ""
+    val approx = approxState
     traceIndented(s"${show(tp1)}  <:  ${show(tp2)}$moreInfo${approx.show}$frozenNotice") {
-      super.isSubType(tp1, tp2, approx)
-    }
-
-  override def recur(tp1: Type, tp2: Type): Boolean =
-    traceIndented(s"${show(tp1)}  <:  ${show(tp2)} (recurring)$frozenNotice") {
       super.recur(tp1, tp2)
     }
 
@@ -2995,7 +2992,7 @@ class ExplainingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
     }
 
   override def addConstraint(param: TypeParamRef, bound: Type, fromBelow: Boolean)(using Context): Boolean =
-    traceIndented(i"add constraint $param ${if (fromBelow) ">:" else "<:"} $bound $frozenConstraint, constraint = ${ctx.typerState.constraint}") {
+    traceIndented(s"add constraint ${show(param)} ${if (fromBelow) ">:" else "<:"} ${show(bound)} $frozenNotice, constraint = ${show(ctx.typerState.constraint)}") {
       super.addConstraint(param, bound, fromBelow)
     }
 

compiler/src/dotty/tools/dotc/reporting/trace.scala

@@ -76,7 +76,7 @@ trait TraceSyntax:
     else
       // Avoid evaluating question multiple time, since each evaluation
       // may cause some extra logging output.
-      val q = question
+      val q = question.replace('\n', ' ')
       val leading = s"==> $q?"
       val trailing = (res: T) => s"<== $q = ${showOp(res)}"
       var finalized = false