Give GadtConstraint a nicer toText

Instead of showing the underlying constraint info, which just looks bad,
takes up lots of space, and shows details that aren't relevant and often
includes leaked type constructors, just show the symbols and their full
bounds, nice and compact.  And do that as a part of toText instead of
having a side "debugBoundsDescription" alternative.

Author: dwijnand

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

@@ -55,8 +55,6 @@ sealed abstract class GadtConstraint extends Showable {
 
   /** Restore the state from other [[GadtConstraint]], probably copied using [[fresh]] */
   def restore(other: GadtConstraint): Unit
-
-  def debugBoundsDescription(using Context): String
 }
 
 final class ProperGadtConstraint private(
@@ -134,7 +132,7 @@ final class ProperGadtConstraint private(
 
     // The replaced symbols are picked up here.
     addToConstraint(poly1, tvars)
-      .showing(i"added to constraint: [$poly1] $params%, %\n$debugBoundsDescription", gadts)
+      .showing(i"added to constraint: [$poly1] $params%, % gadt = $this", gadts)
   }
 
   override def addBound(sym: Symbol, bound: Type, isUpper: Boolean)(using Context): Boolean = {
@@ -291,17 +289,7 @@ final class ProperGadtConstraint private(
 
   override def constr = gadtsConstr
 
-  override def toText(printer: Printer): Texts.Text = constraint.toText(printer)
-
-  override def debugBoundsDescription(using Context): String = {
-    val sb = new mutable.StringBuilder
-    sb ++= constraint.show
-    sb += '\n'
-    mapping.foreachBinding { case (sym, _) =>
-      sb ++= i"$sym: ${fullBounds(sym)}\n"
-    }
-    sb.result
-  }
+  override def toText(printer: Printer): Texts.Text = printer.toText(this)
 }
 
 @sharable object EmptyGadtConstraint extends GadtConstraint {
@@ -325,7 +313,5 @@ final class ProperGadtConstraint private(
   override def restore(other: GadtConstraint): Unit =
     assert(!other.isNarrowing, "cannot restore a non-empty GADTMap")
 
-  override def debugBoundsDescription(using Context): String = "EmptyGadtConstraint"
-
-  override def toText(printer: Printer): Texts.Text = "EmptyGadtConstraint"
+  override def toText(printer: Printer): Texts.Text = printer.toText(this)
 }

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

@@ -261,7 +261,7 @@ trait PatternTypeConstrainer { self: TypeComparer =>
     val assumeInvariantRefinement =
       migrateTo3 || forceInvariantRefinement || refinementIsInvariant(patternTp)
 
-    trace(i"constraining simple pattern type $tp >:< $pt", gadts, res => s"$res\ngadt = ${ctx.gadt.debugBoundsDescription}") {
+    trace(i"constraining simple pattern type $tp >:< $pt", gadts, (res: Boolean) => i"$res gadt = ${ctx.gadt}") {
       (tp, pt) match {
         case (AppliedType(tyconS, argsS), AppliedType(tyconP, argsP)) =>
           val saved = state.nn.constraint

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

@@ -3226,7 +3226,7 @@ class ExplainingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
     }
 
   override def gadtAddBound(sym: Symbol, b: Type, isUpper: Boolean): Boolean =
-    traceIndented(s"add GADT constraint ${show(sym)} ${if isUpper then "<:" else ">:"} ${show(b)} $frozenNotice, GADT constraint = ${show(ctx.gadt.debugBoundsDescription)}") {
+    traceIndented(s"add GADT constraint ${show(sym)} ${if isUpper then "<:" else ">:"} ${show(b)} $frozenNotice, GADT constraint = ${show(ctx.gadt)}") {
       super.gadtAddBound(sym, b, isUpper)
     }
 

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -693,6 +693,14 @@ class PlainPrinter(_ctx: Context) extends Printer {
     finally
       ctx.typerState.constraint = savedConstraint
 
+  def toText(g: GadtConstraint): Text = g match
+    case EmptyGadtConstraint     => "EmptyGadtConstraint"
+    case g: ProperGadtConstraint =>
+      val deps = for sym <- g.symbols yield
+        val bound = g.fullBounds(sym).nn
+        (typeText(toText(sym.typeRef)) ~ toText(bound)).close
+      ("GadtConstraint(" ~ Text(deps, ", ") ~ ")").close
+
   def plain: PlainPrinter = this
 
   protected def keywordStr(text: String): String = coloredStr(text, SyntaxHighlighting.KeywordColor)

compiler/src/dotty/tools/dotc/printing/Printer.scala

@@ -163,6 +163,9 @@ abstract class Printer {
   /** Textual representation of a constraint */
   def toText(c: OrderingConstraint): Text
 
+  /** Textual representation of a GADT constraint */
+  def toText(c: GadtConstraint): Text
+
   /** Render element within highest precedence */
   def toTextLocal(elem: Showable): Text =
     atPrec(DotPrec) { elem.toText(this) }