Give GadtConstraint a nicer toText (#16085)

Author: dwijnand

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

@@ -56,6 +56,7 @@ sealed abstract class GadtConstraint extends Showable {
   /** Restore the state from other [[GadtConstraint]], probably copied using [[fresh]] */
   def restore(other: GadtConstraint): Unit
 
+  /** Provides more information than toText, by showing the underlying Constraint details. */
   def debugBoundsDescription(using Context): String
 }
 
@@ -134,7 +135,7 @@ 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 +292,9 @@ final class ProperGadtConstraint private(
 
   override def constr = gadtsConstr
 
-  override def toText(printer: Printer): Texts.Text = constraint.toText(printer)
+  override def toText(printer: Printer): Texts.Text = printer.toText(this)
 
-  override def debugBoundsDescription(using Context): String = {
-    val sb = new mutable.StringBuilder
-    sb ++= constraint.show
-    sb += '\n'
-    mapping.foreachBinding { case (sym, _) =>
-      sb ++= i"$sym: ${fullBounds(sym)}\n"
-    }
-    sb.result
-  }
+  override def debugBoundsDescription(using Context): String = i"$this\n$constraint"
 }
 
 @sharable object EmptyGadtConstraint extends GadtConstraint {
@@ -325,7 +318,6 @@ 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)
+  override def debugBoundsDescription(using Context): String = i"$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) }