Print capture variable dependencies under -Ydebug-cc

Author: odersky

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -12,8 +12,9 @@ import annotation.internal.sharable
 import reporting.trace
 import printing.{Showable, Printer}
 import printing.Texts.*
-import util.SimpleIdentitySet
+import util.{SimpleIdentitySet, Property}
 import util.common.alwaysTrue
+import scala.collection.mutable
 
 /** A class for capture sets. Capture sets can be constants or variables.
  *  Capture sets support inclusion constraints <:< where <:< is subcapturing.
@@ -317,6 +318,7 @@ object CaptureSet:
       s"$id${getClass.getSimpleName.take(1)}$trail"
 
     override def toText(printer: Printer): Text = inContext(printer.printerContext) {
+      for vars <- ctx.property(ShownVars) do vars += this
       super.toText(printer) ~ (Str(ids) provided !isConst && ctx.settings.YccDebug.value)
     }
 
@@ -528,4 +530,34 @@ object CaptureSet:
         empty
     recur(tp)
       .showing(i"capture set of $tp = $result", capt)
+
+  private val ShownVars: Property.Key[mutable.Set[Var]] = Property.Key()
+
+  def withCaptureSetsExplained[T](op: Context ?=> T)(using ctx: Context): T =
+    if ctx.settings.YccDebug.value then
+      val shownVars = mutable.Set[Var]()
+      inContext(ctx.withProperty(ShownVars, Some(shownVars))) {
+        try op
+        finally
+          val reachable = mutable.Set[Var]()
+          val todo = mutable.Queue[Var]() ++= shownVars
+          def incl(cv: Var): Unit =
+            if !reachable.contains(cv) then todo += cv
+          while todo.nonEmpty do
+            val cv = todo.dequeue()
+            if !reachable.contains(cv) then
+              reachable += cv
+              cv.deps.foreach {
+                case cv: Var => incl(cv)
+                case _ =>
+              }
+              cv match
+                case cv: DerivedVar => incl(cv.source)
+                case _ =>
+          val allVars = reachable.toArray.sortBy(_.id)
+          println(i"Capture set dependencies:")
+          for cv <- allVars do
+            println(i"  ${cv.show.padTo(20, ' ')} :: ${cv.deps.toList}%, %")
+      }
+    else op
 end CaptureSet

compiler/src/dotty/tools/dotc/typer/CheckCaptures.scala

@@ -16,14 +16,14 @@ import Trees._
 import scala.util.control.NonFatal
 import typer.ErrorReporting._
 import util.Spans.Span
-import util.{SimpleIdentitySet, EqHashMap, SrcPos, Property}
+import util.{SimpleIdentitySet, EqHashMap, SrcPos}
 import util.Chars.*
 import transform.*
 import transform.SymUtils.*
 import scala.collection.mutable
 import reporting._
 import dotty.tools.backend.jvm.DottyBackendInterface.symExtensions
-import CaptureSet.CompareResult
+import CaptureSet.{CompareResult, withCaptureSetsExplained}
 
 object CheckCaptures:
   import ast.tpd.*
@@ -359,8 +359,12 @@ class CheckCaptures extends Recheck:
       res
 
     override def checkUnit(unit: CompilationUnit)(using Context): Unit =
-      super.checkUnit(unit)
-      PostRefinerCheck.traverse(unit.tpdTree)
+      withCaptureSetsExplained {
+        super.checkUnit(unit)
+        PostRefinerCheck.traverse(unit.tpdTree)
+        if ctx.settings.YccDebug.value then
+          show(unit.tpdTree) // this dows not print tree, but makes its variables visible for dependency printing
+      }
 
     def checkNotGlobal(tree: Tree, allArgs: Tree*)(using Context): Unit =
       if disallowGlobal then