First version of capture checker based on rechecking

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -4,6 +4,7 @@ package dotc
 import core._
 import Contexts._
 import typer.{TyperPhase, RefChecks}
+import cc.CheckCaptures
 import parsing.Parser
 import Phases.Phase
 import transform._
@@ -78,6 +79,8 @@ class Compiler {
          new RefChecks,              // Various checks mostly related to abstract members and overriding
          new TryCatchPatterns,       // Compile cases in try/catch
          new PatternMatcher) ::      // Compile pattern matches
+    List(new PreRecheck) ::          // Preparations for check captures phase, enabled under -Ycc
+    List(new CheckCaptures) ::       // Check captures, enabled under -Ycc
     List(new ElimOpaque,             // Turn opaque into normal aliases
          new sjs.ExplicitJSClasses,  // Make all JS classes explicit (Scala.js only)
          new ExplicitOuter,          // Add accessors to outer classes from nested ones.
@@ -101,8 +104,6 @@ class Compiler {
          new TupleOptimizations,     // Optimize generic operations on tuples
          new LetOverApply,           // Lift blocks from receivers of applications
          new ArrayConstructors) ::   // Intercept creation of (non-generic) arrays and intrinsify.
-    List(new PreRecheck) ::          // Preparations for recheck phase, enabled under -Yrecheck
-    List(new TestRecheck) ::         // Test rechecking, enabled under -Yrecheck
     List(new Erasure) ::             // Rewrite types to JVM model, erasing all type parameters, abstract types and refinements.
     List(new ElimErasedValueType,    // Expand erased value types to their underlying implmementation types
          new PureStats,              // Remove pure stats from blocks

compiler/src/dotty/tools/dotc/Run.scala

@@ -20,9 +20,7 @@ import reporting.{Reporter, Suppression, Action}
 import reporting.Diagnostic
 import reporting.Diagnostic.Warning
 import rewrites.Rewrites
-
 import profile.Profiler
-import printing.XprintMode
 import parsing.Parsers.Parser
 import parsing.JavaParsers.JavaParser
 import typer.ImplicitRunInfo
@@ -328,7 +326,7 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
     val fusedPhase = ctx.base.fusedContaining(prevPhase)
     val echoHeader = f"[[syntax trees at end of $fusedPhase%25s]] // ${unit.source}"
     val tree = if ctx.isAfterTyper then unit.tpdTree else unit.untpdTree
-    val treeString = tree.show(using ctx.withProperty(XprintMode, Some(())))
+    val treeString = fusedPhase.show(tree)
 
     last match {
       case SomePrintedTree(phase, lastTreeString) if lastTreeString == treeString =>

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1752,6 +1752,9 @@ object desugar {
         flatTree(pats1 map (makePatDef(tree, mods, _, rhs)))
       case ext: ExtMethods =>
         Block(List(ext), Literal(Constant(())).withSpan(ext.span))
+      case CapturingTypeTree(refs, parent) =>
+        val annot = New(scalaDot(tpnme.retains), List(refs))
+        Annotated(parent, annot)
     }
     desugared.withSpan(tree.span)
   }
@@ -1890,6 +1893,8 @@ object desugar {
             case _ => traverseChildren(tree)
           }
         }.traverse(expr)
+      case CapturingTypeTree(refs, parent) =>
+        collect(parent)
       case _ =>
     }
     collect(tree)

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -260,16 +260,10 @@ object Trees {
   /** Tree's denotation can be derived from its type */
   abstract class DenotingTree[-T >: Untyped](implicit @constructorOnly src: SourceFile) extends Tree[T] {
     type ThisTree[-T >: Untyped] <: DenotingTree[T]
-    override def denot(using Context): Denotation = typeOpt match {
+    override def denot(using Context): Denotation = typeOpt.stripped match
       case tpe: NamedType => tpe.denot
       case tpe: ThisType => tpe.cls.denot
-      case tpe: AnnotatedType => tpe.stripAnnots match {
-        case tpe: NamedType => tpe.denot
-        case tpe: ThisType => tpe.cls.denot
-        case _ => NoDenotation
-      }
       case _ => NoDenotation
-    }
   }
 
   /** Tree's denot/isType/isTerm properties come from a subtree

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -147,6 +147,9 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     case Floating
   }
 
+  /** {x1, ..., xN} T   (only relevant under -Ycc) */
+  case class CapturingTypeTree(refs: List[Tree], parent: Tree)(implicit @constructorOnly src: SourceFile) extends TypTree
+
   /** Short-lived usage in typer, does not need copy/transform/fold infrastructure */
   case class DependentTypeTree(tp: List[Symbol] => Type)(implicit @constructorOnly src: SourceFile) extends Tree
 
@@ -458,7 +461,11 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   def AppliedTypeTree(tpt: Tree, arg: Tree)(implicit src: SourceFile): AppliedTypeTree =
     AppliedTypeTree(tpt, arg :: Nil)
 
-  def TypeTree(tpe: Type)(using Context): TypedSplice = TypedSplice(TypeTree().withTypeUnchecked(tpe))
+  def TypeTree(tpe: Type)(using Context): TypedSplice =
+    TypedSplice(TypeTree().withTypeUnchecked(tpe))
+
+  def InferredTypeTree(tpe: Type)(using Context): TypedSplice =
+    TypedSplice(new InferredTypeTree().withTypeUnchecked(tpe))
 
   def unitLiteral(implicit src: SourceFile): Literal = Literal(Constant(()))
 
@@ -646,6 +653,10 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
       case tree: Number if (digits == tree.digits) && (kind == tree.kind) => tree
       case _ => finalize(tree, untpd.Number(digits, kind))
     }
+    def CapturingTypeTree(tree: Tree)(refs: List[Tree], parent: Tree)(using Context): Tree = tree match
+      case tree: CapturingTypeTree if (refs eq tree.refs) && (parent eq tree.parent) => tree
+      case _ => finalize(tree, untpd.CapturingTypeTree(refs, parent))
+
     def TypedSplice(tree: Tree)(splice: tpd.Tree)(using Context): ProxyTree = tree match {
       case tree: TypedSplice if splice `eq` tree.splice => tree
       case _ => finalize(tree, untpd.TypedSplice(splice)(using ctx))
@@ -711,6 +722,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
         tree
       case MacroTree(expr) =>
         cpy.MacroTree(tree)(transform(expr))
+      case CapturingTypeTree(refs, parent) =>
+        cpy.CapturingTypeTree(tree)(transform(refs), transform(parent))
       case _ =>
         super.transformMoreCases(tree)
     }
@@ -772,6 +785,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
         this(x, splice)
       case MacroTree(expr) =>
         this(x, expr)
+      case CapturingTypeTree(refs, parent) =>
+        this(this(x, refs), parent)
       case _ =>
         super.foldMoreCases(x, tree)
     }

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

@@ -0,0 +1,63 @@
+package dotty.tools
+package dotc
+package cc
+
+import core.*
+import Types.*, Symbols.*, Contexts.*, Annotations.*
+import ast.Trees.*
+import ast.{tpd, untpd}
+import Decorators.*
+import config.Printers.capt
+import printing.Printer
+import printing.Texts.Text
+
+
+case class CaptureAnnotation(refs: CaptureSet, boxed: Boolean) extends Annotation:
+  import CaptureAnnotation.*
+  import tpd.*
+
+  override def tree(using Context) =
+    val elems = refs.elems.toList.map {
+      case cr: TermRef => ref(cr)
+      case cr: TermParamRef => untpd.Ident(cr.paramName).withType(cr)
+      case cr: ThisType => This(cr.cls)
+    }
+    val arg = repeated(elems, TypeTree(defn.AnyType))
+    New(symbol.typeRef, arg :: Nil)
+
+  override def symbol(using Context) = defn.RetainsAnnot
+
+  override def derivedAnnotation(tree: Tree)(using Context): Annotation =
+    unsupported("derivedAnnotation(Tree)")
+
+  def derivedAnnotation(refs: CaptureSet, boxed: Boolean)(using Context): Annotation =
+    if (this.refs eq refs) && (this.boxed == boxed) then this
+    else CaptureAnnotation(refs, boxed)
+
+  override def sameAnnotation(that: Annotation)(using Context): Boolean = that match
+    case CaptureAnnotation(refs2, boxed2) => refs == refs2 && boxed == boxed2
+    case _ => false
+
+  override def mapWith(tp: TypeMap)(using Context) =
+    val elems = refs.elems.toList
+    val elems1 = elems.mapConserve(tp)
+    if elems1 eq elems then this
+    else if elems1.forall(_.isInstanceOf[CaptureRef])
+    then derivedAnnotation(CaptureSet(elems1.asInstanceOf[List[CaptureRef]]*), boxed)
+    else EmptyAnnotation
+
+  override def refersToParamOf(tl: TermLambda)(using Context): Boolean =
+    refs.elems.exists {
+      case TermParamRef(tl1, _) => tl eq tl1
+      case _ => false
+    }
+
+  override def toText(printer: Printer): Text = refs.toText(printer)
+
+  override def hash: Int = (refs.hashCode << 1) | (if boxed then 1 else 0)
+
+  override def eql(that: Annotation) = that match
+    case that: CaptureAnnotation => (this.refs eq that.refs) && (this.boxed == boxed)
+    case _ => false
+
+end CaptureAnnotation

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

@@ -0,0 +1,82 @@
+package dotty.tools
+package dotc
+package cc
+
+import core.*
+import Types.*, Symbols.*, Contexts.*, Annotations.*
+import ast.{tpd, untpd}
+import Decorators.*
+import config.Printers.capt
+import util.Property.Key
+import tpd.*
+
+private val Captures: Key[CaptureSet] = Key()
+private val IsBoxed: Key[Unit] = Key()
+
+def retainedElems(tree: Tree)(using Context): List[Tree] = tree match
+  case Apply(_, Typed(SeqLiteral(elems, _), _) :: Nil) => elems
+  case _ => Nil
+
+extension (tree: Tree)
+
+  def toCaptureRef(using Context): CaptureRef = tree.tpe.asInstanceOf[CaptureRef]
+
+  def toCaptureSet(using Context): CaptureSet =
+    tree.getAttachment(Captures) match
+      case Some(refs) => refs
+      case None =>
+        val refs = CaptureSet(retainedElems(tree).map(_.toCaptureRef)*)
+          .showing(i"toCaptureSet $tree --> $result", capt)
+        tree.putAttachment(Captures, refs)
+        refs
+
+  def isBoxedCapturing(using Context): Boolean =
+    tree.hasAttachment(IsBoxed)
+
+  def setBoxedCapturing()(using Context): Unit =
+    tree.putAttachment(IsBoxed, ())
+
+extension (tp: Type)
+
+  def derivedCapturingType(parent: Type, refs: CaptureSet)(using Context): Type = tp match
+    case CapturingType(p, r, b) =>
+      if (parent eq p) && (refs eq r) then tp
+      else CapturingType(parent, refs, b)
+
+  /** If this is  type variable instantiated or upper bounded with a capturing type,
+   *  the capture set associated with that type. Extended to and-or types and
+   *  type proxies in the obvious way. If a term has a type with a boxed captureset,
+   *  that captureset counts towards the capture variables of the envirionment.
+   */
+  def boxedCaptured(using Context): CaptureSet =
+    def getBoxed(tp: Type): CaptureSet = tp match
+      case CapturingType(_, refs, boxed) => if boxed then refs else CaptureSet.empty
+      case tp: TypeProxy => getBoxed(tp.superType)
+      case tp: AndType => getBoxed(tp.tp1) ++ getBoxed(tp.tp2)
+      case tp: OrType => getBoxed(tp.tp1) ** getBoxed(tp.tp2)
+      case _ => CaptureSet.empty
+    getBoxed(tp)
+
+  def isBoxedCapturing(using Context) = !tp.boxedCaptured.isAlwaysEmpty
+
+  def canHaveInferredCapture(using Context): Boolean = tp match
+    case tp: TypeRef if tp.symbol.isClass =>
+      !tp.symbol.isValueClass && tp.symbol != defn.AnyClass
+    case _: TypeVar | _: TypeParamRef =>
+      false
+    case tp: TypeProxy =>
+      tp.superType.canHaveInferredCapture
+    case tp: AndType =>
+      tp.tp1.canHaveInferredCapture && tp.tp2.canHaveInferredCapture
+    case tp: OrType =>
+      tp.tp1.canHaveInferredCapture || tp.tp2.canHaveInferredCapture
+    case _ =>
+      false
+
+  def stripCapturing(using Context): Type = tp.dealiasKeepAnnots match
+    case CapturingType(parent, _, _) =>
+      parent.stripCapturing
+    case atd @ AnnotatedType(parent, annot) =>
+      atd.derivedAnnotatedType(parent.stripCapturing, annot)
+    case _ =>
+      tp