Add experimental capture checking

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 +79,10 @@ class Compiler {
          new SpecializeApplyMethods, // Adds specialized methods to FunctionN
          new TryCatchPatterns,       // Compile cases in try/catch
          new PatternMatcher) ::      // Compile pattern matches
+    List(new TestRecheck.Pre) ::     // Test only: run rechecker, enabled under -Yrecheck-test
+    List(new TestRecheck) ::         // Test only: run rechecker, enabled under -Yrecheck-test
+    List(new CheckCaptures.Pre) ::   // 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.

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

@@ -19,7 +19,6 @@ import reporting.{Suppression, Action, Profile, ActiveProfile, NoProfile}
 import reporting.Diagnostic
 import reporting.Diagnostic.Warning
 import rewrites.Rewrites
-
 import profile.Profiler
 import printing.XprintMode
 import typer.ImplicitRunInfo
@@ -294,7 +293,7 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
     val fusedPhase = ctx.phase.prevMega
     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

@@ -468,7 +468,7 @@ object desugar {
 
     if mods.is(Trait) then
       for vparams <- originalVparamss; vparam <- vparams do
-        if vparam.tpt.isInstanceOf[ByNameTypeTree] then
+        if isByNameType(vparam.tpt) then
           report.error(em"implementation restriction: traits cannot have by name parameters", vparam.srcPos)
 
     // Annotations on class _type_ parameters are set on the derived parameters
@@ -576,9 +576,8 @@ object desugar {
       appliedTypeTree(tycon, targs)
     }
 
-    def isRepeated(tree: Tree): Boolean = tree match {
+    def isRepeated(tree: Tree): Boolean = stripByNameType(tree) match {
       case PostfixOp(_, Ident(tpnme.raw.STAR)) => true
-      case ByNameTypeTree(tree1) => isRepeated(tree1)
       case _ => false
     }
 
@@ -1810,6 +1809,16 @@ object desugar {
         flatTree(pats1 map (makePatDef(tree, mods, _, rhs)))
       case ext: ExtMethods =>
         Block(List(ext), Literal(Constant(())).withSpan(ext.span))
+      case CapturingTypeTree(refs, parent) =>
+        // convert   `{refs} T`   to `T @retains refs`
+        //           `{refs}-> T` to `-> (T @retainsByName refs)`
+        def annotate(annotName: TypeName, tp: Tree) =
+          Annotated(tp, New(scalaAnnotationDot(annotName), List(refs)))
+        parent match
+          case ByNameTypeTree(restpt) =>
+            cpy.ByNameTypeTree(parent)(annotate(tpnme.retainsByName, restpt))
+          case _ =>
+            annotate(tpnme.retains, parent)
     }
     desugared.withSpan(tree.span)
   }
@@ -1946,6 +1955,8 @@ object desugar {
             case _ => traverseChildren(tree)
           }
         }.traverse(expr)
+      case CapturingTypeTree(refs, parent) =>
+        collect(parent)
       case _ =>
     }
     collect(tree)

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

@@ -178,8 +178,7 @@ trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
   }
 
   /** Is tpt a vararg type of the form T* or => T*? */
-  def isRepeatedParamType(tpt: Tree)(using Context): Boolean = tpt match {
-    case ByNameTypeTree(tpt1) => isRepeatedParamType(tpt1)
+  def isRepeatedParamType(tpt: Tree)(using Context): Boolean = stripByNameType(tpt) match {
     case tpt: TypeTree => tpt.typeOpt.isRepeatedParam
     case AppliedTypeTree(Select(_, tpnme.REPEATED_PARAM_CLASS), _) => true
     case _ => false
@@ -196,6 +195,18 @@ trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
     case arg => arg.typeOpt.widen.isRepeatedParam
   }
 
+  /** Is tree a type tree of the form `=> T` or (under -Ycc) `{refs}-> T`? */
+  def isByNameType(tree: Tree)(using Context): Boolean =
+    stripByNameType(tree) ne tree
+
+  /** Strip `=> T` to `T` and (under -Ycc) `{refs}-> T` to `T` */
+  def stripByNameType(tree: Tree)(using Context): Tree = unsplice(tree) match
+    case ByNameTypeTree(t1) => t1
+    case untpd.CapturingTypeTree(_, parent) =>
+      val parent1 = stripByNameType(parent)
+      if parent1 eq parent then tree else parent1
+    case _ => tree
+
   /** All type and value parameter symbols of this DefDef */
   def allParamSyms(ddef: DefDef)(using Context): List[Symbol] =
     ddef.paramss.flatten.map(_.symbol)
@@ -388,6 +399,22 @@ trait UntypedTreeInfo extends TreeInfo[Untyped] { self: Trees.Instance[Untyped]
       case _ => None
     }
   }
+
+  /** Under -Ycc: A builder and extractor for `=> T`, which is an alias for `{*}-> T`.
+   *  Only trees of the form `=> T` are matched; trees written directly as `{*}-> T`
+   *  are ignored by the extractor.
+   */
+  object ImpureByNameTypeTree:
+
+    def apply(tp: ByNameTypeTree)(using Context): untpd.CapturingTypeTree =
+      untpd.CapturingTypeTree(
+        Ident(nme.CAPTURE_ROOT).withSpan(tp.span.startPos) :: Nil, tp)
+
+    def unapply(tp: Tree)(using Context): Option[ByNameTypeTree] = tp match
+      case untpd.CapturingTypeTree(id @ Ident(nme.CAPTURE_ROOT) :: Nil, bntp: ByNameTypeTree)
+      if id.span == bntp.span.startPos => Some(bntp)
+      case _ => None
+  end ImpureByNameTypeTree
 }
 
 trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>

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

@@ -253,16 +253,10 @@ 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/tpd.scala

@@ -164,8 +164,8 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   def Inlined(call: Tree, bindings: List[MemberDef], expansion: Tree)(using Context): Inlined =
     ta.assignType(untpd.Inlined(call, bindings, expansion), bindings, expansion)
 
-  def TypeTree(tp: Type)(using Context): TypeTree =
-    untpd.TypeTree().withType(tp)
+  def TypeTree(tp: Type, inferred: Boolean = false)(using Context): TypeTree =
+    (if inferred then untpd.InferredTypeTree() else untpd.TypeTree()).withType(tp)
 
   def SingletonTypeTree(ref: Tree)(using Context): SingletonTypeTree =
     ta.assignType(untpd.SingletonTypeTree(ref), ref)
@@ -203,8 +203,8 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
     ta.assignType(untpd.UnApply(fun, implicits, patterns), proto)
   }
 
-  def ValDef(sym: TermSymbol, rhs: LazyTree = EmptyTree)(using Context): ValDef =
-    ta.assignType(untpd.ValDef(sym.name, TypeTree(sym.info), rhs), sym)
+  def ValDef(sym: TermSymbol, rhs: LazyTree = EmptyTree, inferred: Boolean = false)(using Context): ValDef =
+    ta.assignType(untpd.ValDef(sym.name, TypeTree(sym.info, inferred), rhs), sym)
 
   def SyntheticValDef(name: TermName, rhs: Tree, flags: FlagSet = EmptyFlags)(using Context): ValDef =
     ValDef(newSymbol(ctx.owner, name, Synthetic | flags, rhs.tpe.widen, coord = rhs.span), rhs)

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

@@ -69,13 +69,13 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   case class InterpolatedString(id: TermName, segments: List[Tree])(implicit @constructorOnly src: SourceFile)
     extends TermTree
 
-  /** A function type */
+  /** A function type or closure */
   case class Function(args: List[Tree], body: Tree)(implicit @constructorOnly src: SourceFile) extends Tree {
     override def isTerm: Boolean = body.isTerm
     override def isType: Boolean = body.isType
   }
 
-  /** A function type with `implicit`, `erased`, or `given` modifiers */
+  /** A function type or closure with `implicit`, `erased`, or `given` modifiers */
   class FunctionWithMods(args: List[Tree], body: Tree, val mods: Modifiers)(implicit @constructorOnly src: SourceFile)
     extends Function(args, body)
 
@@ -145,6 +145,9 @@ 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
 
@@ -213,6 +216,9 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     case class Transparent()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Transparent)
 
     case class Infix()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Infix)
+
+    /** Used under -Ycc to mark impure function types `A => B` in `FunctionWithMods` */
+    case class Impure()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Impure)
   }
 
   /** Modifiers and annotations for definitions
@@ -390,6 +396,7 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   def JavaSeqLiteral(elems: List[Tree], elemtpt: Tree)(implicit src: SourceFile): JavaSeqLiteral = new JavaSeqLiteral(elems, elemtpt)
   def Inlined(call: tpd.Tree, bindings: List[MemberDef], expansion: Tree)(implicit src: SourceFile): Inlined = new Inlined(call, bindings, expansion)
   def TypeTree()(implicit src: SourceFile): TypeTree = new TypeTree()
+  def InferredTypeTree()(implicit src: SourceFile): TypeTree = new InferredTypeTree()
   def SingletonTypeTree(ref: Tree)(implicit src: SourceFile): SingletonTypeTree = new SingletonTypeTree(ref)
   def RefinedTypeTree(tpt: Tree, refinements: List[Tree])(implicit src: SourceFile): RefinedTypeTree = new RefinedTypeTree(tpt, refinements)
   def AppliedTypeTree(tpt: Tree, args: List[Tree])(implicit src: SourceFile): AppliedTypeTree = new AppliedTypeTree(tpt, args)
@@ -647,6 +654,10 @@ 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))
@@ -710,6 +721,8 @@ 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)
     }
@@ -769,6 +782,8 @@ 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/BoxedTypeCache.scala

@@ -0,0 +1,19 @@
+package dotty.tools
+package dotc
+package cc
+
+import core.*
+import Types.*, Symbols.*, Contexts.*
+
+/** A one-element cache for the boxed version of an unboxed capturing type */
+class BoxedTypeCache:
+  private var boxed: Type = compiletime.uninitialized
+  private var unboxed: Type = NoType
+
+  def apply(tp: AnnotatedType)(using Context): Type =
+    if tp ne unboxed then
+      unboxed = tp
+      val CapturingType(parent, refs) = tp: @unchecked
+      boxed = CapturingType(parent, refs, boxed = true)
+    boxed
+end BoxedTypeCache

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

@@ -0,0 +1,77 @@
+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
+
+/** An annotation representing a capture set and whether it is boxed.
+ *  It simulates a normal @retains annotation except that it is more efficient,
+ *  supports variables as capture sets, and adds a `boxed` flag.
+ *  These annotations are created during capture checking. Before that
+ *  there are only regular @retains and @retainsByName annotations.
+ *  @param refs    the capture set
+ *  @param boxed   whether the type carrying the annotation is boxed
+ *  @param cls     the underlying class (either annotation.retains or annotation.retainsByName)
+ */
+case class CaptureAnnotation(refs: CaptureSet, boxed: Boolean)(cls: Symbol) extends Annotation:
+  import CaptureAnnotation.*
+  import tpd.*
+
+  /** A cache for boxed version of a capturing type with this annotation */
+  val boxedType = BoxedTypeCache()
+
+  /** Reconstitute annotation tree from capture set */
+  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) = cls
+
+  override def derivedAnnotation(tree: Tree)(using Context): Annotation =
+    unsupported(i"derivedAnnotation(Tree), $tree, $refs")
+
+  def derivedAnnotation(refs: CaptureSet, boxed: Boolean)(using Context): Annotation =
+    if (this.refs eq refs) && (this.boxed == boxed) then this
+    else CaptureAnnotation(refs, boxed)(cls)
+
+  override def sameAnnotation(that: Annotation)(using Context): Boolean = that match
+    case CaptureAnnotation(refs, boxed) =>
+      this.refs == refs && this.boxed == boxed && this.symbol == that.symbol
+    case _ => false
+
+  override def mapWith(tm: TypeMap)(using Context) =
+    val elems = refs.elems.toList
+    val elems1 = elems.mapConserve(tm)
+    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 == that.boxed)
+    case _ => false
+
+end CaptureAnnotation