Avoid cycles when loading standard library under cc

Two measures

 - Introduce a new annotation @retainsCap that's equivalent to
   @retains(cap) but that does not need to load Seq to analyze a
   vararg argument.
 - Don't use `&` when computing self types of classes with
   explicitly declared capturing self types.

Fixes #19568

Author: odersky

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

@@ -98,10 +98,14 @@ extension (tree: Tree)
         tree.putAttachment(Captures, refs)
         refs
 
-  /** The arguments of a @retains or @retainsByName annotation */
+  /** The arguments of a @retains, @retainsCap or @retainsByName annotation */
   def retainedElems(using Context): List[Tree] = tree match
-    case Apply(_, Typed(SeqLiteral(elems, _), _) :: Nil) => elems
-    case _ => Nil
+    case Apply(_, Typed(SeqLiteral(elems, _), _) :: Nil) =>
+      elems
+    case _ =>
+      if tree.symbol.maybeOwner == defn.RetainsCapAnnot
+      then ref(defn.captureRoot.termRef) :: Nil
+      else Nil
 
 extension (tp: Type)
 
@@ -207,7 +211,7 @@ extension (tp: Type)
   def dropAllRetains(using Context): Type = // TODO we should drop retains from inferred types before unpickling
     val tm = new TypeMap:
       def apply(t: Type) = t match
-        case AnnotatedType(parent, annot) if annot.symbol == defn.RetainsAnnot =>
+        case AnnotatedType(parent, annot) if annot.symbol.isRetains =>
           apply(parent)
         case _ =>
           mapOver(t)
@@ -326,6 +330,14 @@ extension (cls: ClassSymbol)
 
 extension (sym: Symbol)
 
+  /** This symbol is one of `retains` or `retainsCap` */
+  def isRetains(using Context): Boolean =
+    sym == defn.RetainsAnnot || sym == defn.RetainsCapAnnot
+
+  /** This symbol is one of `retains`, `retainsCap`, or`retainsByName` */
+  def isRetainsLike(using Context): Boolean =
+    isRetains || sym == defn.RetainsByNameAnnot
+
   /** A class is pure if:
    *   - one its base types has an explicitly declared self type with an empty capture set
    *   - or it is a value class

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

@@ -58,8 +58,7 @@ object CapturingType:
     case AnnotatedType(parent, ann: CaptureAnnotation)
     if isCaptureCheckingOrSetup =>
       Some((parent, ann.refs))
-    case AnnotatedType(parent, ann)
-    if ann.symbol == defn.RetainsAnnot && isCaptureChecking =>
+    case AnnotatedType(parent, ann) if ann.symbol.isRetains && isCaptureChecking =>
       // There are some circumstances where we cannot map annotated types
       // with retains annotations to capturing types, so this second recognizer
       // path still has to exist. One example is when checking capture sets

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

@@ -23,7 +23,7 @@ object RetainingType:
 
   def unapply(tp: AnnotatedType)(using Context): Option[(Type, List[Tree])] =
     val sym = tp.annot.symbol
-    if sym == defn.RetainsAnnot || sym == defn.RetainsByNameAnnot then
+    if sym.isRetainsLike then
       tp.annot match
         case _: CaptureAnnotation =>
           assert(ctx.mode.is(Mode.IgnoreCaptures), s"bad retains $tp at ${ctx.phase}")

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

@@ -120,7 +120,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
       case tp @ CapturingType(parent, refs) =>
         if tp.isBoxed then tp else tp.boxed
       case tp @ AnnotatedType(parent, ann) =>
-        if ann.symbol == defn.RetainsAnnot
+        if ann.symbol.isRetains
         then CapturingType(parent, ann.tree.toCaptureSet, boxed = true)
         else tp.derivedAnnotatedType(box(parent), ann)
       case tp1 @ AppliedType(tycon, args) if defn.isNonRefinedFunction(tp1) =>
@@ -192,7 +192,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
 
       def apply(tp: Type) =
         val tp1 = tp match
-          case AnnotatedType(parent, annot) if annot.symbol == defn.RetainsAnnot =>
+          case AnnotatedType(parent, annot) if annot.symbol.isRetains =>
             // Drop explicit retains annotations
             apply(parent)
           case tp @ AppliedType(tycon, args) =>
@@ -283,7 +283,7 @@ class Setup extends PreRecheck, SymTransformer, SetupAPI:
             t.derivedCapturingType(this(parent), refs)
           case t @ AnnotatedType(parent, ann) =>
             val parent1 = this(parent)
-            if ann.symbol == defn.RetainsAnnot then
+            if ann.symbol.isRetains then
               for tpt <- tptToCheck do
                 checkWellformedLater(parent1, ann.tree, tpt)
               CapturingType(parent1, ann.tree.toCaptureSet)

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

@@ -1057,6 +1057,7 @@ class Definitions {
   @tu lazy val ReachCapabilityAnnot = requiredClass("scala.annotation.internal.reachCapability")
   @tu lazy val RequiresCapabilityAnnot: ClassSymbol = requiredClass("scala.annotation.internal.requiresCapability")
   @tu lazy val RetainsAnnot: ClassSymbol = requiredClass("scala.annotation.retains")
+  @tu lazy val RetainsCapAnnot: ClassSymbol = requiredClass("scala.annotation.retainsCap")
   @tu lazy val RetainsByNameAnnot: ClassSymbol = requiredClass("scala.annotation.retainsByName")
   @tu lazy val PublicInBinaryAnnot: ClassSymbol = requiredClass("scala.annotation.publicInBinary")
 
@@ -2008,7 +2009,7 @@ class Definitions {
   @tu lazy val ccExperimental: Set[Symbol] = Set(
     CapsModule, CapsModule.moduleClass, PureClass,
     CapabilityAnnot, RequiresCapabilityAnnot,
-    RetainsAnnot, RetainsByNameAnnot)
+    RetainsAnnot, RetainsCapAnnot, RetainsByNameAnnot)
 
   // ----- primitive value class machinery ------------------------------------------
 

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

@@ -583,6 +583,7 @@ object StdNames {
     val releaseFence : N        = "releaseFence"
     val retains: N              = "retains"
     val retainsByName: N        = "retainsByName"
+    val retainsCap: N           = "retainsCap"
     val rootMirror : N          = "rootMirror"
     val run: N                  = "run"
     val runOrElse: N            = "runOrElse"

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

@@ -38,7 +38,7 @@ import config.Printers.{core, typr, matchTypes}
 import reporting.{trace, Message}
 import java.lang.ref.WeakReference
 import compiletime.uninitialized
-import cc.{CapturingType, CaptureSet, derivedCapturingType, isBoxedCapturing, RetainingType, isCaptureChecking}
+import cc.{CapturingType, CaptureSet, derivedCapturingType, isBoxedCapturing, isCaptureChecking, isRetains, isRetainsLike}
 import CaptureSet.{CompareResult, IdempotentCaptRefMap, IdentityCaptRefMap}
 
 import scala.annotation.internal.sharable
@@ -4025,7 +4025,7 @@ object Types extends TypeUtils {
               mapOver(tp)
             case AnnotatedType(parent, ann) if ann.refersToParamOf(thisLambdaType) =>
               val parent1 = mapOver(parent)
-              if ann.symbol == defn.RetainsAnnot || ann.symbol == defn.RetainsByNameAnnot then
+              if ann.symbol.isRetainsLike then
                 range(
                   AnnotatedType(parent1, CaptureSet.empty.toRegularAnnotation(ann.symbol)),
                   AnnotatedType(parent1, CaptureSet.universal.toRegularAnnotation(ann.symbol)))
@@ -5315,10 +5315,10 @@ object Types extends TypeUtils {
           else if (clsd.is(Module)) givenSelf
           else if (ctx.erasedTypes) appliedRef
           else givenSelf.dealiasKeepAnnots match
-            case givenSelf1 @ AnnotatedType(tp, ann) if ann.symbol == defn.RetainsAnnot =>
-              givenSelf1.derivedAnnotatedType(tp & appliedRef, ann)
+            case givenSelf1 @ AnnotatedType(tp, ann) if ann.symbol.isRetains =>
+              givenSelf1.derivedAnnotatedType(AndType.make(tp, appliedRef), ann)
             case _ =>
-              AndType(givenSelf, appliedRef)
+              AndType.make(givenSelf, appliedRef)
         }
       selfTypeCache.nn
     }

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1765,12 +1765,11 @@ object Parsers {
           RefinedTypeTree(rejectWildcardType(t), refinement(indentOK = true))
         })
       else if Feature.ccEnabled && in.isIdent(nme.UPARROW) && isCaptureUpArrow then
-        val upArrowStart = in.offset
-        in.nextToken()
-        def cs =
-          if in.token == LBRACE then captureSet()
-          else atSpan(upArrowStart)(captureRoot) :: Nil
-        makeRetaining(t, cs, tpnme.retains)
+        atSpan(t.span.start):
+          in.nextToken()
+          if in.token == LBRACE
+          then makeRetaining(t, captureSet(), tpnme.retains)
+          else makeRetaining(t, Nil, tpnme.retainsCap)
       else
         t
     }

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

@@ -27,7 +27,7 @@ import config.{Config, Feature}
 
 import dotty.tools.dotc.util.SourcePosition
 import dotty.tools.dotc.ast.untpd.{MemberDef, Modifiers, PackageDef, RefTree, Template, TypeDef, ValOrDefDef}
-import cc.{CaptureSet, CapturingType, toCaptureSet, IllegalCaptureRef}
+import cc.{CaptureSet, CapturingType, toCaptureSet, IllegalCaptureRef, isRetains}
 import dotty.tools.dotc.parsing.JavaParsers
 
 class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
@@ -643,7 +643,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         def toTextRetainsAnnot =
           try changePrec(GlobalPrec)(toText(arg) ~ "^" ~ toTextCaptureSet(captureSet))
           catch case ex: IllegalCaptureRef => toTextAnnot
-        if annot.symbol.maybeOwner == defn.RetainsAnnot
+        if annot.symbol.maybeOwner.isRetains
             && Feature.ccEnabled && !printDebug
             && Phases.checkCapturesPhase.exists // might be missing on -Ytest-pickler
         then toTextRetainsAnnot

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

@@ -38,7 +38,7 @@ import config.Feature.sourceVersion
 import config.SourceVersion.*
 import config.MigrationVersion
 import printing.Formatting.hlAsKeyword
-import cc.isCaptureChecking
+import cc.{isCaptureChecking, isRetainsLike}
 
 import collection.mutable
 import reporting.*
@@ -705,8 +705,7 @@ object Checking {
             declaredParents =
               tp.declaredParents.map(p => transformedParent(apply(p)))
             )
-        case tp @ AnnotatedType(underlying, annot)
-        if annot.symbol == defn.RetainsAnnot || annot.symbol == defn.RetainsByNameAnnot =>
+        case tp @ AnnotatedType(underlying, annot) if annot.symbol.isRetainsLike =>
           val underlying1 = this(underlying)
           val saved = inCaptureSet
           inCaptureSet = true

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

@@ -48,7 +48,7 @@ import staging.StagingLevel
 import reporting.*
 import Nullables.*
 import NullOpsDecorator.*
-import cc.CheckCaptures
+import cc.{CheckCaptures, isRetainsLike}
 import config.Config
 import config.MigrationVersion
 
@@ -2940,9 +2940,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val arg1 = typed(tree.arg, pt)
     if (ctx.mode is Mode.Type) {
       val cls = annot1.symbol.maybeOwner
-      if Feature.ccEnabled
-          && (cls == defn.RetainsAnnot || cls == defn.RetainsByNameAnnot)
-      then
+      if Feature.ccEnabled && cls.isRetainsLike then
         CheckCaptures.checkWellformed(arg1, annot1)
       if arg1.isType then
         assignType(cpy.Annotated(tree)(arg1, annot1), arg1, annot1)

library/src/scala/annotation/retains.scala

@@ -13,3 +13,11 @@ package scala.annotation
  */
 @experimental
 class retains(xs: Any*) extends annotation.StaticAnnotation
+
+/** Equivalent in meaning to `@retains(cap)`, but consumes less bytecode. 
+ */
+@experimental
+class retainsCap() extends annotation.StaticAnnotation
+  // This special case is needed to be able to load standard library modules without
+  // cyclic reference errors. Specifically, load sequences involving IterableOnce.
+

tests/run-tasty-inspector/stdlibExperimentalDefinitions.scala

@@ -51,6 +51,7 @@ val experimentalDefinitionInLibrary = Set(
   "scala.annotation.capability",
   "scala.annotation.retains",
   "scala.annotation.retainsByName",
+  "scala.annotation.retainsCap",
   "scala.Pure",
   "scala.caps",
   "scala.caps$",