Fix nesting level to account for block scopes

Previously, the nesting level of a symbol was always equal to the
nesting level of its owner incremented by one, but in a situation like
i8900a.scala where we have:

...
  val a = unwrap[?Outer]({
    class Local
...

the owner of both `?Outer` and `Local` is `a`, and so they ended up with
the same level even though `Local` should not leak outside of the block
that defines it (i8900a.scala compiled regardless due to the disabled
check for forward references in TreePickler re-enabled later in this PR).

We rectify this by associating each scope with a level which is always
greated than the level of the enclosing scope (repurposing the existing
Scope#nestingLevel method which wasn't used for anything), newly created
symbols then simply take the level of the current scope (this required
tweaking typedCase so that the pattern symbols were typed in the scope
were they end up being entered).

Also add a `-Yprint-level` option for debugging level-related issues.

Author: smarter

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

@@ -70,7 +70,7 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
   protected def rootContext(using Context): Context = {
     ctx.initialize()
     ctx.base.setPhasePlan(comp.phases)
-    val rootScope = new MutableScope
+    val rootScope = new MutableScope(0)
     val bootstrap = ctx.fresh
       .setPeriod(Period(comp.nextRunId, FirstPhaseId))
       .setScope(rootScope)

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

@@ -7,6 +7,7 @@ import typer.ProtoTypes
 import transform.SymUtils._
 import transform.TypeUtils._
 import core._
+import Scopes.newScope
 import util.Spans._, Types._, Contexts._, Constants._, Names._, Flags._, NameOps._
 import Symbols._, StdNames._, Annotations._, Trees._, Symbols._
 import Decorators._, DenotTransformers._
@@ -344,7 +345,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
       }
       else parents
     val cls = newNormalizedClassSymbol(owner, tpnme.ANON_CLASS, Synthetic | Final, parents1,
-        coord = fns.map(_.span).reduceLeft(_ union _))
+        newScope, coord = fns.map(_.span).reduceLeft(_ union _))
     val constr = newConstructor(cls, Synthetic, Nil, Nil).entered
     def forwarder(fn: TermSymbol, name: TermName) = {
       val fwdMeth = fn.copy(cls, name, Synthetic | Method | Final).entered.asTerm

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -274,6 +274,7 @@ private sealed trait YSettings:
   val YprintSyms: Setting[Boolean] = BooleanSetting("-Yprint-syms", "When printing trees print info in symbols instead of corresponding info in trees.")
   val YprintDebug: Setting[Boolean] = BooleanSetting("-Yprint-debug", "When printing trees, print some extra information useful for debugging.")
   val YprintDebugOwners: Setting[Boolean] = BooleanSetting("-Yprint-debug-owners", "When printing trees, print owners of definitions.")
+  val YprintLevel: Setting[Boolean] = BooleanSetting("-Yprint-level", "print nesting levels of symbols and type variables.")
   val YshowPrintErrors: Setting[Boolean] = BooleanSetting("-Yshow-print-errors", "Don't suppress exceptions thrown during tree printing.")
   val YtestPickler: Setting[Boolean] = BooleanSetting("-Ytest-pickler", "Self-test for pickling functionality; should be used with -Ystop-after:pickler.")
   val YcheckReentrant: Setting[Boolean] = BooleanSetting("-Ycheck-reentrant", "Check that compiled program does not contain vars that can be accessed from a global root.")

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

@@ -270,6 +270,10 @@ object Contexts {
       if owner != null && owner.isClass then owner.asClass.unforcedDecls
       else scope
 
+    def nestingLevel: Int =
+      val sc = effectiveScope
+      if sc != null then sc.nestingLevel else 0
+
     /** Sourcefile corresponding to given abstract file, memoized */
     def getSource(file: AbstractFile, codec: => Codec = Codec(settings.encoding.value)) = {
       util.Stats.record("Context.getSource")

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

@@ -49,7 +49,10 @@ class Definitions {
   private def newPermanentClassSymbol(owner: Symbol, name: TypeName, flags: FlagSet, infoFn: ClassSymbol => Type) =
     newClassSymbol(owner, name, flags | Permanent | NoInits | Open, infoFn)
 
-  private def enterCompleteClassSymbol(owner: Symbol, name: TypeName, flags: FlagSet, parents: List[TypeRef], decls: Scope = newScope) =
+  private def enterCompleteClassSymbol(owner: Symbol, name: TypeName, flags: FlagSet, parents: List[TypeRef]): ClassSymbol =
+    enterCompleteClassSymbol(owner, name, flags, parents, newScope(owner.nestingLevel + 1))
+
+  private def enterCompleteClassSymbol(owner: Symbol, name: TypeName, flags: FlagSet, parents: List[TypeRef], decls: Scope) =
     newCompleteClassSymbol(owner, name, flags | Permanent | NoInits | Open, parents, decls).entered
 
   private def enterTypeField(cls: ClassSymbol, name: TypeName, flags: FlagSet, scope: MutableScope) =
@@ -433,7 +436,7 @@ class Definitions {
     Any_toString, Any_##, Any_getClass, Any_isInstanceOf, Any_typeTest, Object_eq, Object_ne)
 
   @tu lazy val AnyKindClass: ClassSymbol = {
-    val cls = newCompleteClassSymbol(ScalaPackageClass, tpnme.AnyKind, AbstractFinal | Permanent, Nil)
+    val cls = newCompleteClassSymbol(ScalaPackageClass, tpnme.AnyKind, AbstractFinal | Permanent, Nil, newScope(0))
     if (!ctx.settings.YnoKindPolymorphism.value)
       // Enable kind-polymorphism by exposing scala.AnyKind
       cls.entered

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

@@ -75,7 +75,7 @@ object Scopes {
      */
     def size: Int
 
-    /** The number of outer scopes from which symbols are inherited */
+    /** The number of scopes enclosing this scope. */
     def nestingLevel: Int
 
     /** The symbols in this scope in the order they were entered;
@@ -193,15 +193,15 @@ object Scopes {
    *  This is necessary because when run from reflection every scope needs to have a
    *  SynchronizedScope as mixin.
    */
-  class MutableScope protected[Scopes](initElems: ScopeEntry, initSize: Int, val nestingLevel: Int = 0)
+  class MutableScope protected[Scopes](initElems: ScopeEntry, initSize: Int, val nestingLevel: Int)
       extends Scope {
 
     /** Scope shares elements with `base` */
     protected[Scopes] def this(base: Scope)(using Context) =
       this(base.lastEntry, base.size, base.nestingLevel + 1)
       ensureCapacity(MinHashedScopeSize)
 
-    def this() = this(null, 0, 0)
+    def this(nestingLevel: Int) = this(null, 0, nestingLevel)
 
     private[dotc] var lastEntry: ScopeEntry = initElems
 
@@ -225,7 +225,7 @@ object Scopes {
     /** Use specified synthesize for this scope */
     def useSynthesizer(s: SymbolSynthesizer): Unit = synthesize = s
 
-    protected def newScopeLikeThis(): MutableScope = new MutableScope()
+    protected def newScopeLikeThis(): MutableScope = new MutableScope(nestingLevel)
 
     /** Clone scope, taking care not to force the denotations of any symbols in the scope.
      */
@@ -440,7 +440,10 @@ object Scopes {
   }
 
   /** Create a new scope */
-  def newScope: MutableScope = new MutableScope()
+  def newScope(using Context): MutableScope =
+    new MutableScope(ctx.nestingLevel + 1)
+
+  def newScope(nestingLevel: Int): MutableScope = new MutableScope(nestingLevel)
 
   /** Create a new scope nested in another one with which it shares its elements */
   def newNestedScope(outer: Scope)(using Context): MutableScope = new MutableScope(outer)
@@ -468,8 +471,4 @@ object Scopes {
     override def lookupEntry(name: Name)(using Context): ScopeEntry = null
     override def lookupNextEntry(entry: ScopeEntry)(using Context): ScopeEntry = null
   }
-
-  /** A class for error scopes (mutable)
-   */
-  class ErrorScope(owner: Symbol) extends MutableScope
 }

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

@@ -1477,14 +1477,6 @@ object SymDenotations {
       else if is(Contravariant) then Contravariant
       else EmptyFlags
 
-    /** The length of the owner chain of this symbol. 1 for _root_, 0 for NoSymbol */
-    def nestingLevel(using Context): Int =
-      @tailrec def recur(d: SymDenotation, n: Int): Int = d match
-        case NoDenotation => n
-        case d: ClassDenotation => d.nestingLevel + n // profit from the cache in ClassDenotation
-        case _ => recur(d.owner, n + 1)
-      recur(this, 0)
-
     /** The flags to be used for a type parameter owned by this symbol.
      *  Overridden by ClassDenotation.
      */
@@ -2323,12 +2315,6 @@ object SymDenotations {
 
     override def registeredCompanion_=(c: Symbol) =
       myCompanion = c
-
-    private var myNestingLevel = -1
-
-    override def nestingLevel(using Context) =
-      if myNestingLevel == -1 then myNestingLevel = owner.nestingLevel + 1
-      myNestingLevel
   }
 
   /** The denotation of a package class.

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

@@ -221,7 +221,7 @@ object SymbolLoaders {
     /** The scope of a package. This is different from a normal scope
   	 *  in that names of scope entries are kept in mangled form.
   	 */
-    final class PackageScope extends MutableScope {
+    final class PackageScope extends MutableScope(0) {
       override def newScopeEntry(name: Name, sym: Symbol)(using Context): ScopeEntry =
         super.newScopeEntry(name.mangled, sym)
 

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

@@ -46,7 +46,7 @@ object Symbols {
    *  @param coord  The coordinates of the symbol (a position or an index)
    *  @param id     A unique identifier of the symbol (unique per ContextBase)
    */
-  class Symbol private[Symbols] (private var myCoord: Coord, val id: Int)
+  class Symbol private[Symbols] (private var myCoord: Coord, val id: Int, val nestingLevel: Int)
     extends Designator, ParamInfo, SrcPos, printing.Showable {
 
     type ThisName <: Name
@@ -368,8 +368,8 @@ object Symbols {
   type TermSymbol = Symbol { type ThisName = TermName }
   type TypeSymbol = Symbol { type ThisName = TypeName }
 
-  class ClassSymbol private[Symbols] (coord: Coord, val assocFile: AbstractFile, id: Int)
-    extends Symbol(coord, id) {
+  class ClassSymbol private[Symbols] (coord: Coord, val assocFile: AbstractFile, id: Int, nestingLevel: Int)
+    extends Symbol(coord, id, nestingLevel) {
 
     type ThisName = TypeName
 
@@ -459,7 +459,7 @@ object Symbols {
     override protected def prefixString: String = "ClassSymbol"
   }
 
-  @sharable object NoSymbol extends Symbol(NoCoord, 0) {
+  @sharable object NoSymbol extends Symbol(NoCoord, 0, 0) {
     override def associatedFile(using Context): AbstractFile = NoSource.file
     override def recomputeDenot(lastd: SymDenotation)(using Context): SymDenotation = NoDenotation
   }
@@ -516,7 +516,7 @@ object Symbols {
       info: Type,
       privateWithin: Symbol = NoSymbol,
       coord: Coord = NoCoord)(using Context): Symbol { type ThisName = N } = {
-    val sym = new Symbol(coord, ctx.base.nextSymId).asInstanceOf[Symbol { type ThisName = N }]
+    val sym = new Symbol(coord, ctx.base.nextSymId, ctx.nestingLevel).asInstanceOf[Symbol { type ThisName = N }]
     val denot = SymDenotation(sym, owner, name, flags, info, privateWithin)
     sym.denot = denot
     sym
@@ -534,7 +534,7 @@ object Symbols {
       coord: Coord = NoCoord,
       assocFile: AbstractFile = null)(using Context): ClassSymbol
   = {
-    val cls = new ClassSymbol(coord, assocFile, ctx.base.nextSymId)
+    val cls = new ClassSymbol(coord, assocFile, ctx.base.nextSymId, ctx.nestingLevel)
     val denot = SymDenotation(cls, owner, name, flags, infoFn(cls), privateWithin)
     cls.denot = denot
     cls
@@ -546,7 +546,7 @@ object Symbols {
       name: TypeName,
       flags: FlagSet,
       parents: List[TypeRef],
-      decls: Scope = newScope,
+      decls: Scope,
       selfInfo: Type = NoType,
       privateWithin: Symbol = NoSymbol,
       coord: Coord = NoCoord,
@@ -564,7 +564,7 @@ object Symbols {
       name: TypeName,
       flags: FlagSet,
       parentTypes: List[Type],
-      decls: Scope = newScope,
+      decls: Scope,
       selfInfo: Type = NoType,
       privateWithin: Symbol = NoSymbol,
       coord: Coord = NoCoord,
@@ -580,7 +580,7 @@ object Symbols {
   }
 
   def newRefinedClassSymbol(coord: Coord = NoCoord)(using Context): ClassSymbol =
-    newCompleteClassSymbol(ctx.owner, tpnme.REFINE_CLASS, NonMember, parents = Nil, coord = coord)
+    newCompleteClassSymbol(ctx.owner, tpnme.REFINE_CLASS, NonMember, parents = Nil, newScope, coord = coord)
 
   /** Create a module symbol with associated module class
    *  from its non-info fields and a function producing the info
@@ -646,7 +646,7 @@ object Symbols {
       name: TermName,
       modFlags: FlagSet = EmptyFlags,
       clsFlags: FlagSet = EmptyFlags,
-      decls: Scope = newScope)(using Context): TermSymbol =
+      decls: Scope = newScope(0))(using Context): TermSymbol =
     newCompleteModuleSymbol(
       owner, name,
       modFlags | PackageCreationFlags, clsFlags | PackageCreationFlags,

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

@@ -4667,7 +4667,7 @@ object Types {
    *  @param  origin        The parameter that's tracked by the type variable.
    *  @param  creatorState  The typer state in which the variable was created.
    */
-  final class TypeVar private(initOrigin: TypeParamRef, creatorState: TyperState, nestingLevel: Int) extends CachedProxyType with ValueType {
+  final class TypeVar private(initOrigin: TypeParamRef, creatorState: TyperState, val nestingLevel: Int) extends CachedProxyType with ValueType {
 
     private var currentOrigin = initOrigin
 
@@ -4713,6 +4713,8 @@ object Types {
      *  are nested more deeply than the type variable itself.
      */
     private def avoidCaptures(tp: Type)(using Context): Type =
+      if ctx.isAfterTyper then
+        return tp
       val problemSyms = new TypeAccumulator[Set[Symbol]]:
         def apply(syms: Set[Symbol], t: Type): Set[Symbol] = t match
           case ref: NamedType
@@ -4806,7 +4808,7 @@ object Types {
   }
   object TypeVar:
     def apply(initOrigin: TypeParamRef, creatorState: TyperState)(using Context) =
-      new TypeVar(initOrigin, creatorState, ctx.owner.nestingLevel)
+      new TypeVar(initOrigin, creatorState, ctx.nestingLevel)
 
   type TypeVars = SimpleIdentitySet[TypeVar]