Better handling of Contexts in RunInfo and Implicits

In several situations, instead of the common pattern in the compiler of
using the enclosing Context available coming an implicit method
parameter, we used some other Context. So far, this wasn't an issue but
starting with the next commit it is important for FunProto#typedArgs to
be called in a Context whose owner chain contains the Context where the
FunProto was created. This motivated the following changes:

- RunInfo does not actually need to keep a reference to the root Context:
  It was previously used by `ImplicitRunInfo#implicitScope`, but can be
  replaced by using the Context from the enclosing scope (which was already
  used in this method with the name `liftingCtx`).
- OfTypeImplicits does not need to keep a reference to the root Context
  either, we can just replace its lazy vals by defs that cache their results.
- ContextualImplicits does need to keep track of the Context from which
  its implicit references come from, but this Context itself does not have
  to be available implicitly in the class, instead the Context from the
  enclosing scope is used.

This also requires replacing OfTypeImplicits#toString and
ContextualImplicits#toString by a toText method in Printer to get access
to a Context.

Author: smarter

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

@@ -141,7 +141,7 @@ class Compiler {
     ctx.initialize()(start) // re-initialize the base context with start
     def addImport(ctx: Context, refFn: () => TermRef) =
       ctx.fresh.setImportInfo(ImportInfo.rootImport(refFn)(ctx))
-    (start.setRunInfo(new RunInfo(start)) /: defn.RootImportFns)(addImport)
+    (start.setRunInfo(new RunInfo) /: defn.RootImportFns)(addImport)
   }
 
   def reset()(implicit ctx: Context): Unit = {

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

@@ -219,7 +219,7 @@ object Contexts {
             else
               outer.implicits
           if (implicitRefs.isEmpty) outerImplicits
-          else new ContextualImplicits(implicitRefs, outerImplicits)(this)
+          else new ContextualImplicits(implicitRefs, outerImplicits, this)
         }
       implicitsCache
     }
@@ -522,7 +522,7 @@ object Contexts {
     sstate = settings.defaultState
     tree = untpd.EmptyTree
     typeAssigner = TypeAssigner
-    runInfo = new RunInfo(this)
+    runInfo = new RunInfo
     diagnostics = None
     freshNames = new FreshNameCreator.Default
     moreProperties = Map.empty
@@ -533,7 +533,7 @@ object Contexts {
 
   @sharable object NoContext extends Context {
     val base = null
-    override val implicits: ContextualImplicits = new ContextualImplicits(Nil, null)(this)
+    override val implicits: ContextualImplicits = new ContextualImplicits(Nil, null, this)
   }
 
   /** A context base defines state and associated methods that exist once per
@@ -695,9 +695,7 @@ object Contexts {
   }
 
   /** Info that changes on each compiler run */
-  class RunInfo(initctx: Context) extends ImplicitRunInfo with ConstraintRunInfo {
-    implicit val ctx: Context = initctx
-  }
+  class RunInfo extends ImplicitRunInfo with ConstraintRunInfo
 
   class GADTMap(initBounds: SimpleMap[Symbol, TypeBounds]) {
     private var myBounds = initBounds

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

@@ -491,6 +491,18 @@ class PlainPrinter(_ctx: Context) extends Printer {
     }
   }.close // todo: override in refined printer
 
+
+  // This is only in Printer to have access to a Context
+  def toText(irefs: ImplicitRefs): Text = irefs match {
+    case irefs: OfTypeImplicits =>
+      s"""OfTypeImplicits(${irefs.tp.show}):
+         |  companions = ${irefs.companionRefs.toList.map(_.show).mkString(", ")}
+         |  refs = ${irefs.refs.map(_.show).mkString(", ")}.""".stripMargin
+    case irefs: ContextualImplicits =>
+      val own = s"ContextualImplicits: ${irefs.refs.map(_.show).mkString(", ")}"
+      if (irefs.isOuterMost) own else own + "\n " + irefs.outerImplicits.show
+  }
+
   def toText(result: SearchResult): Text = result match {
     case result: SearchSuccess =>
       "SearchSuccess: " ~ toText(result.ref) ~ " via " ~ toText(result.tree)

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

@@ -5,7 +5,7 @@ import core._
 import Texts._, ast.Trees._
 import Types.Type, Symbols.Symbol, Contexts.Context, Scopes.Scope, Constants.Constant,
        Names.Name, Denotations._, Annotations.Annotation
-import typer.Implicits.SearchResult
+import typer.Implicits.{ImplicitRefs, SearchResult}
 import typer.ImportInfo
 
 /** The base class of all printers
@@ -96,6 +96,9 @@ abstract class Printer {
   /** Textual representation of tree */
   def toText[T >: Untyped](tree: Tree[T]): Text
 
+  /** Textual representation of implicit references */
+  def toText(irefs: ImplicitRefs): Text
+
   /** Textual representation of implicit search result */
   def toText(result: SearchResult): Text
 

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

@@ -48,15 +48,12 @@ object Implicits {
   /** A common base class of contextual implicits and of-type implicits which
    *  represents a set of implicit references.
    */
-  abstract class ImplicitRefs(initctx: Context) {
-    implicit val ctx: Context =
-      if (initctx == NoContext) initctx else initctx retractMode Mode.ImplicitsEnabled
-
+  abstract class ImplicitRefs extends Showable {
     /** The nesting level of this context. Non-zero only in ContextialImplicits */
     def level: Int = 0
 
     /** The implicit references */
-    def refs: List[TermRef]
+    def refs(implicit ctx: Context): List[TermRef]
 
     /** Return those references in `refs` that are compatible with type `pt`. */
     protected def filterMatching(pt: Type)(implicit ctx: Context): List[Candidate] = track("filterMatching") {
@@ -131,67 +128,80 @@ object Implicits {
       }
 
       if (refs.isEmpty) Nil
-      else refs.filter(refMatches(_)(ctx.fresh.addMode(Mode.TypevarsMissContext).setExploreTyperState)) // create a defensive copy of ctx to avoid constraint pollution
-               .map(Candidate(_, level))
+      else {
+        val filteringCtx = ctx.fresh
+          .retractMode(Mode.ImplicitsEnabled)
+          .addMode(Mode.TypevarsMissContext)
+          .setExploreTyperState // create a defensive copy of ctx to avoid constraint pollution
+        refs.filter(refMatches(_)(filteringCtx)).map(Candidate(_, level))
+      }
     }
+
+    def toText(printer: Printer): Text = printer.toText(this)
   }
 
   /** The implicit references coming from the implicit scope of a type.
    *  @param tp              the type determining the implicit scope
    *  @param companionRefs   the companion objects in the implicit scope.
    */
-  class OfTypeImplicits(tp: Type, val companionRefs: TermRefSet)(initctx: Context) extends ImplicitRefs(initctx) {
-    assert(initctx.typer != null)
-    lazy val refs: List[TermRef] = {
-      val buf = new mutable.ListBuffer[TermRef]
-      for (companion <- companionRefs) buf ++= companion.implicitMembers
-      buf.toList
+  class OfTypeImplicits(val tp: Type, val companionRefs: TermRefSet) extends ImplicitRefs {
+    private[this] var myRefs: List[TermRef] = _
+    def refs(implicit ctx: Context): List[TermRef] = {
+      if (myRefs == null) {
+        val buf = new mutable.ListBuffer[TermRef]
+        for (companion <- companionRefs) buf ++= companion.implicitMembers
+        myRefs = buf.toList
+      }
+      myRefs
     }
 
+    private[this] var myEligible: List[Candidate] = _
     /** The candidates that are eligible for expected type `tp` */
-    lazy val eligible: List[Candidate] =
-      /*>|>*/ track("eligible in tpe") /*<|<*/ {
-        /*>|>*/ ctx.traceIndented(i"eligible($tp), companions = ${companionRefs.toList}%, %", implicitsDetailed, show = true) /*<|<*/ {
-          if (refs.nonEmpty && monitored) record(s"check eligible refs in tpe", refs.length)
-          filterMatching(tp)
+    def eligible(implicit ctx: Context): List[Candidate] = {
+      if (myEligible == null) {
+        /*>|>*/ track("eligible in tpe") /*<|<*/ {
+          /*>|>*/ ctx.traceIndented(i"eligible($tp), companions = ${companionRefs.toList}%, %", implicitsDetailed, show = true) /*<|<*/ {
+            if (refs.nonEmpty && monitored) record(s"check eligible refs in tpe", refs.length)
+            myEligible = filterMatching(tp)
+          }
         }
       }
-
-    override def toString =
-      i"OfTypeImplicits($tp), companions = ${companionRefs.toList}%, %; refs = $refs%, %."
+      myEligible
+    }
   }
 
   /** The implicit references coming from the context.
-   *  @param refs      the implicit references made visible by the current context.
+   *  @param myRefs    the implicit references made visible by the current context.
    *                   Note: The name of the reference might be different from the name of its symbol.
    *                   In the case of a renaming import a => b, the name of the reference is the renamed
    *                   name, b, whereas the name of the symbol is the original name, a.
-   *  @param outerCtx  the next outer context that makes visible further implicits
    */
-  class ContextualImplicits(val refs: List[TermRef], val outerImplicits: ContextualImplicits)(initctx: Context) extends ImplicitRefs(initctx) {
+  class ContextualImplicits(myRefs: List[TermRef], val outerImplicits: ContextualImplicits, private val ictx: Context) extends ImplicitRefs {
     private val eligibleCache = new mutable.AnyRefMap[Type, List[Candidate]]
 
+    override def refs(implicit ctx: Context) = myRefs
+
     /** The level increases if current context has a different owner or scope than
      *  the context of the next-outer ImplicitRefs. This is however disabled under
      *  Scala2 mode, since we do not want to change the implicit disambiguation then.
      */
     override val level: Int =
       if (outerImplicits == null) 1
-      else if (ctx.scala2Mode ||
-               (ctx.owner eq outerImplicits.ctx.owner) &&
-               (ctx.scope eq outerImplicits.ctx.scope)) outerImplicits.level
+      else if (ictx.scala2Mode ||
+               (ictx.owner eq outerImplicits.ictx.owner) &&
+               (ictx.scope eq outerImplicits.ictx.scope)) outerImplicits.level
       else outerImplicits.level + 1
 
     /** Is this the outermost implicits? This is the case if it either the implicits
      *  of NoContext, or the last one before it.
      */
-    private def isOuterMost = {
+    def isOuterMost = {
       val finalImplicits = NoContext.implicits
       (this eq finalImplicits) || (outerImplicits eq finalImplicits)
     }
 
     /** The implicit references that are eligible for type `tp`. */
-    def eligible(tp: Type): List[Candidate] = /*>|>*/ track(s"eligible in ctx") /*<|<*/ {
+    def eligible(tp: Type)(implicit ctx: Context): List[Candidate] = /*>|>*/ track(s"eligible in ctx") /*<|<*/ {
       if (tp.hash == NotCached) computeEligible(tp)
       else eligibleCache get tp match {
         case Some(eligibles) =>
@@ -203,7 +213,7 @@ object Implicits {
           if (monitored) record(s"elided eligible refs", elided(this))
           eligibles
         case None =>
-          if (ctx eq NoContext) Nil
+          if (ictx eq NoContext) Nil
           else {
             val savedEphemeral = ctx.typerState.ephemeral
             ctx.typerState.ephemeral = false
@@ -217,8 +227,8 @@ object Implicits {
       }
     }
 
-    private def computeEligible(tp: Type): List[Candidate] = /*>|>*/ ctx.traceIndented(i"computeEligible $tp in $refs%, %", implicitsDetailed) /*<|<*/ {
-      if (monitored) record(s"check eligible refs in ctx", refs.length)
+    private def computeEligible(tp: Type)(implicit ctx: Context): List[Candidate] = /*>|>*/ ctx.traceIndented(i"computeEligible $tp in $refs%, %", implicitsDetailed) /*<|<*/ {
+      if (monitored) record(s"check eligible refs in ictx", refs.length)
       val ownEligible = filterMatching(tp)
       if (isOuterMost) ownEligible
       else ownEligible ::: {
@@ -227,21 +237,16 @@ object Implicits {
       }
     }
 
-    override def toString = {
-      val own = i"(implicits: $refs%, %)"
-      if (isOuterMost) own else own + "\n " + outerImplicits
-    }
-
     /** This context, or a copy, ensuring root import from symbol `root`
      *  is not present in outer implicits.
      */
     def exclude(root: Symbol): ContextualImplicits =
       if (this == NoContext.implicits) this
       else {
         val outerExcluded = outerImplicits exclude root
-        if (ctx.importInfo.site.termSymbol == root) outerExcluded
+        if (ictx.importInfo.site(ictx).termSymbol(ictx) == root) outerExcluded
         else if (outerExcluded eq outerImplicits) this
-        else new ContextualImplicits(refs, outerExcluded)(ctx)
+        else new ContextualImplicits(myRefs, outerExcluded, ictx)
       }
   }
 
@@ -355,14 +360,8 @@ trait ImplicitRunInfo { self: RunInfo =>
 
   private val implicitScopeCache = mutable.AnyRefMap[Type, OfTypeImplicits]()
 
-  /** The implicit scope of a type `tp`
-   *  @param liftingCtx   A context to be used when computing the class symbols of
-   *                      a type. Types may contain type variables with their instances
-   *                      recorded in the current context. To find out the instance of
-   *                      a type variable, we need the current context, the current
-   *                      runinfo context does not do.
-   */
-  def implicitScope(rootTp: Type, liftingCtx: Context): OfTypeImplicits = {
+  /** The implicit scope of a type `tp` */
+  def implicitScope(rootTp: Type)(implicit ctx: Context): OfTypeImplicits = {
 
     val seen: mutable.Set[Type] = mutable.Set()
     val incomplete: mutable.Set[Type] = mutable.Set()
@@ -374,7 +373,6 @@ trait ImplicitRunInfo { self: RunInfo =>
      *  abstract types are eliminated.
      */
     object liftToClasses extends TypeMap {
-      override implicit protected val ctx: Context = liftingCtx
       override def stopAtStatic = true
       def apply(tp: Type) = tp match {
         case tp: TypeRef if tp.symbol.isAbstractOrAliasType =>
@@ -440,7 +438,7 @@ trait ImplicitRunInfo { self: RunInfo =>
               if (companion.exists) addRef(companion.valRef)
               cls.classParents foreach addParentScope
             }
-            tp.classSymbols(liftingCtx) foreach addClassScope
+            tp.classSymbols foreach addClassScope
           case _ =>
             // We exclude lower bounds to conform to SLS 7.2:
             // "The parts of a type T are: [...] if T is an abstract type, the parts of its upper bound"
@@ -466,7 +464,8 @@ trait ImplicitRunInfo { self: RunInfo =>
               iscope(liftedTp, isLifted = true).companionRefs
             else
               collectCompanions(tp)
-          val result = new OfTypeImplicits(tp, refs)(ctx)
+          val result = new OfTypeImplicits(tp, refs)
+
           if (ctx.typerState.ephemeral)
             record("ephemeral cache miss: implicitScope")
           else if (canCache &&
@@ -600,7 +599,7 @@ trait Implicits { self: Typer =>
     def lazyImplicitCtx(lazyImplicit: Symbol): Context = {
       val lctx = ctx.fresh
       for (delayedRef <- ctx.property(DelayedImplicit))
-        lctx.setImplicits(new ContextualImplicits(delayedRef :: Nil, ctx.implicits)(ctx))
+        lctx.setImplicits(new ContextualImplicits(delayedRef :: Nil, ctx.implicits, ctx))
       lctx.setProperty(DelayedImplicit, lazyImplicit.termRef)
     }
 
@@ -900,7 +899,7 @@ trait Implicits { self: Typer =>
       }
     }
 
-    def implicitScope(tp: Type): OfTypeImplicits = ctx.runInfo.implicitScope(tp, ctx)
+    def implicitScope(tp: Type): OfTypeImplicits = ctx.runInfo.implicitScope(tp)
   }
 
   final class ExplainedImplicitSearch(pt: Type, argument: Tree, pos: Position)(implicit ctx: Context)