Avoid double context creation when modes change on a fresh context.

Move addMode and friends to two decorators, one for Context, the other
for FreshContext. Implement behavior accordingly.

This avoids creating two contexts in situations like:

     c.fresh.setxploreTyperState.addMode(...)

Mow we can write

     c.fresh.addMode(...).setExploreTyperState

Because addMode returns a fresh context when applied to a fresh context.
Note that we specifically do not want virtual dispatch of addMode, that's
why it was moved to a decorator.

Also: removed mention of ".fresh: when just forllowed by an addMode, because
that one is redundant.

Author: odersky

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

@@ -386,13 +386,6 @@ object Contexts {
     final def withOwner(owner: Symbol): Context =
       if (owner ne this.owner) fresh.setOwner(owner) else this
 
-    final def withMode(mode: Mode): Context =
-      if (mode != this.mode) fresh.setMode(mode) else this
-
-    final def addMode(mode: Mode): Context = withMode(this.mode | mode)
-    final def maskMode(mode: Mode): Context = withMode(this.mode & mode)
-    final def retractMode(mode: Mode): Context = withMode(this.mode &~ mode)
-
     override def toString =
       "Context(\n" +
       (outersIterator map ( ctx => s"  owner = ${ctx.owner}, scope = ${ctx.scope}") mkString "\n")
@@ -444,6 +437,21 @@ object Contexts {
     def setDebug = setSetting(base.settings.debug, true)
   }
 
+  implicit class ModeChanges(val c: Context) extends AnyVal {
+    final def withMode(mode: Mode): Context =
+      if (mode != c.mode) c.fresh.setMode(mode) else c
+
+    final def addMode(mode: Mode): Context = withMode(c.mode | mode)
+    final def maskMode(mode: Mode): Context = withMode(c.mode & mode)
+    final def retractMode(mode: Mode): Context = withMode(c.mode &~ mode)
+  }
+
+  implicit class FreshModeChanges(val c: FreshContext) extends AnyVal {
+    final def addMode(mode: Mode): c.type = c.setMode(c.mode | mode)
+    final def maskMode(mode: Mode): c.type = c.setMode(c.mode & mode)
+    final def retractMode(mode: Mode): c.type = c.setMode(c.mode &~ mode)
+  }
+
   /** A class defining the initial context with given context base
    *  and set of possible settings.
    */

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

@@ -12,7 +12,7 @@ import typer.Mode
 import scala.annotation.switch
 
 class PlainPrinter(_ctx: Context) extends Printer {
-  protected[this] implicit def ctx: Context = _ctx.fresh.addMode(Mode.Printing)
+  protected[this] implicit def ctx: Context = _ctx.addMode(Mode.Printing)
 
   protected def maxToTextRecursions = 100
 

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

@@ -90,7 +90,7 @@ object Implicits {
       }
 
       if (refs.isEmpty) refs
-      else refs filter (refMatches(_)(ctx.fresh.setExploreTyperState.addMode(Mode.TypevarsMissContext))) // create a defensive copy of ctx to avoid constraint pollution
+      else refs filter (refMatches(_)(ctx.fresh.addMode(Mode.TypevarsMissContext).setExploreTyperState)) // create a defensive copy of ctx to avoid constraint pollution
     }
   }
 
@@ -491,7 +491,8 @@ trait Implicits { self: Typer =>
             pt)
         val generated1 = adapt(generated, pt)
         lazy val shadowing =
-          typed(untpd.Ident(ref.name) withPos pos.toSynthetic, funProto)(nestedContext.setNewTyperState.addMode(Mode.ImplicitShadowing))
+          typed(untpd.Ident(ref.name) withPos pos.toSynthetic, funProto)
+               (nestedContext.addMode(Mode.ImplicitShadowing).setNewTyperState)
         def refMatches(shadowing: Tree): Boolean =
           ref.symbol == closureBody(shadowing).symbol || {
             shadowing match {

src/dotty/tools/dotc/typer/Namer.scala

@@ -676,7 +676,7 @@ class Namer { typer: Typer =>
 
       // println(s"final inherited for $sym: ${inherited.toString}") !!!
       // println(s"owner = ${sym.owner}, decls = ${sym.owner.info.decls.show}")
-      val rhsCtx = ctx.fresh addMode Mode.InferringReturnType
+      val rhsCtx: Context = ctx.addMode(Mode.InferringReturnType)
       def rhsType = typedAheadExpr(mdef.rhs, rhsProto)(rhsCtx).tpe.widen.approximateUnion
       def lhsType = fullyDefinedType(rhsType, "right-hand side", mdef.pos)
       if (inherited.exists) inherited