Reduce context creations in implicits

Author: odersky

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

@@ -674,7 +674,7 @@ object Contexts {
     final def retractMode(mode: Mode): c.type = c.setMode(c.mode &~ mode)
   }
 
-  private def exploreCtx(using Context): Context =
+  private def exploreCtx(using Context): FreshContext =
     util.Stats.record("explore")
     val base = ctx.base
     import base._
@@ -701,6 +701,10 @@ object Contexts {
     val ectx = exploreCtx
     try op(using ectx) finally wrapUpExplore(ectx)
 
+  inline def exploreInFreshCtx[T](inline op: FreshContext ?=> T)(using Context): T =
+    val ectx = exploreCtx
+    try op(using ectx) finally wrapUpExplore(ectx)
+
   private def changeOwnerCtx(owner: Symbol)(using Context): Context =
     val base = ctx.base
     import base._

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

@@ -245,10 +245,12 @@ object Implicits:
       if refs.isEmpty && (!considerExtension || companionRefs.isEmpty) then
         Nil
       else
-        val nestedCtx = ctx.fresh.addMode(Mode.TypevarsMissContext)
         val candidates = new mutable.ListBuffer[Candidate]
         def tryCandidate(extensionOnly: Boolean)(ref: ImplicitRef) =
-          var ckind = explore(candidateKind(ref.underlyingRef))(using nestedCtx)
+          var ckind = exploreInFreshCtx { (using ctx: FreshContext) =>
+            ctx.setMode(ctx.mode | Mode.TypevarsMissContext)
+            candidateKind(ref.underlyingRef)
+          }
           if extensionOnly then ckind &= Candidate.Extension
           if ckind != Candidate.None then
             candidates += Candidate(ref, ckind, level)