Use simpler desugaring for splices

This allows writing explicitly the context function of a splice. It will make the desugaring simpler to explain as one can write it explicitly.

The downside is that each nested context will get a generic "evidence" name which might be visible to the users. Though this is general limitation of context functions that might need a general fix.

Author: nicolasstucki

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

@@ -99,24 +99,10 @@ trait QuotesAndSplices {
 
       val (outerQctx, ctx1) = popQuoteContext()
 
-      // Explicitly provide the given QuoteContext of the splice.
-      //  * Avoids leaking implementation details of scala.internal.quoted.CompileTime.exprSplice,
-      //    such as exprSplice taking a ?=> function argument
-      //  * Provide meaningful names for QuoteContext synthesized by within `${ ... }`
-      //  * If within a quote, provide a QuoteContext is linked typewise with the outer QuoteContext
-      val qctxParamName = NameKinds.UniqueName.fresh(s"qctx${if level > 0 then level - 1 else ""}_".toTermName)
-      val qctxParamTpe = outerQctx match {
-        case Some(qctxRef) => qctxRef.tpe.select("NestedContext".toTypeName)
-        case _ => defn.QuoteContextClass.typeRef // splice at level 0 (or lower)
-      }
-      val qctxParamTpt = untpd.TypedSplice(TypeTree(qctxParamTpe))
-      val qctxParam = untpd.makeParameter(qctxParamName, qctxParamTpt, untpd.Modifiers(Given))
-      val expr = untpd.Function(List(qctxParam), tree.expr).withSpan(tree.span)
-
       val internalSplice =
         outerQctx match
-          case Some(qctxRef) => untpd.Apply(untpd.Apply(untpd.ref(defn.InternalQuoted_exprNestedSplice.termRef), qctxRef), expr)
-          case _ => untpd.Apply(untpd.ref(defn.InternalQuoted_exprSplice.termRef), expr)
+          case Some(qctxRef) => untpd.Apply(untpd.Apply(untpd.ref(defn.InternalQuoted_exprNestedSplice.termRef), qctxRef), tree.expr)
+          case _ => untpd.Apply(untpd.ref(defn.InternalQuoted_exprSplice.termRef), tree.expr)
 
       typedApply(internalSplice, pt)(using ctx1).withSpan(tree.span)
     }

tests/neg/i8052.scala

@@ -11,7 +11,7 @@ object Macro2 {
     def derived[T: Type](ev: Expr[Mirror.Of[T]])(using qctx: QuoteContext): Expr[TC[T]] = '{
       new TC[T] {
         def encode(): Unit = $ev match {
-          case '{ $m: Mirror.ProductOf[T] } => ??? // error
+          case '{ $m: Mirror.ProductOf[T] } => ??? // error // error
         }
       }
     }

tests/pos/splice-with-explicit-context.scala

@@ -0,0 +1,7 @@
+import scala.quoted._
+
+def f(a: Expr[Int])(using qctx: QuoteContext): Unit =
+
+  '{ val x: Int = ${ (using qctx2) => a } }
+
+  '{ val x: Int = ${ (using qctx2: qctx.NestedContext) => a } }

tests/run-staging/quote-nested-2.check

@@ -1,4 +1,4 @@
 ((qctx: scala.quoted.QuoteContext) ?=> {
   val a: scala.quoted.Expr[scala.Int] = scala.internal.quoted.CompileTime.exprQuote[scala.Int](4).apply(using qctx)
-  ((qctx1_$1: qctx.NestedContext) ?=> a).asInstanceOf[scala.ContextFunction1[scala.quoted.QuoteContext, scala.quoted.Expr[scala.Int]]].apply(using qctx)
+  ((evidence$2: qctx.NestedContext) ?=> a).asInstanceOf[scala.ContextFunction1[scala.quoted.QuoteContext, scala.quoted.Expr[scala.Int]]].apply(using qctx)
 })

tests/run-staging/quote-nested-5.check

@@ -1,4 +1,4 @@
 ((qctx: scala.quoted.QuoteContext) ?=> {
   val a: scala.quoted.Expr[scala.Int] = scala.internal.quoted.CompileTime.exprQuote[scala.Int](4).apply(using qctx)
-  ((qctx2: scala.quoted.QuoteContext) ?=> ((qctx1_$1: qctx2.NestedContext) ?=> a).asInstanceOf[scala.ContextFunction1[scala.quoted.QuoteContext, scala.quoted.Expr[scala.Int]]].apply(using qctx2)).apply(using qctx)
+  ((qctx2: scala.quoted.QuoteContext) ?=> ((evidence$3: qctx2.NestedContext) ?=> a).asInstanceOf[scala.ContextFunction1[scala.quoted.QuoteContext, scala.quoted.Expr[scala.Int]]].apply(using qctx2)).apply(using qctx)
 })