Address review comments

* Simplify grabbing of rooted handle to StringContext
* Simplify StringContext tree unapply
* Remove check file for test and use an assert instead

Author: lloydmeta

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

@@ -1110,12 +1110,7 @@ object desugar {
           case t => t
         }
         // This is a deliberate departure from scalac, where StringContext is not rooted (See #4732)
-        val rootedStringContext = {
-          val rootPkg = Ident(nme.ROOTPKG)
-          val scalaPkg = Select(rootPkg, nme.scala_)
-          Select(scalaPkg, nme.StringContext)
-        }
-        Apply(Select(Apply(rootedStringContext, strs), id), elems)
+        Apply(Select(Apply(scalaDot(nme.StringContext), strs), id), elems)
       case InfixOp(l, op, r) =>
         if (ctx.mode is Mode.Type)
           AppliedTypeTree(op, l :: r :: Nil) // op[l, r]

compiler/src/dotty/tools/dotc/transform/localopt/StringInterpolatorOpt.scala

@@ -34,10 +34,8 @@ class StringInterpolatorOpt extends MiniPhase {
 
   private object StringContextApply {
     def unapply(tree: Select)(implicit ctx: Context): Boolean = {
-      tree.symbol.eq(defn.StringContextModule_apply) && {
-        val qualifier = tree.qualifier
-        qualifier.isInstanceOf[Select] && qualifier.symbol.eq(defn.StringContextModule)
-      }
+      tree.symbol.eq(defn.StringContextModule_apply) &&
+      tree.qualifier.symbol.eq(defn.StringContextModule)
     }
   }
 

tests/run/rooted_stringcontext.check

@@ -5,6 +5,6 @@ object StringContext {
 
 object Test {
   def main(args: Array[String]): Unit = {
-    println(StringContext.s)
+    assert(StringContext.s == "Answer: 42")
   }
 }