Revert eager use of escape in interpolator

Author: som-snytt

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -1253,7 +1253,7 @@ object Scanners {
               nextChar()
             }
           }
-          val alt = if oct == LF then raw"\n" else f"\\u$oct%04x"
+          val alt = if oct == LF then raw"\n" else f"${"\\"}u$oct%04x"
           error(s"octal escape literals are unsupported: use $alt instead", start)
           putChar(oct.toChar)
         }

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

@@ -542,7 +542,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
     case '"' => "\\\""
     case '\'' => "\\\'"
     case '\\' => "\\\\"
-    case _ => if ch.isControl then f"\\u${ch.toInt}%04x" else String.valueOf(ch)
+    case _ => if ch.isControl then f"${"\\"}u${ch.toInt}%04x" else String.valueOf(ch)
   }
 
   def toText(const: Constant): Text = const.tag match {

compiler/src/scala/quoted/runtime/impl/printers/SourceCode.scala

@@ -1423,7 +1423,7 @@ object SourceCode {
       case '"' => "\\\""
       case '\'' => "\\\'"
       case '\\' => "\\\\"
-      case _ => if ch.isControl then f"\\u${ch.toInt}%04x" else String.valueOf(ch)
+      case _ => if ch.isControl then f"${"\\"}u${ch.toInt}%04x" else String.valueOf(ch)
     }
 
     private def escapedString(str: String): String = str flatMap escapedChar