Inline extra object to StringInterpOpt

Author: som-snytt

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

@@ -137,10 +137,38 @@ class StringInterpolatorOpt extends MiniPhase:
         case _        => false
     // Perform format checking and normalization, then make it StringOps(fmt).format(args1) with tweaked args
     def transformF(fun: Tree, args: Tree): Tree =
-      val (fmt, args1) = FormatInterpolatorTransform.checked(fun, args)
+      // For f"${arg}%xpart", check format conversions and return (format, args) for String.format(format, args).
+      def checked(args0: Tree)(using Context): (Tree, Tree) =
+        val (partsExpr, parts) = fun match
+          case TypeApply(Select(Apply(_, (parts: SeqLiteral) :: Nil), _), _) =>
+            (parts.elems, parts.elems.map { case Literal(Constant(s: String)) => s })
+          case _ =>
+            report.error("Expected statically known StringContext", fun.srcPos)
+            (Nil, Nil)
+        val (args, elemtpt) = args0 match
+          case seqlit: SeqLiteral => (seqlit.elems, seqlit.elemtpt)
+          case _ =>
+            report.error("Expected statically known argument list", args0.srcPos)
+            (Nil, EmptyTree)
+
+        def literally(s: String) = Literal(Constant(s))
+        if parts.lengthIs != args.length + 1 then
+          val badParts =
+            if parts.isEmpty then "there are no parts"
+            else s"too ${if parts.lengthIs > args.length + 1 then "few" else "many"} arguments for interpolated string"
+          report.error(badParts, fun.srcPos)
+          (literally(""), args0)
+        else
+          val checker = TypedFormatChecker(partsExpr, parts, args)
+          val (format, formatArgs) = checker.checked
+          if format.isEmpty then (literally(parts.mkString), args0) // on error just use unchecked inputs
+          else (literally(format.mkString), SeqLiteral(formatArgs.toList, elemtpt))
+      end checked
+      val (fmt, args1) = checked(args)
       resolveConstructor(defn.StringOps.typeRef, List(fmt))
         .select(nme.format)
         .appliedTo(args1)
+    end transformF
     // Starting with Scala 2.13, s and raw are macros in the standard
     // library, so we need to expand them manually.
     // sc.s(args)    -->   standardInterpolator(processEscapes, args, sc.parts)