message: Fix correctness of optimization
diff: @@ -363,7 +363,7 @@ enum ArgKind<'tcx> {
  ParamEnv::reveal_all(),
  local_decl.source_info.span,
  );
  - if local_decl.mutability == mir::Mutability::Not && internally_mutable {
  + if local_decl.mutability == mir::Mutability::Not && !internally_mutable {
  // We wont mutate this argument, so it is fine to borrow the backing storage
  // of this argument, to prevent a copy.
  