error:
       … while calling the 'toString' builtin
         at /pwd/lang/eval-fail-hashfile-missing.nix:7:1:
            6| in
            7| toString (
             | ^
            8|   builtins.concatLists (

       … while evaluating the first argument passed to builtins.toString

       … while calling the 'hashFile' builtin

       error: opening file '/pwd/lang/this-file-is-definitely-not-there-7392097': No such file or directory