error:
       … while evaluating the attribute 'body'
         at /pwd/lang/eval-fail-assert.nix:4:3:
            3|
            4|   body = x "x";
             |   ^
            5| }

       … from call site
         at /pwd/lang/eval-fail-assert.nix:4:10:
            3|
            4|   body = x "x";
             |          ^
            5| }

       … while calling 'x'
         at /pwd/lang/eval-fail-assert.nix:2:7:
            1| let {
            2|   x = arg: assert arg == "y"; 123;
             |       ^
            3|

       … while evaluating the condition of the assertion '(arg == "y")'
         at /pwd/lang/eval-fail-assert.nix:2:12:
            1| let {
            2|   x = arg: assert arg == "y"; 123;
             |            ^
            3|

       error: string '"x"' is not equal to string '"y"'