error: Only field names, constants, integers, basic arithmetic expressions (+ - * / %) and parentheses are allowed in the "length" attribute
  --> $DIR/length_expr.rs:16:16
   |
16 |     #[length = "banana + 7.5"] //~ ERROR Only field names, constants, integers, basic arithmetic expressions (+ - * / %) and parentheses ...
   |                ^^^^^^^^^^^^^^