error: the only supported receiver is exactly `&self`
 --> tests/ui/bad_receiver.rs:5:13
  |
5 |     fn load(&mut self) -> OperationResult<String>;
  |             ^^^^^^^^^

warning: unused import: `OperationResult`
 --> tests/ui/bad_receiver.rs:1:21
  |
1 | use wip_interface::{OperationResult, interface};
  |                     ^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default