guzt

   Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
 --> src/lib.rs:3:20
  |
3 |         return Err("c".to_string());
  |                    ^^^^^^^^^^^^^^^ expected struct `failure::error::Error`, found struct `std::string::String`
  |
  = note: expected type `failure::error::Error`
             found type `std::string::String`