Macro custom_error_expression 

macro_rules! custom_error_expression {
    (bad_request, $code:expr, $msg:expr) => { ... };
    (invalid_request, $code:expr, $msg:expr) => { ... };
    (internal_error, $code:expr, $msg:expr) => { ... };
    (unauthorized, $code:expr, $msg:expr) => { ... };
    (not_found, $code:expr, $msg:expr) => { ... };
    (conflict, $code:expr, $msg:expr) => { ... };
}