Macro validation_errors 

macro_rules! validation_errors {
    ( $( $key:expr => $val:tt ),* $(,)? ) => { ... };
    (@insert $m:ident, $key:expr, [ $( ($code:expr, $val:expr) ),* $(,)? ]) => { ... };
    (@insert $m:ident, $key:expr, ($code:expr, $val:expr)) => { ... };
}