Macro delegate_config 

macro_rules! delegate_config {
    ($wrapper:ident<$($lt:lifetime,)? $backend:ident $(, $rest:ident)* $(,)?>, $field:ident) => { ... };
}

This macro is used to implement the BackendConfig trait for a wrapper type that contains a backend field. It delegates the associated types and methods to the inner backend field, allowing the wrapper type to behave like a BackendConfig without having to manually implement each item.