Macro delegate_deref 

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

This macro is used to implement the Deref and DerefMut traits for a wrapper type that contains a backend field. It delegates to the inner backend field, allowing the wrapper type to behave like the backend without having to manually implement each method.