Macro wrap_download_handler 

macro_rules! wrap_download_handler {
    ($ vis : vis struct $ name : ident ; impl DownloadHandler { $ ($ (# [$ attrs_name : meta]) * fn $ method_name : ident (& $ self : ident $ (, $ arg_name : ident : $ arg_type : ty) * $ (,) ?) $ (-> $ return_type : ty) ? { $ ($ body : tt) * }) * }) => { ... };
    ($ vis : vis struct $ name : ident $ (< $ ($ generic_type : ident : $ first_generic_type_bound : tt $ (+ $ generic_type_bound : tt) *) , + $ (,) ? >) ? { $ ($ field_vis : vis $ field_name : ident : $ field_type : ty) , * $ (,) ? } impl DownloadHandler { $ ($ (# [$ attrs_name : meta]) * fn $ method_name : ident (& $ self : ident $ (, $ arg_name : ident : $ arg_type : ty) * $ (,) ?) $ (-> $ return_type : ty) ? { $ ($ body : tt) * }) * }) => { ... };
}

Implement the WrapDownloadHandler trait for the specified struct. You can declare more members for your struct, and in the impl DownloadHandler block you can override default methods implemented by the ImplDownloadHandler trait.

Example

wrap_download_handler! {
    struct MyDownloadHandler {
        payload: String,
    }

    impl DownloadHandler {
        // ...
    }
}

fn make_my_struct() -> DownloadHandler {
    MyDownloadHandler::new("payload".to_string())
}