macro_rules! impl_from_error {
($from:ty, $dest:ident::$item:ident) => { ... };
($from:ty, $nest_dest:ident::$nest_item:ident=>$dest:ident::$item:ident) => { ... };
}
macro_rules! impl_from_error {
($from:ty, $dest:ident::$item:ident) => { ... };
($from:ty, $nest_dest:ident::$nest_item:ident=>$dest:ident::$item:ident) => { ... };
}