impl<Id> Item for FileDownloadItem<Id>
{
    type Params<'exec> = FileDownloadParams<Id>;
    type State = FileDownloadState;
    // ..
}

Input:

pub struct FileDownloadParams<Id> {
    src: Url,
    dest: PathBuf,
    marker: PhantomData<Id>,
}

Output:

pub enum FileDownloadState {
    None,
    Some {
        path: PathBuf,
        md5: Md5Sum,
    },
}