Consider adding type for partial parsing

[Kestrer]

Sometimes I need to parse one item, and then keep the rest of the tokens for later. I would like a struct in syn::parse:

pub struct ParseStart<T: Parse> {
    pub item: T,
    pub rest: TokenStream,
}

I'm not certain about the name, it could also be called PartialParse or ParseOne, and there is also the question of whether to have a separate ParseStart and ParseStart2 for proc_macro::TokenStream and proc_macro2::TokenStream respectively. We could even blanket impl Parse for (T, U) for maximum flexibility and minimum API surface.

Previous discussion has happened here and here but this is a slightly different approach.

If you think this is a good idea and can answer my above questions I'm happy to go ahead and make a PR.

[dtolnay]

I think I would prefer not to build this into Syn. But if there is an abstraction along these lines that is reasonably widely used in downstream crates, I would be interested to know (with links).

[tqwewe]

Is this still not possible currently? It would be very useful in my case, and I'm not sure what the alternatives approaches are