1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50

extern crate proc_macro;
extern crate syn;
#[macro_use]
extern crate quote;

use proc_macro::TokenStream;

#[proc_macro_derive(Future)]
pub fn future(input: TokenStream) -> TokenStream {
    let s = input.to_string();
    let ast = syn::parse_macro_input(&s).unwrap();
    let gen = impl_future(&ast);
    gen.parse().unwrap()
}

fn impl_future(ast: &syn::MacroInput) -> quote::Tokens {
    let name = &ast.ident;
    match ast.body {
        syn::Body::Enum(ref variants) => {
            let constraints = variants.iter().map(|var| {
                let ty = &var.data.fields()[0].ty;
                quote! {
                    #ty : Future<Item = It, Error = Err>
                }
            });
            let types = variants.iter().map(|var| {
                let t = &var.data.fields()[0].ty;
                quote! { #t }
            });
            let match_clauses = variants.iter().map(|var| {
                let var_name = &var.ident;
                quote! {
                    #name::#var_name(ref mut a) => a.poll()
                }
            });
            quote! {
                impl<It, Err, #(#constraints),*> Future for #name<#(#types),*> {
                    type Item = It;
                    type Error = Err;
                    fn poll(&mut self) -> futures::Poll<Self::Item, Self::Error> {
                        match *self {
                            #(#match_clauses),*
                        }
                    }
                }
            }
        },
        _ => panic!("Only enums can derive EnumFrom")
    }
}