rand_macros/src/lib.rs

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

use proc_macro::TokenStream;

#[proc_macro_derive(Rand)]
pub fn rand_derive(input: TokenStream) -> TokenStream {
    let s = input.to_string();
    let ast = syn::parse_derive_input(&s).unwrap();
    let gen = impl_rand_derive(&ast);
    gen.parse().unwrap()
}

fn impl_rand_derive(ast: &syn::MacroInput) -> quote::Tokens {
    let name = &ast.ident;
    let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();

    let rand = match ast.body {
        syn::Body::Struct(syn::VariantData::Struct(ref body)) => {
            let fields = body
                .iter()
                .filter_map(|field| field.ident.as_ref())
                .map(|ident| quote! { #ident: rng.gen() })
                .collect::<Vec<_>>();

            quote! { #name { #(#fields,)* } }
        },
        syn::Body::Struct(syn::VariantData::Tuple(ref body)) => {
            let fields = (0..body.len())
                .map(|_| quote! { rng.gen() })
                .collect::<Vec<_>>();

            quote! { #name (#(#fields),*) }
        },
        syn::Body::Struct(syn::VariantData::Unit) => {
            quote! { #name }
        },
        syn::Body::Enum(ref body) => {
            if body.is_empty() {
                panic!("`Rand` cannot be derived for enums with no variants");
            }

            let len = body.len();
            let mut variants = body
                .iter()
                .enumerate()
                .map(|(index, variant)| {
                    let ident = &variant.ident;
                    let arm = match variant.data {
                        syn::VariantData::Struct(ref body) => {
                            let fields = body
                                .iter()
                                .filter_map(|field| field.ident.as_ref())
                                .map(|ident| quote! { #ident: rng.gen() })
                                .collect::<Vec<_>>();
                            quote! { #name::#ident { #(#fields,)* } }
                        },
                        syn::VariantData::Tuple(ref body) => {
                            let fields = (0..body.len())
                                .map(|_| quote! { rng.gen() })
                                .collect::<Vec<_>>();

                            quote! { #name::#ident (#(#fields),*) }
                        },
                        syn::VariantData::Unit => quote! { #name::#ident }
                    };

                    quote! { #index => #arm }
                })
                .collect::<Vec<_>>();
            variants.push(quote! { _ => unreachable!() });
            quote! { match rng.gen_range(0, #len) { #(#variants,)* } }
        }
    };

    quote! {
        impl #impl_generics ::rand::Rand for #name #ty_generics #where_clause {
            #[inline]
            fn rand<R: ::rand::Rng>(rng: &mut R) -> Self {
                #rand
            }
        }
    }
}