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#![crate_type = "proc-macro"]
extern crate syn;
#[macro_use]
extern crate quote;
extern crate proc_macro;
use proc_macro::TokenStream;
use syn::Body::Enum;
use syn::VariantData::Unit;
#[proc_macro_derive(FromPrimitive)]
pub fn from_primitive(input: TokenStream) -> TokenStream {
let source = input.to_string();
let ast = syn::parse_macro_input(&source).unwrap();
let name = &ast.ident;
let variants = match ast.body {
Enum(ref variants) => variants,
_ => panic!("`FromPrimitive` can be applied only to the enums, {} is not an enum", name)
};
let mut idx = 0;
let variants: Vec<_> = variants.iter()
.map(|variant| {
let ident = &variant.ident;
match variant.data {
Unit => (),
_ => {
panic!("`FromPrimitive` can be applied only to unitary enums, {}::{} is either struct or tuple", name, ident)
},
}
if let Some(val) = variant.discriminant {
idx = val.value;
}
let tt = quote!(#idx => Some(#name::#ident));
idx += 1;
tt
})
.collect();
let res = quote! {
impl ::num::traits::FromPrimitive for #name {
fn from_i64(n: i64) -> Option<Self> {
Self::from_u64(n as u64)
}
fn from_u64(n: u64) -> Option<Self> {
match n {
#(variants,)*
_ => None,
}
}
}
};
res.to_string().parse().unwrap()
}