num-traits/derive/src/lib.rs

// Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

#![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()
}