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extern crate proc_macro;
extern crate syn;
#[macro_use]
extern crate quote;
use proc_macro::TokenStream;
#[proc_macro_derive(Display)]
#[doc(hidden)]
pub fn display(input: TokenStream) -> TokenStream {
let s = input.to_string();
let ast = syn::parse_macro_input(&s).unwrap();
let gen = match ast.body {
syn::Body::Enum(ref variants) => {
let name = &ast.ident;
impl_display(name, variants)
}
_ => panic!("#[derive(Display)] works only on enums"),
};
gen.parse().unwrap()
}
fn impl_display(name: &syn::Ident, variants: &[syn::Variant]) -> quote::Tokens {
let variants = variants.iter()
.map(|variant| impl_display_for_variant(name, variant));
quote! {
impl Display for #name {
fn fmt(&self, f: &mut ::std::fmt::Formatter) -> Result<(), ::std::fmt::Error> {
match *self {
#(#variants)*
}
}
}
}
}
fn impl_display_for_variant(name: &syn::Ident, variant: &syn::Variant) -> quote::Tokens {
let id = &variant.ident;
match variant.data {
syn::VariantData::Unit => {
quote! {
#name::#id => {
f.write_str(stringify!(#id))
}
}
}
syn::VariantData::Tuple(ref fields) => {
match fields.len() {
0 => {
quote! {
#name::#id() => {
f.write_str(stringify!(#id))?;
f.write_str("()")
}
}
}
1 => {
quote! {
#name::#id(ref inner) => {
::std::fmt::Display::fmt(inner, f)
}
}
}
_ => {
panic!("#[derive(Display)] does not support tuple variants with more than one \
fields")
}
}
}
_ => panic!("#[derive(Display)] works only with unit and tuple variants"),
}
}