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extern crate proc_macro;
use proc_macro::TokenStream;
use quote::quote;
#[proc_macro_derive(EnumDefault, attributes(default))]
pub fn enum_default_derive(input: TokenStream) -> TokenStream {
let ast: syn::DeriveInput = syn::parse(input).unwrap();
match ast.data {
syn::Data::Enum(data) => {
if data.variants.is_empty() {
return TokenStream::default();
}
let name = ast.ident;
let iter = data.variants.iter();
for variant in iter {
for attr in &variant.attrs {
if attr.path.is_ident("default") {
return impl_enum_default(&name, &variant.ident)
}
}
}
let first_variant = data.variants.first().unwrap();
let variant = &first_variant.ident;
impl_enum_default(&name, variant)
}
_ => TokenStream::default()
}
}
fn impl_enum_default(name: &syn::Ident, variant: &syn::Ident) -> TokenStream {
let result = quote! {
impl Default for #name {
fn default() -> #name {
#name::#variant
}
}
};
result.into()
}