extern crate proc_macro;
extern crate syn;
#[macro_use]
extern crate quote;
use proc_macro::TokenStream;
use syn::{parse_macro_input, Data, DataStruct, DeriveInput, Fields};
#[proc_macro_attribute]
pub fn key_by(key: TokenStream, input: TokenStream) -> TokenStream {
let item = parse_macro_input!(input as DeriveInput);
let name = &item.ident;
let _fields = match &item.data {
Data::Struct(DataStruct {
fields: Fields::Named(fields),
..
}) => &fields.named,
_ => panic!("expected a struct with named fields"),
};
let key = proc_macro2::TokenStream::from(key);
let output: proc_macro2::TokenStream = {
quote! {
#item
impl ::std::hash::Hash for #name {
fn hash<H: ::std::hash::Hasher>(&self, state: &mut H) {
self.#key.hash(state);
}
}
}
};
proc_macro::TokenStream::from(output)
}