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use proc_macro2::{Span, TokenStream};
use quote::{format_ident, quote, quote_spanned};
use std::iter::FromIterator;
use syn::{parse_macro_input, Data, DeriveInput};
#[proc_macro_derive(CowStruct)]
pub fn make_cow(items: proc_macro::TokenStream) -> proc_macro::TokenStream {
let input = parse_macro_input!(items as DeriveInput);
let cow = format_ident!("Cow{}", input.ident);
let name = input.ident;
let fields = if let Data::Struct(data) = input.data {
data.fields
} else {
panic!("Only struct supported");
};
let set_fields = TokenStream::from_iter(fields.iter().map(|item| {
let field = &item.ident;
quote! { #field: std::borrow::Cow::Borrowed(&self.#field), }
}));
let cow_fields = TokenStream::from_iter(fields.iter().map(|item| {
let field = &item.ident;
let ty = &item.ty;
quote! { #field: std::borrow::Cow<'a, #ty>, }
}));
let ret: TokenStream = quote_spanned! {
Span::call_site() =>
impl #name {
fn to_cow(&self) -> #cow {
#cow {
#set_fields
}
}
}
#[derive(Debug, Default)]
struct #cow<'a> {
#cow_fields
}
impl #cow<'_> {
fn to_cow(&self) -> #cow {
#cow {
#set_fields
}
}
}
}
.into();
ret.into()
}