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#![recursion_limit = "128"]
extern crate proc_macro;
use proc_macro::TokenStream;
use proc_macro2::TokenStream as TokenStream2;
use quote::quote;
use syn::{parse_macro_input, Data, DeriveInput, Fields, Meta, NestedMeta, Type};
type Result<T> = std::result::Result<T, &'static str>;
#[proc_macro_derive(RefCast)]
pub fn derive_ref_cast(input: TokenStream) -> TokenStream {
let input = parse_macro_input!(input as DeriveInput);
let expanded = expand(input).unwrap_or_else(|error| quote! {
compile_error! { #error }
});
TokenStream::from(expanded)
}
fn expand(input: DeriveInput) -> Result<TokenStream2> {
if !has_repr_c(&input) {
return Err("RefCast trait requires #[repr(C)] or #[repr(transparent)]");
}
let name = &input.ident;
let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
let from = only_field_ty(&input)?;
Ok(quote! {
impl #impl_generics ::ref_cast::RefCast for #name #ty_generics #where_clause {
type From = #from;
#[inline]
fn ref_cast(_from: &Self::From) -> &Self {
unsafe {
&*(_from as *const Self::From as *const Self)
}
}
#[inline]
fn ref_cast_mut(_from: &mut Self::From) -> &mut Self {
unsafe {
&mut *(_from as *mut Self::From as *mut Self)
}
}
}
})
}
fn has_repr_c(input: &DeriveInput) -> bool {
for attr in &input.attrs {
if let Ok(Meta::List(meta)) = attr.parse_meta() {
if meta.path.is_ident("repr") && meta.nested.len() == 1 {
if let NestedMeta::Meta(Meta::Path(path)) = &meta.nested[0] {
if path.is_ident("C") || path.is_ident("transparent") {
return true;
}
}
}
}
}
false
}
fn only_field_ty(input: &DeriveInput) -> Result<&Type> {
let fields = match &input.data {
Data::Struct(data) => match &data.fields {
Fields::Named(fields) => &fields.named,
Fields::Unnamed(fields) => &fields.unnamed,
Fields::Unit => {
return Err("RefCast does not support unit structs");
}
},
Data::Enum(_) => {
return Err("RefCast does not support enums");
}
Data::Union(_) => {
return Err("RefCast does not support unions");
}
};
if fields.len() != 1 {
return Err("RefCast requires a struct with a single field");
}
Ok(&fields[0].ty)
}