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extern crate proc_macro;
extern crate proc_macro2;
use proc_macro::TokenStream;
use proc_macro2::{Ident, Span};
use quote::{quote, ToTokens};
use syn::*;
#[proc_macro_derive(Derefable, attributes(deref))]
pub fn derefable_derive(input: TokenStream) -> TokenStream {
let ast = syn::parse(input).unwrap();
impl_derefable(&ast)
}
fn impl_derefable(ast: &syn::DeriveInput) -> TokenStream {
let name = &ast.ident;
let data = match ast.data {
Data::Struct(ref s) => s,
_ => {
panic!("`#[derive(Derefable)]` is only available for structs")
}
};
match data.fields {
Fields::Unit => {
panic!("`#[derive(Derefable)]` requires a field to be able to deref")
}
_ => {}
}
let mut deref_field = None;
let mut is_field_mutable = false;
for (i, field) in data.fields.iter().enumerate() {
for attribute in &field.attrs {
if let Ok(meta) = attribute.parse_meta() {
match meta {
Meta::Word(ref ident) if ident == "deref" => {
if deref_field.is_none() {
deref_field = Some((field.clone(), i as u32));
} else {
panic!("Only one field in a struct can be `#[deref]`")
}
}
Meta::List(MetaList {
ref ident,
ref nested,
..
}) if ident == "deref" => {
is_field_mutable = nested.iter().any(|nested_item| match nested_item {
NestedMeta::Meta(m) => match m {
Meta::Word(ident) => ident == "mutable",
_ => false,
},
_ => false,
});
if deref_field.is_none() {
deref_field = Some((field.clone(), i as u32));
} else {
panic!("Only one field in a struct can be `#[deref]`")
}
}
_ => {}
}
}
}
}
if deref_field.is_none() {
panic!("`#[derive(Derefable)]` requires one field to be marked `#[deref]`");
}
let (field, index) = deref_field.unwrap();
let target = field.ty;
let ident = field
.ident
.map(Ident::into_token_stream)
.unwrap_or_else(|| {
Index {
index,
span: Span::call_site(),
}
.into_token_stream()
});
let mut_gen = if is_field_mutable {
quote! {
impl std::ops::DerefMut for #name {
fn deref_mut(&mut self) -> &mut Self::Target {
&mut self.#ident
}
}
}
} else {
quote! {}
};
let gen = quote! {
impl std::ops::Deref for #name {
type Target = #target;
fn deref(&self) -> &Self::Target {
&self.#ident
}
}
#mut_gen
};
gen.into()
}