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|
//! Implementation of a [`TryFrom`] derive macro.
use proc_macro2::{Literal, TokenStream};
use quote::{format_ident, quote, ToTokens};
use syn::spanned::Spanned as _;
use crate::utils::{
attr::{self, ParseMultiple as _},
Spanning,
};
/// Expands a [`TryFrom`] derive macro.
pub fn expand(input: &syn::DeriveInput, _: &'static str) -> syn::Result<TokenStream> {
match &input.data {
syn::Data::Struct(data) => Err(syn::Error::new(
data.struct_token.span(),
"`TryFrom` cannot be derived for structs",
)),
syn::Data::Enum(data) => Ok(Expansion {
repr: attr::ReprInt::parse_attrs(&input.attrs, &format_ident!("repr"))?
.map(Spanning::into_inner)
.unwrap_or_default(),
attr: ItemAttribute::parse_attrs(&input.attrs, &format_ident!("try_from"))?
.map(|attr| {
if matches!(attr.item, ItemAttribute::Types(_)) {
Err(syn::Error::new(
attr.span,
"`#[try_from(repr(...))]` attribute is not supported yet",
))
} else {
Ok(attr.item)
}
})
.transpose()?,
ident: input.ident.clone(),
generics: input.generics.clone(),
variants: data.variants.clone().into_iter().collect(),
}
.into_token_stream()),
syn::Data::Union(data) => Err(syn::Error::new(
data.union_token.span(),
"`TryFrom` cannot be derived for unions",
)),
}
}
/// Representation of a [`TryFrom`] derive macro struct item attribute.
///
/// ```rust,ignore
/// #[try_from(repr)]
/// #[try_from(repr(<types>))]
/// ```
type ItemAttribute = attr::ReprConversion;
/// Expansion of a macro for generating [`TryFrom`] implementation of an enum.
struct Expansion {
/// `#[repr(u/i*)]` of the enum.
repr: attr::ReprInt,
/// [`ItemAttribute`] of the enum.
attr: Option<ItemAttribute>,
/// [`syn::Ident`] of the enum.
///
/// [`syn::Ident`]: struct@syn::Ident
ident: syn::Ident,
/// [`syn::Generics`] of the enum.
generics: syn::Generics,
/// [`syn::Variant`]s of the enum.
variants: Vec<syn::Variant>,
}
impl ToTokens for Expansion {
/// Expands [`TryFrom`] implementations for a struct.
fn to_tokens(&self, tokens: &mut TokenStream) {
if self.attr.is_none() {
return;
}
let ident = &self.ident;
let (impl_generics, ty_generics, where_clause) = self.generics.split_for_impl();
let repr_ty = &self.repr.ty();
let mut last_discriminant = quote! { 0 };
let mut inc = 0usize;
let (consts, (discriminants, variants)): (
Vec<syn::Ident>,
(Vec<TokenStream>, Vec<TokenStream>),
) = self
.variants
.iter()
.filter_map(
|syn::Variant {
ident,
fields,
discriminant,
..
}| {
if let Some(d) = discriminant {
last_discriminant = d.1.to_token_stream();
inc = 0;
}
let ret = {
let inc = Literal::usize_unsuffixed(inc);
fields.is_empty().then_some((
format_ident!("__DISCRIMINANT_{ident}"),
(
quote! { #last_discriminant + #inc },
quote! { #ident #fields },
),
))
};
inc += 1;
ret
},
)
.unzip();
let error = quote! { derive_more::TryFromReprError<#repr_ty> };
quote! {
#[automatically_derived]
impl #impl_generics derive_more::core::convert::TryFrom<#repr_ty #ty_generics>
for #ident #where_clause {
type Error = #error;
#[allow(non_upper_case_globals)]
#[inline]
fn try_from(val: #repr_ty) -> derive_more::core::result::Result<Self, #error> {
#( const #consts: #repr_ty = #discriminants; )*
match val {
#(#consts => derive_more::core::result::Result::Ok(#ident::#variants),)*
_ => derive_more::core::result::Result::Err(
derive_more::TryFromReprError::new(val)
),
}
}
}
}.to_tokens(tokens);
}
}
|
