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use proc_macro2::{Span, TokenTree};
use syn::parse::{Error, Parse, ParseStream, Result};
use syn::{token, Attribute, Data, DeriveInput, Expr, Fields, Ident, Meta, Token};
pub struct Input {
pub ident: Ident,
pub repr: Ident,
pub variants: Vec<Variant>,
pub default_variant: Option<Variant>,
}
#[derive(Clone)]
pub struct Variant {
pub ident: Ident,
pub attrs: VariantAttrs,
}
#[derive(Clone)]
pub struct VariantAttrs {
pub is_default: bool,
}
fn parse_serde_attr(attrs: &mut VariantAttrs, attr: &Attribute) {
if let Meta::List(_) = attr.meta {
let _ = attr.parse_nested_meta(|meta| {
if meta.input.peek(Token![=]) {
let _value: Expr = meta.value()?.parse()?;
} else if meta.input.peek(token::Paren) {
let _group: TokenTree = meta.input.parse()?;
} else if meta.path.is_ident("other") {
attrs.is_default = true;
}
Ok(())
});
}
}
fn parse_attrs(variant: &syn::Variant) -> VariantAttrs {
let mut attrs = VariantAttrs { is_default: false };
for attr in &variant.attrs {
if attr.path().is_ident("serde") {
parse_serde_attr(&mut attrs, attr);
}
}
attrs
}
impl Parse for Input {
fn parse(input: ParseStream) -> Result<Self> {
let call_site = Span::call_site();
let derive_input = DeriveInput::parse(input)?;
let data = match derive_input.data {
Data::Enum(data) => data,
_ => {
return Err(Error::new(call_site, "input must be an enum"));
}
};
let variants = data
.variants
.into_iter()
.map(|variant| match variant.fields {
Fields::Unit => {
let attrs = parse_attrs(&variant);
Ok(Variant {
ident: variant.ident,
attrs,
})
}
Fields::Named(_) | Fields::Unnamed(_) => {
Err(Error::new(variant.ident.span(), "must be a unit variant"))
}
})
.collect::<Result<Vec<Variant>>>()?;
if variants.is_empty() {
return Err(Error::new(call_site, "there must be at least one variant"));
}
let generics = derive_input.generics;
if !generics.params.is_empty() || generics.where_clause.is_some() {
return Err(Error::new(call_site, "generic enum is not supported"));
}
let mut repr = None;
for attr in derive_input.attrs {
if attr.path().is_ident("repr") {
if let Meta::List(_) = &attr.meta {
let ty: Ident = attr.parse_args()?;
repr = Some(ty);
break;
}
}
}
let repr = repr.ok_or_else(|| Error::new(call_site, "missing #[repr(...)] attribute"))?;
let mut default_variants = variants.iter().filter(|x| x.attrs.is_default);
let default_variant = default_variants.next().cloned();
if default_variants.next().is_some() {
return Err(Error::new(
call_site,
"only one variant can be #[serde(other)]",
));
}
Ok(Input {
ident: derive_input.ident,
repr,
variants,
default_variant,
})
}
}