Function derive_discriminant_impl

proc-macros/src/discriminant.rs:6–143

(input_item: TokenStream)

use syn::{Attribute, Data, DeriveInput, Field, Fields, ItemEnum, MetaList};

pub fn derive_discriminant_impl(input_item: TokenStream) -> syn::Result<TokenStream> {
	let input = syn::parse2::<DeriveInput>(input_item).unwrap();

	let mut data = match input.data {
		Data::Enum(data) => data,
		_ => return Err(syn::Error::new(Span::call_site(), "Tried to derive a discriminant for non-enum")),
	};

	let mut is_sub_discriminant = vec![];
	let mut attr_errs = vec![];

	for var in &mut data.variants {
		if var.attrs.iter().any(|a| a.path().is_ident("sub_discriminant")) {
			match var.fields.len() {
				1 => {
					let Field { ty, .. } = var.fields.iter_mut().next().unwrap();
					*ty = syn::parse_quote! {
						<#ty as ToDiscriminant>::Discriminant
					};
					is_sub_discriminant.push(true);
				}
				n => unimplemented!("#[sub_discriminant] on variants with {n} fields is not supported (for now)"),
			}
		} else {
			var.fields = Fields::Unit;
			is_sub_discriminant.push(false);
		}
		let mut retain = vec![];
		for (i, a) in var.attrs.iter_mut().enumerate() {
			if a.path().is_ident("discriminant_attr") {
				match a.meta.require_list() {
					Ok(MetaList { tokens, .. }) => {
						let attr: Attribute = syn::parse_quote! {
							#[#tokens]
						};
						*a = attr;
						retain.push(i);
					}
					Err(e) => {
						attr_errs.push(syn::Error::new(a.span(), e));
					}
				}
			}
		}
		var.attrs = var.attrs.iter().enumerate().filter(|(i, _)| retain.contains(i)).map(|(_, x)| x.clone()).collect();
	}

	let attrs = input
		.attrs
		.iter()
		.cloned()
		.filter_map(|a| {
			let a_span = a.span();
			a.path()
				.is_ident("discriminant_attr")
				.then(|| match a.meta.require_list() {
					Ok(MetaList { tokens, .. }) => {
						let attr: Attribute = syn::parse_quote! {