proc-macros/src/discriminant.rs

use crate::helpers::call_site_ident;
use proc_macro2::{Ident, Span, TokenStream};
use syn::spanned::Spanned;
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! {
							#[#tokens]
						};
						Some(attr)
					}
					Err(e) => {
						attr_errs.push(syn::Error::new(a_span, e));
						None
					}
				})
				.and_then(|opt| opt)
		})
		.collect::<Vec<Attribute>>();

	if !attr_errs.is_empty() {
		return Err(attr_errs
			.into_iter()
			.reduce(|mut l, r| {
				l.combine(r);
				l
			})
			.unwrap());
	}

	let discriminant = ItemEnum {
		attrs,
		vis: input.vis,
		enum_token: data.enum_token,
		ident: call_site_ident(format!("{}Discriminant", input.ident)),
		generics: input.generics,
		brace_token: data.brace_token,
		variants: data.variants,
	};

	let input_type = &input.ident;
	let discriminant_type = &discriminant.ident;
	let variant = &discriminant.variants.iter().map(|var| &var.ident).collect::<Vec<&Ident>>();

	let (pattern, value) = is_sub_discriminant
		.into_iter()
		.map(|b| {
			(
				if b {
					quote::quote! { (x) }
				} else {
					quote::quote! { { .. } }
				},
				b.then(|| quote::quote! { (x.to_discriminant()) }).unwrap_or_default(),
			)
		})
		.unzip::<_, _, Vec<_>, Vec<_>>();
	#[cfg(feature = "serde-discriminant")]
	let serde = quote::quote! {
		#[derive(serde::Serialize, serde::Deserialize)]
	};

	#[cfg(not(feature = "serde-discriminant"))]
	let serde = quote::quote! {};

	let res = quote::quote! {
		#serde
		#discriminant

		impl ToDiscriminant for #input_type {
			type Discriminant = #discriminant_type;

			fn to_discriminant(&self) -> #discriminant_type {
				match self {
					#(
						#input_type::#variant #pattern => #discriminant_type::#variant #value
					),*
				}
			}
		}

		impl From<&#input_type> for #discriminant_type {
			fn from(x: &#input_type) -> #discriminant_type {
				x.to_discriminant()
			}
		}
	};

	Ok(res)
}