enum-kinds 0.5.1

#![doc = include_str!("../README.md")]

extern crate proc_macro;
extern crate proc_macro2;
extern crate quote;
#[macro_use]
extern crate syn;

use proc_macro2::TokenStream;
use quote::quote;
use std::collections::HashSet;
use syn::punctuated::Punctuated;
use syn::{
    Data, DataEnum, DeriveInput, Fields, GenericParam, Lifetime, LifetimeDef, Meta, MetaList,
    MetaNameValue, NestedMeta, Path,
};

#[proc_macro_derive(EnumKind, attributes(enum_kind))]
pub fn enum_kind(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let ast = syn::parse(input).expect("#[derive(EnumKind)] failed to parse input");
    let (name, traits) = get_enum_specification(&ast);
    let enum_ = create_kind_enum(&ast, &name, traits);
    let impl_ = create_impl(&ast, &name);
    let code = quote! {
        #enum_
        #impl_
    };
    proc_macro::TokenStream::from(code)
}

fn find_attribute(
    definition: &DeriveInput,
    name: &str,
) -> Option<Punctuated<NestedMeta, syn::token::Comma>> {
    for attr in definition.attrs.iter() {
        match attr.parse_meta() {
            Ok(Meta::List(MetaList {
                ref path,
                ref nested,
                ..
            })) if path.is_ident(name) => return Some(nested.clone()),
            _ => continue,
        }
    }
    None
}

fn get_enum_specification(definition: &DeriveInput) -> (Path, Vec<NestedMeta>) {
    let params = find_attribute(definition, "enum_kind")
        .expect("#[derive(EnumKind)] requires an associated enum_kind attribute to be specified");
    let mut iter = params.iter();
    if let Some(&NestedMeta::Meta(Meta::Path(ref path))) = iter.next() {
        return (path.to_owned(), iter.cloned().collect());
    } else {
        panic!("#[enum_kind(NAME)] attribute requires NAME to be specified");
    }
}

fn has_docs(traits: &[NestedMeta]) -> bool {
    traits.iter().any(|attr| {
        if let NestedMeta::Meta(Meta::NameValue(MetaNameValue { path, .. })) = attr {
            path.is_ident("doc")
        } else {
            false
        }
    })
}

fn create_kind_enum(
    definition: &DeriveInput,
    kind_ident: &Path,
    traits: Vec<NestedMeta>,
) -> TokenStream {
    let variant_idents = match &definition.data {
        &Data::Enum(DataEnum { ref variants, .. }) => variants.iter().map(|ref v| v.ident.clone()),
        _ => {
            panic!("#[derive(EnumKind)] is only allowed for enums");
        }
    };
    let visibility = &definition.vis;
    let docs_attr = if !has_docs(traits.as_ref()) {
        quote! {#[allow(missing_docs)]}
    } else {
        quote! {}
    };
    let code = quote! {
        #[derive(Debug, Clone, Copy, PartialEq, Eq)]
        #[allow(dead_code)]
        #docs_attr
        #( #[#traits] )*
        #visibility enum #kind_ident {
            #(#variant_idents),*
        }
    };
    TokenStream::from(code)
}

fn is_uninhabited_enum(definition: &DeriveInput) -> bool {
    if let Data::Enum(ref data) = definition.data {
        return data.variants.len() == 0;
    }
    return false;
}

fn create_impl(definition: &DeriveInput, kind_ident: &Path) -> TokenStream {
    let (_, ty_generics, where_clause) = definition.generics.split_for_impl();
    let ident = &definition.ident;

    let arms = match &definition.data {
        &Data::Enum(DataEnum { ref variants, .. }) => variants.iter().map(|ref v| {
            let variant = &v.ident;
            match v.fields {
                Fields::Unit => quote! {
                    &#ident::#variant => #kind_ident::#variant,
                },
                Fields::Unnamed(_) => quote! {
                    &#ident::#variant(..) => #kind_ident::#variant,
                },
                Fields::Named(_) => quote! {
                    &#ident::#variant{..} => #kind_ident::#variant,
                },
            }
        }),
        _ => {
            panic!("#[derive(EnumKind)] is only allowed for enums");
        }
    };

    let trait_: Path = if cfg!(feature = "no-stdlib") {
        parse_quote!(::core::convert::From)
    } else {
        parse_quote!(::std::convert::From)
    };

    let mut counter: u32 = 1;
    let used: HashSet<Lifetime> = definition
        .generics
        .lifetimes()
        .map(|ld| ld.lifetime.clone())
        .collect();
    let a = loop {
        let lifetime: Lifetime = syn::parse_str(&format!("'__enum_kinds{}", counter)).unwrap();
        if !used.contains(&lifetime) {
            break LifetimeDef::new(lifetime);
        }
        counter += 1;
    };

    let mut generics = definition.generics.clone();
    generics.params.insert(0, GenericParam::Lifetime(a.clone()));
    let (impl_generics, _, _) = generics.split_for_impl();

    let impl_ = if is_uninhabited_enum(definition) {
        quote! {
            unreachable!();
        }
    } else {
        quote! {
            match _value {
                #(#arms)*
            }
        }
    };

    let tokens = quote! {
        #[automatically_derived]
        #[allow(unused_attributes)]
        impl #impl_generics #trait_<&#a #ident#ty_generics> for #kind_ident #where_clause {
            fn from(_value: &#a #ident#ty_generics) -> Self {
                #impl_
            }
        }

        #[automatically_derived]
        #[allow(unused_attributes)]
        impl #impl_generics #trait_<#ident#ty_generics> for #kind_ident #where_clause {
            fn from(value: #ident#ty_generics) -> Self {
                #kind_ident::from(&value)
            }
        }
    };
    TokenStream::from(tokens)
}