1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145

//! Generate enums with matching variants, but without any of the associated data.
//! `enum-kinds-traits` crate contains trait definitions used by this crate.
//! 
//! In other words, `enum-kinds-macros` automatically generates `enum`s that have
//! the same set of variants as the original `enum`, but with all the embedded data
//! stripped away (that is, all the variants are unit variants). Additionally,
//! `enum-kinds-macros` implements `ToKind` trait for the original `enum` allowing
//! one to get the associated unit variant.
//! 
//! The crates are compatible with stable Rust releases.
//! 
//! # Example
//! 
//! ```rust,ignore
//! #[macro_use]
//! extern crate enum_kinds_macros;
//! extern crate enum_kinds_traits;
//! 
//! use enum_kinds_traits::ToKind;
//! 
//! #[derive(EnumKind)]
//! #[enum_kind_name(SomeEnumKind)]
//! enum SomeEnum {
//!     First(String, u32),
//!     Second(char),
//!     Third
//! }
//! 
//! #[test]
//! fn test_enum_kind() {
//!     let first = SomeEnum::First("Example".to_owned(), 32);
//!     assert_eq!(first.kind(), SomeEnumKind::First);
//! }
//! ```
//! 
//! The `#[derive(EnumKind)]` attribute automatically creates another `enum` named
//! `SomeEnumKind` that contains matching unit variant for each of the variants in
//! `SomeEnum`. Additionally, `SomeEnum` implements `ToKind` trait that provides the
//! `kind` method for constructing matching values from `SomeEnumKind`.
//!

#![no_std]

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

use proc_macro::TokenStream;
use quote::Tokens;
use syn::{DeriveInput, Meta, NestedMeta, Ident, Data, MetaList, DataEnum, Fields};
use syn::punctuated::Pair;

#[proc_macro_derive(EnumKind, attributes(enum_kind_name))]
pub fn enum_kind(input: TokenStream) -> TokenStream {
    let ast = syn::parse(input).expect("#[derive(EnumKind)] failed to parse input");
    let name = get_enum_name(&ast)
        .expect("#[derive(EnumKind)] requires an associated #[enum_kind_name(NAME)] to be specified");
    let enum_ = create_kind_enum(&ast, &name);
    let impl_ = create_impl(&ast, &name);
    let code = quote! {
        #enum_
        #impl_
    };
    code.into()
}

fn get_enum_name(definition: &DeriveInput) -> Option<Ident> {
    for attr in definition.attrs.iter() {
        match attr.interpret_meta() {
            Some(Meta::List(MetaList { ident, ref nested, .. })) if ident == "enum_kind_name" => {
                if let Some(Pair::End(&NestedMeta::Meta(Meta::Word(ident)))) = nested.pairs().next() {
                    return Some(ident.clone());
                } else {
                    panic!("#[enum_kind_name(NAME)] requires an identifier NAME to be specified");
                }
            },
            _ => continue
        }
    }
    return None;
}

fn create_kind_enum(definition: &DeriveInput, kind_ident: &Ident) -> Tokens {
    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;
    quote! {
        #[derive(Debug, Clone, Copy, PartialEq, Eq)]
        #[allow(dead_code)]
        #[allow(non_snake_case)]
        #visibility enum #kind_ident {
            #(#variant_idents),*
        }
    }
}

fn create_impl(definition: &DeriveInput, kind_ident: &Ident) -> Tokens {
    let (impl_generics, 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 defined for enums");
        }
    };

    quote! {
        #[automatically_derived]
        #[allow(unused_attributes)]
        impl #impl_generics ::enum_kinds_traits::ToKind
            for #ident #ty_generics #where_clause {
            type Kind = #kind_ident;

            #[inline]
            fn kind(&self) -> Self::Kind {
                match self {
                    #(#arms)*
                }
            }
        }
    }
}