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
146
147
148
149
150
151
152
153
154

//! # arg_enum_proc_macro
//!
//! This crate consists in a procedural macro derive that provides the
//! same implementations that clap the [`clap::arg_enum`][1] macro provides:
//! [`std::fmt::Display`], [`std::str::FromStr`] and a `variants()` function.
//!
//! By using a procedural macro it allows documenting the enum fields
//! correctly and avoids the requirement of expanding the macro to use
//! the structure with [cbindgen](https://crates.io/crates/cbindgen).
//!
//! [1]: https://docs.rs/clap/2.32.0/clap/macro.arg_enum.html
//!

#![recursion_limit = "128"]

extern crate proc_macro;

use proc_macro2::{Literal, Punct, Span, TokenStream, TokenTree};
use quote::{quote, quote_spanned};
use syn::{parse_macro_input, Data, DeriveInput};
use std::iter::FromIterator;

/// Implement [`std::fmt::Display`], [`std::str::FromStr`] and `variants()`.
///
/// The invocation:
/// ``` no_run
/// use arg_enum_proc_macro::ArgEnum;
///
/// #[derive(ArgEnum)]
/// enum Foo {
///     A,
///     B
/// }
/// ```
///
/// produces:
/// ``` no_run
/// enum Foo {
///     A,
///     B
/// }
/// impl ::std::str::FromStr for Foo {
///     type Err = String;
///
///     fn from_str(s: &str) -> ::std::result::Result<Self,Self::Err> {
///         match s {
///             "A" | _ if s.eq_ignore_ascii_case("A") => Ok(Foo::A),
///             "B" | _ if s.eq_ignore_ascii_case("B") => Ok(Foo::B),
///             _ => Err({
///                 let v = vec![ "A", "B" ];
///                 format!("valid values: {}", v.join(" ,"))
///             }),
///         }
///     }
/// }
/// impl ::std::fmt::Display for Foo {
///     fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
///         match *self {
///             Foo::A => write!(f, "A"),
///             Foo::B => write!(f, "B"),
///         }
///     }
/// }
///
/// impl Foo {
///     #[allow(dead_code)]
///     pub fn variants() -> [&'static str; 2] {
///         [ "A", "B" ]
///     }
/// }
/// ```
#[proc_macro_derive(ArgEnum)]
pub fn arg_enum(items: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let input = parse_macro_input!(items as DeriveInput);

    let name = input.ident;
    let variants = if let Data::Enum(data) = input.data {
        data.variants
    } else {
        panic!("Only enum supported");
    };

    let len = variants.len();

    let from_str_match = variants.iter().flat_map(|item| {
        let id = &item.ident;
        let lit: TokenTree = Literal::string(&id.to_string()).into();

        let pat: TokenStream = quote! {
            #lit | _ if s.eq_ignore_ascii_case(#lit) => Ok(#name::#id),
        }
        .into();

        pat.into_iter()
    });

    let from_str_match = TokenStream::from_iter(from_str_match);

    let display_match = variants.iter().flat_map(|item| {
        let id = &item.ident;
        let lit: TokenTree = Literal::string(&id.to_string()).into();

        let pat: TokenStream = quote! {
            #name::#id => write!(f, #lit),
        }
        .into();

        pat.into_iter()
    });

    let display_match = TokenStream::from_iter(display_match);

    let array_items = variants.iter().flat_map(|item| {
        let tok: TokenTree = Literal::string(&item.ident.to_string()).into();
        let comma: TokenTree = Punct::new(',', proc_macro2::Spacing::Alone).into();

        vec![tok, comma].into_iter()
    });

    let array_items = TokenStream::from_iter(array_items);

    let ret: TokenStream = quote_spanned! {
        Span::call_site() =>
        impl ::std::str::FromStr for #name {
            type Err = String;

            fn from_str(s: &str) -> ::std::result::Result<Self,Self::Err> {
                match s {
                    #from_str_match
                    _ => {
                        let values = vec![ #array_items ];

                        Err(format!("valid values: {}", values.join(" ,")))
                    }
                }
            }
        }
        impl ::std::fmt::Display for #name {
            fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
                match *self {
                    #display_match
                }
            }
        }
        impl #name {
            pub fn variants() -> [&'static str; #len] {
                [ #array_items ]
            }
        }
    }
    .into();

    ret.into()
}