variante 0.2.0

Statically-verified enum variant names as strings.
Documentation 
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

// Copyright 2023 yamato6537.
//
// Licensed under either of MIT (https://opensource.org/license/mit/) or Apache 2.0 (https://opensource.org/license/apache-2-0/), at your option.
// This file may not be copied, modified, or distributed except according to those terms.

//! Statically-verified `enum` variant names as strings.
//!
//! The [`variant!`] macro evaluates to a variant's name after verifying that the variant exists on the specified `enum`.
//! The type must be in scope, and the variant must be visible (accessible) at the point of invocation of the macro.
//!
//! # Usage
//!
//! Invoke the macro as `variant!(Variant @ Enum)`:
//! ```
//! use variante::*;
//!
//! enum Enum {
//!     Foo,
//!     Bar(u8),
//!     Baz { x: i32 },
//! }
//!
//! let foo = variant!(Foo @ Enum);
//! assert_eq!(foo, "Foo");
//!
//! let bar = variant!(Bar @ Enum);
//! assert_eq!(bar, "Bar");
//! 
//! let baz = variant!(Baz @ Enum);
//! assert_eq!(baz, "Baz")
//! ```
//!
//! A variant that is not on the specified `enum` or a type that is not in scope will cause a compilation error:
//! ```compile_fail
//! // This fails because there is no variant named "Hoge" on "Enum"
//! let hoge = variant!(Hoge @ Enum);
//!
//! // This fails because their is no enum named "NonExistent"
//! let foo = variant!(Foo @ NonExistent);
//! ```
//!
//! # Generics
//!
//! [`variant!`] also works with generic types, as long as concrete type parameters are provided:
//! ```
//! use variante::*;
//!
//! enum GenericEnum<T, U> {
//!     Foo(T),
//!     Bar(U),
//! }
//!
//! let foo = variant!(Foo @ GenericEnum<(), ()>);
//! assert_eq!(foo, "Foo");
//!
//! // Any type can be used for the type parameter(s)
//! let bar = variant!(Bar @ GenericEnum<i32, i64>);
//! assert_eq!(bar, "Bar");
//! ```
//!
//! # Paths
//!
//! That's right, [`variant!`] also works with path syntax:
//! ```
//! use variante::*;
//!
//! mod fuga {
//!     pub enum Enum<T> {
//!         Foo(T), // Must be pub so that it is visible at the point of invocation
//!     }
//! }
//!
//! let foo = variant!(Foo @ fuga::Enum<()>);
//! assert_eq!(foo, "Foo");
//! ```
//!
//! # Dependencies
//!
//! This crate is completely dependency-free.
//! `#[no_std]` is also supported by default.

#![warn(missing_docs)]
#![no_std]

/// Checks for the presence of a variant on a `enum` at compile-time and returns the field's name as a `&'static str`.
///
/// Invoked as `variant!(Variant @ Enum)`, this macro verifies that a variant named `Variant` exists on the type `Enum`, and is visible at the point of invocation.
/// It then stringifies the variant name, evaluating to `"Variant"`.
///
/// Refer to the [crate-level documentation](https://docs.rs/variante) for more information.
#[macro_export]
macro_rules! variant {
    ( $variant:ident @ $($enumeration:ident)::+ $(<$($generics:ty),+>)? ) => {{
        #[allow(unused)]
        fn assert_variant_exists( on: $($enumeration)::+ $(<$($generics),+>)? ) {
            matches!(on, $($enumeration)::+ :: $variant {..});
        }
        ::core::stringify!($variant)
    }};
}

#[cfg(test)]
mod tests {
    #[test]
    fn normal() {
        #[allow(unused)]
        enum Enum {
            Foo,
            Bar(u8),
            Baz { x: i32 }
        }

        let foo = variant!(Foo @ Enum);
        assert_eq!(foo, "Foo");

        let bar = variant!(Bar @ Enum);
        assert_eq!(bar, "Bar");

        let baz = variant!(Baz @ Enum);
        assert_eq!(baz, "Baz");
    }

    #[test]
    fn generic() {
        #[allow(unused)]
        enum GenericEnum<T, U> {
            Foo(T),
            Bar(U),
        }

        let foo = variant!(Foo @ GenericEnum<(), ()>);
        assert_eq!(foo, "Foo");

        let bar = variant!(Bar @ GenericEnum<(), ()>);
        assert_eq!(bar, "Bar");
    }

    // #[test]
    // fn fails() {
    //     enum Enum {
    //         Foo,
    //     }
    //     let _ = variant!(Bar @ Enum);
    // }
}