static_assertions 0.3.2

Compile-time assertions to ensure that invariants are met.
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

/// Asserts that constant expressions evaluate to `true`.
///
/// Constant expressions can be ensured to have certain properties via this
/// macro If the expression evaluates to `false`, the file will fail to compile.
/// This is synonymous to [`static_assert` in C++][static_assert].
///
/// # Alternatives
///
/// There also exists [`const_assert_eq`](macro.const_assert_eq.html) for
/// validating whether a sequence of expressions are equal to one another.
///
/// # Examples
///
/// Some expressions are blatantly false:
///
/// ```compile_fail
/// # #[macro_use] extern crate static_assertions;
/// # fn main() {
/// const_assert!(1 >= 2);
/// # }
/// ```
///
/// Inputs are type-checked as booleans:
///
/// ```compile_fail
#[cfg_attr(feature = "nightly", doc = "#![feature(underscore_const_names)]")]
/// # #[macro_use] extern crate static_assertions;
/// # fn main() {
/// const_assert!(!0);
/// # }
/// ```
///
/// Despite this being a macro, we see this produces a type error:
///
/// ```txt
///   | const_assert!(!0);
///   |               ^^ expected bool, found integral variable
///   |
///   = note: expected type `bool`
///              found type `{integer}`
/// ```
///
/// On stable Rust, using the macro requires a unique “label” when used in a
/// module scope:
///
#[cfg_attr(feature = "nightly", doc = "```ignore")]
#[cfg_attr(not(feature = "nightly"), doc = "```")]
/// # #[macro_use]
/// # extern crate static_assertions;
/// # fn main() {}
/// const_assert!(meaning_of_life; 42 == !!42);
/// ```
///
/// The [labeling limitation](index.html#limitations) is not necessary if
/// compiling on nightly Rust with the `nightly` feature enabled:
///
#[cfg_attr(feature = "nightly", doc = "```")]
#[cfg_attr(not(feature = "nightly"), doc = "```ignore")]
/// #![feature(underscore_const_names)]
/// # #[macro_use] extern crate static_assertions;
///
/// const FIVE: usize = 5;
///
/// const_assert!(FIVE * FIVE == 25);
///
/// fn main() {
///     const_assert!(2 + 2 == 4);
///     const_assert!(FIVE - FIVE == 0);
/// }
/// ```
///
/// [static_assert]: http://en.cppreference.com/w/cpp/language/static_assert
#[macro_export(local_inner_macros)]
macro_rules! const_assert {
    ($($xs:tt)+) => { _const_assert!($($xs)+); };
}

#[doc(hidden)]
#[cfg(feature = "nightly")]
#[macro_export(local_inner_macros)]
#[allow(dead_code)]
macro_rules! _const_assert {
    ($($xs:expr),+ $(,)*) => {
        #[allow(unknown_lints, eq_op)]
        const _: [(); 0 - !($({ const B: bool = $xs; B })&&+) as usize] = [];
    };
}

#[doc(hidden)]
#[cfg(not(feature = "nightly"))]
#[macro_export(local_inner_macros)]
macro_rules! _const_assert {
    ($($xs:expr),+ $(,)*) => {
        #[allow(unknown_lints, eq_op)]
        let _ = [(); 0 - !($({ const B: bool = $xs; B })&&+) as usize];
    };
    ($label:ident; $($xs:tt)+) => {
        #[allow(dead_code, non_snake_case)]
        fn $label() { const_assert!($($xs)+); }
    };
}

/// Asserts that constants are equal in value.
///
/// # Examples
///
/// Works as a shorthand for `const_assert!(a == b)`:
///
#[cfg_attr(feature = "nightly", doc = "```ignore")]
#[cfg_attr(not(feature = "nightly"), doc = "```")]
/// # #[macro_use]
/// # extern crate static_assertions;
/// const TWO: usize = 2;
/// const_assert_eq!(two; TWO * TWO, TWO + TWO, 4);
///
/// fn main() {
///     const NUM: usize = 32;
///     const_assert_eq!(NUM + NUM, 64);
/// }
/// ```
///
/// Just because 2 × 2 = 2 + 2 doesn't mean it holds true for other numbers:
///
/// ```compile_fail
/// # #[macro_use] extern crate static_assertions;
/// # fn main() {
/// const_assert_eq!(4 + 4, 4 * 4);
/// # }
/// ```
#[macro_export(local_inner_macros)]
macro_rules! const_assert_eq {
    ($x:expr, $($xs:expr),+ $(,)*) => {
        const_assert!($($x == $xs),+);
    };
    ($label:ident; $x:expr, $($xs:expr),+ $(,)*) => {
        const_assert!($label; $($x == $xs),+);
    };
}