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| </pre><pre class="rust"><code><span class="doccomment">/// 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 |
| /// |
| /// A common use case is to guarantee properties about a constant value that's |
| /// generated via meta-programming. |
| /// |
| /// ``` |
| /// # #[macro_use] extern crate static_assertions; fn main() {} |
| /// const VALUE: i32 = // ... |
| /// # 3; |
| /// |
| /// const_assert!(VALUE >= 2); |
| /// ``` |
| /// |
| /// Inputs are type-checked as booleans: |
| /// |
| /// ```compile_fail |
| /// # #[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}` |
| /// ``` |
| /// |
| /// The following fails to compile because multiplying by 5 does not have an |
| /// identity property: |
| /// |
| /// ```compile_fail |
| /// # #[macro_use] extern crate static_assertions; fn main() {} |
| /// const_assert!(5 * 5 == 5); |
| /// ``` |
| /// |
| /// [static_assert]: http://en.cppreference.com/w/cpp/language/static_assert |
| </span><span class="attribute">#[macro_export] |
| </span><span class="macro">macro_rules! </span>const_assert { |
| (<span class="macro-nonterminal">$x</span>:expr $(,)<span class="question-mark">?</span>) => { |
| <span class="attribute">#[allow(unknown_lints, eq_op)] |
| </span><span class="kw">const _</span>: [(); <span class="number">0 </span>- !{ <span class="kw">const </span>ASSERT: bool = <span class="macro-nonterminal">$x</span>; ASSERT } <span class="kw">as </span>usize] = []; |
| }; |
| } |
| |
| <span class="doccomment">/// Asserts that constants are equal in value. |
| /// |
| /// # Examples |
| /// |
| /// This works as a shorthand for `const_assert!(a == b)`: |
| /// |
| /// ``` |
| /// # #[macro_use] extern crate static_assertions; fn main() {} |
| /// const TWO: usize = 2; |
| /// |
| /// const_assert_eq!(TWO * TWO, TWO + TWO); |
| /// ``` |
| /// |
| /// 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); |
| /// ``` |
| </span><span class="attribute">#[macro_export(local_inner_macros)] |
| </span><span class="macro">macro_rules! </span>const_assert_eq { |
| (<span class="macro-nonterminal">$x</span>:expr, <span class="macro-nonterminal">$y</span>:expr $(,)<span class="question-mark">?</span>) => { |
| <span class="macro">const_assert!</span>(<span class="macro-nonterminal">$x </span>== <span class="macro-nonterminal">$y</span>); |
| }; |
| } |
| |
| <span class="doccomment">/// Asserts that constants are **not** equal in value. |
| /// |
| /// # Examples |
| /// |
| /// This works as a shorthand for `const_assert!(a != b)`: |
| /// |
| /// ``` |
| /// # #[macro_use] extern crate static_assertions; fn main() {} |
| /// const NUM: usize = 32; |
| /// |
| /// const_assert_ne!(NUM * NUM, 64); |
| /// ``` |
| /// |
| /// The following example fails to compile because 2 is magic and 2 × 2 = 2 + 2: |
| /// |
| /// ```compile_fail |
| /// # #[macro_use] extern crate static_assertions; fn main() {} |
| /// const_assert_ne!(2 + 2, 2 * 2); |
| /// ``` |
| </span><span class="attribute">#[macro_export(local_inner_macros)] |
| </span><span class="macro">macro_rules! </span>const_assert_ne { |
| (<span class="macro-nonterminal">$x</span>:expr, <span class="macro-nonterminal">$y</span>:expr $(,)<span class="question-mark">?</span>) => { |
| <span class="macro">const_assert!</span>(<span class="macro-nonterminal">$x </span>!= <span class="macro-nonterminal">$y</span>); |
| }; |
| } |
| </code></pre></div> |
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