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/// 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
/// # #[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:
///
/// # #[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:
///
/// #![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
/// Asserts that constants are equal in value.
///
/// # Examples
///
/// Works as a shorthand for `const_assert!(a == b)`:
///
/// # #[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);
/// # }
/// ```
/// Asserts that constants are _not_ equal in value.
///
/// # Examples
///
/// Works as a shorthand for `const_assert!(a != b)`:
///
/// # #[macro_use]
/// # extern crate static_assertions;
/// const_assert_ne!(nums; 1, 2, 3, 4);
///
/// fn main() {
/// const NUM: usize = 32;
/// const_assert_ne!(NUM * NUM, 64);
/// }
/// ```
///
/// The magic number 2, where 2 × 2 = 2 + 2:
///
/// ```compile_fail
/// # #[macro_use] extern crate static_assertions;
/// # fn main() {
/// const_assert_ne!(2 + 2, 2 * 2);
/// # }
/// ```