1/// Asserts that a given configuration is set.
2///
3/// # Examples
4///
5/// A project will simply fail to compile if the given configuration is not set.
6///
7/// ```
8/// # #[macro_use] extern crate static_assertions; fn main() {}
9/// // We're not masochists
10/// # #[cfg(not(target_pointer_width = "16"))] // Just in case
11/// assert_cfg!(not(target_pointer_width = "16"));
12/// ```
13///
14/// If a project does not support a set of configurations, you may want to
15/// report why. There is the option of providing a compile error message string:
16///
17/// ```
18/// # #[macro_use] extern crate static_assertions; fn main() {}
19/// # #[cfg(any(unix, windows))]
20/// assert_cfg!(any(unix, windows), "There is only support for Unix or Windows");
21///
22/// // User needs to specify a database back-end
23/// # #[cfg(target_pointer_width = "0")] // Impossible
24/// assert_cfg!(all(not(all(feature = "mysql", feature = "mongodb")),
25///                 any(    feature = "mysql", feature = "mongodb")),
26///             "Must exclusively use MySQL or MongoDB as database back-end");
27/// ```
28///
29/// Some configurations are impossible. For example, we can't be compiling for
30/// both macOS _and_ Windows simultaneously:
31///
32/// ```compile_fail
33/// # #[macro_use] extern crate static_assertions; fn main() {}
34/// assert_cfg!(all(target_os = "macos",
35///                 target_os = "windows"),
36///             "No, that's not how it works! ಠ_ಠ");
37/// ```
38#[macro_export]
39macro_rules! assert_cfg {
40    () => {};
41    ($($cfg:meta)+, $msg:expr $(,)?) => {
42        #[cfg(not($($cfg)+))]
43        compile_error!($msg);
44    };
45    ($($cfg:tt)*) => {
46        #[cfg(not($($cfg)*))]
47        compile_error!(concat!("Cfg does not pass: ", stringify!($($cfg)*)));
48    };
49}
50