Macro warning

macro_rules! warning {
    ($($tokens:tt)*) => { ... };
}

The warninginstruction tells Cargo to display a warning after the build script has finished running. Warnings are only shown for path dependencies (that is, those you’re working on locally), so for example warnings printed out in crates.io crates are not emitted by default. The-vv` “very verbose” flag may be used to have Cargo display warnings for all crates.

For more information, see this cargo reference.