Macro compile_error 

macro_rules! compile_error {
    ($msg:expr $(,)?) => { ... };
}

Causes compilation to fail with the given error message when encountered.

This macro should be used when a crate uses a conditional compilation strategy to provide better error messages for erroneous conditions. It’s the compiler-level form of panic!, but emits an error during compilation rather than at runtime.

Examples

Two such examples are macros and #[cfg] environments.

Emit a better compiler error if a macro is passed invalid values. Without the final branch, the compiler would still emit an error, but the error’s message would not mention the two valid values.

macro_rules! give_me_foo_or_bar {
    (foo) => {};
    (bar) => {};
    ($x:ident) => {
        compile_error!("This macro only accepts `foo` or `bar`");
    }
}

give_me_foo_or_bar!(neither);
// ^ will fail at compile time with message "This macro only accepts `foo` or `bar`"

Emit a compiler error if one of a number of features isn’t available.

#[cfg(not(any(feature = "foo", feature = "bar")))]
compile_error!("Either feature \"foo\" or \"bar\" must be enabled for this crate.");