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 providebetter error messages for erroneous conditions. It’s the compiler-level form ofpanic!,but emits an error duringcompilation rather than atruntime.

§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 twovalid 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.");