Macro option_env 

macro_rules! option_env {
    ($name:expr $(,)?) => { ... };
}

Optionally inspects an environment variable at compile time.

If the named environment variable is present at compile time, this will expand into an expression of type Option<&'static str> whose value is Some of the value of the environment variable (a compilation error will be emitted if the environment variable is not a valid Unicode string). If the environment variable is not present, then this will expand to None. See Option<T> for more information on this type. Use std::env::var instead if you want to read the value at runtime.

A compile time error is only emitted when using this macro if the environment variable exists and is not a valid Unicode string. To also emit a compile error if the environment variable is not present, use the env! macro instead.

Examples

let key: Option<&'static str> = option_env!("SECRET_KEY");
println!("the secret key might be: {key:?}");