Macro env

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

🔬This is a nightly-only experimental API. (prelude_2024)

Inspects an environment variable at compile time.

This macro will expand to the value of the named environment variable at compile time, yielding an expression of type &'static str. Use std::env::var instead if you want to read the value at runtime.

If the environment variable is not defined, then a compilation error will be emitted. To not emit a compile error, use the option_env! macro instead. A compilation error will also be emitted if the environment variable is not a valid Unicode string.

Examples

let path: &'static str = env!("PATH");
println!("the $PATH variable at the time of compiling was: {path}");

You can customize the error message by passing a string as the second parameter:

let doc: &'static str = env!("documentation", "what's that?!");

If the documentation environment variable is not defined, you’ll get the following error:

error: what's that?!