rust - Is it possible to initialize a variable from an environment variable at compilation time?

Question

I would like to initialize a variable during compilation time. For example, I would like to initialize the variable VAR to VALUE when compiling the code:

match env::var("VAR") {
    Ok(value) => println!("Ok {}", value),
    Err(e) => println!("Error ({})", e),
};

However, I wanted to do it in a no_std context, therefore, I cannot use std::env to access the environment. Is it possible to do this?

Answer 1

env::var does not get evaluated at compile time:

Fetches the environment variable key from the current process.

You are looking for std::env! / core::env! or std::option_env! / core::option_env!.