default-env

default_env! is a macro like env! that returns a default value if the environment variable is not found. Unlike option_env!, the output of default_env! can be used in macros (because who doesn't love macros in their macros?).

Example

macro_rules! long_str {
  () => {
    concat!(
        "Hello, ", default_env!("USER", "anonymous user"), ".",
        "Today is ", default_env!("WEEKDAY", compile_error!("You exist in a land beyond time."))
      )
  }
}