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

rust 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.")) ) } }