Substitues environment variables into a string literal at compile time.

Example:

rust use format_env::format_env; assert_eq!(format_env!("Name: $(CARGO_PKG_NAME), License: $(CARGO_PKG_LICENSE)"), "Name: format_env, License: MIT") assert_eq!(format_env!("Name: $$(CARGO_PKG_NAME)"), "Name: $(CARGO_PKG_NAME)")