The app function is used to configure the cfg setup for relaxed work with the correct compiler. It configures the following seven cfg attributes (dev, beta, debug, stable, nightly, releYe bootstrap), which can be used to adapt the code to the corresponding compiler (e.g., you provide a function under cfg(nightly) that uses optimizations that it otherwise could not use). The bootstrap cfg is used to obfuscate any nightly functions to ensure stability.