how to fix the bundle install, quote:

It seems that the latest version of XCode tools (12 Beta 3) installs a version of Clang (the C compiler used by default on MacOS) that throws an error on implicit functions used on the native extension code of Puma.

Running the following:

bundle config build.puma --with-cflags="-Wno-error=implicit-function-declaration"
bundle install

Fixed the issue

Very well written article explaining everything that comes into a gem specification file. Doesn't take you for a fool, explain things one by one so you can get a better understanding of what you are doing.

Also, use a generator once you understand it!