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.
The workaround as pointed out here is to tell Clang not to treat this behavior as an error.
Try running:
bundle config build.puma --with-cflags="-Wno-error=implicit-function-declaration"
bundle install
and it should work.
