Recently I decided to give an asdf a go as I wanted to switch from my old rvm / nvm / homebrew-for-other-langs setup (plus I wanted to give a dart a try). Unfortunatelly installing ruby 2.6.x for one of the projects under Catalina was a true PITA.

If you encounter some errors like:

file.c:23:10: fatal error: 'CoreFoundation/CFString.h' file not found
#include <CoreFoundation/CFString.h>

or maybe like those:

Last 10 log lines:
int     sigvec(int, struct sigvec *, struct sigvec *);

or even:

ld: symbol(s) not found for architecture x86_64
clang-5.0: error: linker command failed with exit code 1

You can try exporting some magical ENVs before executing asdf install ruby command:

# don't forget $ for bash shell; using fish here
export CPATH=(xcrun --show-sdk-path)/usr/include
export SDKROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
# aaand we're switching compiler here as well
export CC=gcc

Of course no guarantees here. Works on my machine tho 😅.