Build fails if using Xcode 11 toolchain

[CatalinMustata]

Noticed this on my machine and also on CircleCI that build fails when using Xcode 11 command line tools. I've switched the tools to Xcode 10.3 and it works fine on my machine, but it gets a bit more complicated when attempting to build on CircleCI.

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

current directory:
/Users/distiller/project/vendor/bundle/ruby/2.3.0/gems/unf_ext-0.0.7.6/ext/unf_ext
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -r
./siteconf20190912-536-1ctxvha.rb extconf.rb
mkmf.rb can't find header files for ruby at
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/include/ruby.h

[ottoo]

I'm having the same issue as well. Cannot use CircleCI to build, also fails locally when running bundle install.

If anybody finds here, this will help https://discuss.circleci.com/t/xcode-11-gm-seed-2-released/32505/22

[CatalinMustata]

It seems the new toolchain changes the location of the Ruby headers. Have a look at the CircleCI forums for more info and workarounds:
https://discuss.circleci.com/t/xcode-11-gm-seed-2-released/32505/23

[bernikovich]

Installing unf_ext 0.0.7.6 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

:'(

[bernikovich]

Have you tried the following?
open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg