When a gem fails, where do I find the mkmf.log file?

find $GEM_HOME -name mkmf.log

or

$GEM_HOME/extensions/$arch/$ruby_version-static/$gem_name-$gem_version/mkmf.log

chruby, ruby-2.6.3:

GEM_HOME: /home/yuri/.gem/ruby/2.6.3
arch: x86_64-linux
ruby_version: 2.6.0
gem_name: openssl
gem_version: 2.0.0

/home/yuri/.gem/ruby/2.6.3/extensions/x86_64-linux/2.6.0-static/openssl-2.0.0/mkmf.log

rvm, ruby-2.6.3:

GEM_HOME: /home/yuri/.rvm/gems/ruby-2.6.3
arch: x86_64-linux
ruby_version: 2.6.0
gem_name: nokogiri
gem_version: 1.10.3

/home/yuri/.gem/ruby/2.6.3/extensions/x86_64-linux/2.6.0-static/nokogiri-1.10.3/mkmf.log

The mkmf.log file can be found under the gem's installation directory.

rvm

If you are using RVM, then look for a directory similar to

~/.rvm/gems/ruby-1.9.3-p194/gems/some-cool-gem-name/ext/mkmf.log

You can list all the mkmf.log files available with this command:

find ~/.rvm -name mkmf.log

rbenv

find ~/.rbenv -name mkmf.log

Windows solution

As @givanse said, the mkmf logs are in the gem installation directory. Because that can be complicated, you should just search for the mkmf file. You can do that in Windows explorer or in PowerShell

cd C:\Dev\Ruby22 # Change to reflect your Ruby install dir
Get-ChildItem mkmf* -Recurse