The error produced when you don't have ant installed is also rather cryptic. Here it is with full trace output: https://gist.github.com/headius/6c6d83229d5a979ae8cf

The error you get right now looks like this:

$ rake spec:ruby
could not load ant: #<Errno::ENOENT: No such file or directory - ant>
MSPEC: {:compile_mode=>"OFF", :format=>"d", :jit_threshold=>20, :jit_max=>-1, :objectspace_enabled=>true, :thread_pooling=>false, :reflection=>false}
rm -rf rubyspec_temp
rake aborted!
NameError: undefined local variable or method `ant' for main:Object
/Volumes/git/3rd-party/jruby/rakelib/commands.rake:27:in `initialize_paths'
/Volumes/git/3rd-party/jruby/rakelib/commands.rake:45:in `jruby'
/Volumes/git/3rd-party/jruby/rakelib/commands.rake:90:in `mspec'
/Volumes/git/3rd-party/jruby/rakelib/rubyspec.rake:63:in `block in (root)'
/Users/plundberg/.rvm/gems/jruby-9.1.16.0/gems/rake-12.3.1/exe/rake:27:in `<main>'
Tasks: TOP => spec:ruby => spec:ci_interpreted => spec:interpreted
(See full trace by running task with --trace)

(Also, worth mentioning is that if you are using macOS, a simple way to get a working ant command is brew install ant, if you have a working Homebrew install.)

Yeah the main thing keeping Ant in the build is that we're using its Java-launching task throughout our test runs. We need a good replacement for that, one that allows us to customize JRuby and Java opts from callers.

See rakelib/commands.rake for the primary calls to Ant we need to replace. Basically the java task in our Ant integration has a set of properties we can set, like classpath etc. Perhaps we fake them by making a look-alike Struct and then peeling out the piece and building our own command line?