What does 'Don't know how to build task 'bootstrap' mean?

I was trying to build Mirah, following the instructions here. On the step 'rake gem', I got several warnings and a failure notice. A sample of the output follows:

warning: org/dynalang/dynalink/MonomorphicCallSite.class(org/dynalang/dynalink:MonomorphicCallSite.class): major version 51 is newer than 50, the highest major version supported by this compiler. It is recommended that the compiler be upgraded. Note: /Users/paul/dev/interesting/mirah/src/org/mirah/DynalangBootstrap.java uses or overrides a deprecated API. Note: Recompile with -Xlint:deprecation for details. 4 warnings Building 开发者_高级运维jar: /Users/paul/dev/interesting/mirah/javalib/mirah-bootstrap.jar rake aborted! Don't know how to build task 'bootstrap'

How should I begin troubleshooting this? For example, which compiler should be upgraded, and how can that be done?

This issue is fixed on master by https://github.com/mirah/mirah/commit/fac6bd5a13be6d818b9ac362650f5f540d771682, so you should be able to pull master and build just fine now.

If you're using an old tree intentionally, there another option to build the gem:

rake jar:bootstrap
gem build mirah.gemspec

Your error message is happening because prior to Charles' fix, the :gem task erroneously refers to "jar:bootstrap" as just "bootstrap".