If you use RVM, I'm sure you're used to installing your Rubies with it. It's a great way to make sure your build dependencies are up-to-date, to build a Ruby that doesn't overlap with your other stuff, to manage paths and gems...
(And yes, it has some down sides. By and large I still like it quite a bit. But if you use rbenv instead, I have no stones to throw. Rbenv and ruby-build are a great combination too, and will get you where you need to be.)
It's less obvious that you can use RVM to build and install Ruby from a local clone of the Ruby GitHub repo or another similar one -- for instance, if you're messing with Ruby patches yourself, or you're cloning from somebody who is.
Name and Location
Two little-known RVM features help us out here: Named Rubies, and the (undocumented, I think) --url option to "rvm install."
Which means you can do something like this:
rvm install ruby-head-nobu --url https://github.com/nobu/ruby.git --branch round-to-even
Now you have a normal-looking RVM-installed Ruby, in this case called "ruby-head-nobu", from pure-custom source. You can even build it from a local repo (see below.) Do you see errors instead? Try "rvm get head" to upgrade to the latest RVM - they patched a problem with this on November 9th, 2016.
A quick disclaimer: don't use a name like "ruby-head-round-even". Turns out that you can't have any more dashes in the name. It just cuts them off and you get a name like "ruby-head-even". Which is pretty confusing at the time. Don't use dashes in the last part of the name.
Another disclaimer: RVM has only recently updated to support Ruby including Bundler. So you may need to update RVM to latest stable before "rvm install ruby-head" works at all, custom URL or no.
Do It Locally
If you just pass the path to a local repo above as a URL, it doesn't work. It tries to clone your local directory with SVN, which doesn't work. Same problem with a file URL. How do you get it to use Git, which is smart about these things?
I had to read through the RVM source to find this out, but if your URL contains the string "git" anywhere, RVM assumes it should use Git, not SVN, to clone it.
So this worked for me:
rvm install ruby-head-nobu --url file:///Users/noah.gibbs/src/ruby/nobu_ruby/.git --branch round-to-even
It's a bit of a hack -- it only works because the URL happens to contain "git" and that's what RVM checks for as of this writing. And because you're allowed to clone from the .git directory. But it works for me! This saves a very time-consuming "git clone" step over the network. The Ruby repo is pretty big at this point, and Git is smart about how it clones a repo from the local file system.
Made of Win
This is all useful if you're building Ruby locally -- it can be annoying to set up individual install directories and then mess with your PATH and RUBYLIB. And an installed Ruby can be necessary to do certain things like running RubySpec. Look for another post about that soon!