I came across 'bundler/inline' in Bundler pull request 3440, which I'd found from Rails pull request 20429. This now has a little snippet on the What's new page of the Bundler site.
This little example demonstrates how I might use it to create a single file using minitest.
I've just noticed that this script was working for me because I already had minitest installed as a system gem. If I uninstall the gem then the script fails because it can't find minitest. The source code documents an option to install gems if they don't exist (gemfile(true) do) but that doesn't seem to work for me. I see the following error:
Fetching gem metadata from https://rubygems.org/...
Fetching version metadata from https://rubygems.org/..
Resolving dependencies...
Using metaclass 0.0.4