Clarifying when and when not to declare gems in an engine's Gemfile

I am building an engine, and have wanted to use vcr in my tests. The current wording of the comments in the engine Gemfile template suggests that gems should only be declared there on a temporary basis, that is until they can be moved into the gemspec. However, adding a development dependency for vcr in the gemspec leads to vcr being added to the development environment. I then have to specifically disable vcr in my development environment.

After some experimentation, I have found that a better solution is to declare vcr within a test group in the engine’s Gemfile. I have attempted to find a definitive reason why declaring a gem used for testing, in the Gemfile would be a bad thing to do, but all I seem to find are generalised comments that gem declaration should be in the gemspec.

Therefore, I think the wording of the engine Gemfile template should be update so that it is less ambiguous about when you should or should not declare gems in an engine’s Gemfile. I have submitted a pull request comprising a suggested rewording of these comments:

Any thoughts?

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