Prevent gem activation in standalone mode

[composerinteralia]

What was the end-user or developer problem that led to this PR?

As discussed in #6273 (comment)

The gem method behaves awkwardly in standalone mode. Assuming bundler isn't loaded at all, a call to gem might activate a gem that is not part of the bundle (because it's the gem method defined in lib/rubygems/core_ext/kernel_gem.rb and not
lib/bundler/rubygems_integration.rb). And when running with --disable-gems, the gem method won't be defined at all so we'll get a NoMethodError.

Calls to gem can appear in dependencies outside an application's control. To work around this at GitHub we defined our own Kernel#gem that no-ops.

I agree with #6273 (comment)

people using standalone mode don't want to activate gems like Kernel.gem

What is your fix for the problem, implemented in this PR?

This commit redefines Kernel#gem in the standalone script to no-op.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[hsbt]

And when running with --disable-gems, the gem method won't be defined at all so we'll get a NoMethodError.

Question: Why do you use --disable-gems in your application? It's only for Ruby core developper, not users.

[composerinteralia]

Question: Why do you use --disable-gems in your application? It's only for Ruby core developper, not users.

We mostly don't. It looks like GitHub started running some things with --disable-gems back in 2014 or so. I think the way GitHub did git operations back then involved spawning a new Ruby process each time, and so --disable-gems was used to make spawning those processes a tiny bit faster. (I'm not certain about that—I didn't work here at the time and I don't think that's how things work nowadays anyway.) There's a few old scripts that still use --disable-gems, but they might not need to.

Hopefully this PR makes sense anyway, since even without --disable-gems the gem method in standalone mode may activate a gem that's not part of the bundle, which seems generally undesirable.

[deivid-rodriguez]

This looks great, but I just noticed there's a merge conflict now. Could you rebase this @composerinteralia 🙏?

[composerinteralia]

Done!

[deivid-rodriguez]

Thank you @composerinteralia!