stop circular require warnings - guard require "rubygems"

[MSP-Greg]

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

Testing rubygems/rubygems with warnings on, specifically, test_gem.rb. Several 'circular require' warnings.

What was your diagnosis of the problem?

RubyGems gets required, then requires Bundler. Several places in Bundler then require RubyGems.

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

Guard the RubyGems requires. Note that rubygems/specification is one of the few files loaded by RubyGems, most are autoload. I kept adding guards until all warnings were removed.

Why did you choose this fix out of the possible options?

Unaware of other possible options

[MSP-Greg]

To repo, from RubyGems repo:

rake test RUBYOPT=-v TEST=test/rubygems/test_gem.rb

[MSP-Greg]

ruby/ruby just switched on warnings in their tests. See for similar circular require warnings, starting around line 2912:

https://travis-ci.org/ruby/ruby/jobs/474816395

[segiddins]

Is this check safe? Is it possible some other file has defined the Gem constant, without all of the classes and methods in there being defined?

[MSP-Greg]

@segiddins

That's a very deep rabbit hole.

What if someone modified $LOAD_PATH so the require statement loaded a different rubygems.rb file?

I haven't checked, but autoload might have similar issues with a improperly defined constant...

I could update the PR where, instead of using the guard, one could do something similar to:

verbose, $VERBOSE = $VERBOSE, false
require "rubygems"
$VERBOSE = verbose