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Cache `Gem.default_dir` RubyGems do internally the same \[[1]\]. This should help with: 1) Performance (not tested) 2) Running RubyGems test suite (main motivation), which actually overrides the `@default_dir` \[[2]\] to be able to place test content into dedicated directory. [1]: https://github.com/rubygems/rubygems/blob/f8e87eba76526d2609d543eb7774711d345edc61/lib/rubygems/defaults.rb#L38 [2]: https://github.com/rubygems/rubygems/blob/f8e87eba76526d2609d543eb7774711d345edc61/test/rubygems/helper.rb#L372