Nothing to Disable

Ruby's mode of operation can be altered with some --enable-* / --disable-* command-line switches.

By default, all of the following features are activated, except for the frozen strings and the JIT:

Feature CLI Option to Change Description
RubyGems --disable-gems RubyGems is the package manager of Ruby, which is required to load 3rd party Ruby libraries¹.
RUBYOPT --disable-rubyopt The RUBYOPT ENV variable lets you define default CLI options for Ruby. This switch tells Ruby to ignore RUBYOPT.
DidYouMean --disable-did-you-mean² Improves error messages by suggesting auto-corrected method or module names
Frozen strings --enable-frozen-string-literal³ Freezes all string literals everywhere.
Just-in-time compiler --enable-jit Shortcut to enable is just --jit. New compiler architecture, introduced with Ruby 2.6. See @k0kubun's blog for more details.
All of the above --disable-all / --enable-all -

¹ --disable-gems will also prevent did_you_mean from loading, because it is a standard gem
² Variants: --disable-did_you_mean, --disable-did-you_, --disable-did, --disable-d
³ Variants: --enable-frozen_string_literal, --enable-frozen_str, --enable-frozen, …

Also See

More Idiosyncratic Ruby