Install gems automatically on newer Ruby

Library/.rubocop.yml

@@ -296,6 +296,7 @@ Sorbet/StrictSigil:
 Sorbet/TrueSigil:
   Enabled: true
   Exclude:
+    - "Homebrew/standalone/init.rb" # loaded before sorbet-runtime
     - "Taps/**/*"
     - "/**/{Formula,Casks}/**/*.rb"
     - "**/{Formula,Casks}/**/*.rb"

Library/Homebrew/brew.rb

@@ -15,14 +15,6 @@ end
 
 std_trap = trap("INT") { exit! 130 } # no backtrace thanks
 
-# check ruby version before requiring any modules.
-REQUIRED_RUBY_X, REQUIRED_RUBY_Y, = ENV.fetch("HOMEBREW_REQUIRED_RUBY_VERSION").split(".").map(&:to_i)
-RUBY_X, RUBY_Y, = RUBY_VERSION.split(".").map(&:to_i)
-if RUBY_X < REQUIRED_RUBY_X || (RUBY_X == REQUIRED_RUBY_X && RUBY_Y < REQUIRED_RUBY_Y)
-  raise "Homebrew must be run under Ruby #{REQUIRED_RUBY_X}.#{REQUIRED_RUBY_Y}! " \
-        "You're running #{RUBY_VERSION}."
-end
-
 require_relative "global"
 
 begin

Library/Homebrew/standalone/init.rb

@@ -1,6 +1,22 @@
-# typed: true
+# typed: false
 # frozen_string_literal: true
 
+# This file is included before any other files. It intentionally has typing disabled and has minimal use of `require`.
+
+required_ruby_major, required_ruby_minor, = ENV.fetch("HOMEBREW_REQUIRED_RUBY_VERSION", "").split(".").map(&:to_i)
+unsupported_ruby = if required_ruby_minor.nil?
+  # We're probably only running rubocop etc so just assume supported
+  false
+else
+  ruby_major, ruby_minor, = RUBY_VERSION.split(".").map(&:to_i)
+  if ruby_major < required_ruby_major || (ruby_major == required_ruby_major && ruby_minor < required_ruby_minor)
+    raise "Homebrew must be run under Ruby #{required_ruby_major}.#{required_ruby_minor}! " \
+          "You're running #{RUBY_VERSION}."
+  end
+
+  ruby_major != required_ruby_major || ruby_minor != required_ruby_minor
+end.freeze
+
 # We trust base Ruby to provide what we need.
 # Don't look into the user-installed sitedir, which may contain older versions of RubyGems.
 require "rbconfig"
@@ -12,6 +28,9 @@ HOMEBREW_LIBRARY_PATH = Pathname(__dir__).parent.realpath.freeze
 require_relative "../utils/gems"
 Homebrew.setup_gem_environment!(setup_path: false)
 
+# Install gems for Rubies we don't vendor for.
+Homebrew.install_bundler_gems! if unsupported_ruby
+
 $LOAD_PATH.push HOMEBREW_LIBRARY_PATH.to_s unless $LOAD_PATH.include?(HOMEBREW_LIBRARY_PATH.to_s)
 require_relative "../vendor/bundle/bundler/setup"
 $LOAD_PATH.unshift "#{HOMEBREW_LIBRARY_PATH}/vendor/bundle/#{RUBY_ENGINE}/#{Gem.ruby_api_version}/gems/" \