brew/Library/Homebrew/requirements/arch_requirement.rb

require "requirement"

class ArchRequirement < Requirement
  fatal true

  def initialize(tags)
    @arch = tags.shift
    super(tags)
  end

  satisfy(build_env: false) do
    case @arch
    when :x86_64 then MacOS.prefer_64_bit?
    when :intel, :ppc then Hardware::CPU.type == @arch
    end
  end

  def message
    "This formula requires an #{@arch} architecture."
  end
end