gistfile1.rb

# Array#inject_with_index
class Array
  def inject_with_index default=nil, &block
    self.inject(default) do |obj, element|
      block[obj, element, index(element)]
    end
  end
end

a = [:a, :b, :c].inject_with_index({}) do |hash, element, index|
  hash[element] = index + 1
  hash
end

p a
__END__
{:b=>2, :c=>3, :a=>1}

gistfile2.rb

module Enumerable
  def inject_with_index default=nil, &block
    if self.respond_to?(:index)
      inject_block = lambda do |obj, element|
        block[obj, element, index(element)]
      end
    else
      counter = -1
      inject_block = lambda do |obj, element|
        block[obj, element, counter += 1]
      end
    end

    self.inject default, &inject_block
  end
end

a = [:a, :b, :c].inject_with_index({}) do |hash, element, index|
  hash[element] = index + 1
  hash
end

a # => {:c=>3, :b=>2, :a=>1}

b = (0..5).inject_with_index({}) do |hash, element, index|
  hash[element] = index
  hash
end

b # => {5=>5, 0=>0, 1=>1, 2=>2, 3=>3, 4=>4}