hashblock.rb

class Hash
  def self.from_block &block
    Hash[*BlockBuilder.build(&block)]
  end
  
  class BlockBuilder
    @@collected = []
    class << self
      def build(&block)
        instance_eval &block
        copy = @@collected.clone and @@collected.clear and return copy
      end

      def method_missing(method, *args)
        @@collected.concat [method.to_sym, args.first]
      end
    end
  end
end

# Hash.from_block do
#   apple 8
#   banana 2
#   carrot 6
# end
# => {:apple=>8, :banana=>2, :carrot=>6}

I was thinking you should use something like BasicObject for BlockBuilder, but then I realised that there are all sorts of things in the lexical scope of that block that are gonna stuff it up.
Hash.from_block { p 1; q 2; r 3 }

=> { :q => 2, :3 => 3 }

Once people get over Ruby 1.8, you can do { apple: 8, banana: 2, carrot: 6 }.

The point of BlockBuilder above is to sandbox the execution of the block away. So that method_missing there won't get in any other class' way.

Yep, but all the methods in Class get in the way. puts Class.methods. Hash.from_block { extend 10 } raises an exception for example. But it's not just method calls that mess with the lexical scope of the block, there are also global functions like 'p', 'print', etc. that get in the way too, which is not a problem I can see any way around.