class Rubocop::Cop::Style::Lambda

anonymous functions.
anonymous functions and uses of the 1.9 lambda syntax for multi-line
This cop checks for uses of the pre 1.9 lambda syntax for one-line

def lambda_length(block_node) 

def lambda_length(block_node)
  start_line = block_node.loc.begin.line
  end_line = block_node.loc.end.line
  end_line - start_line
end

def on_block(node) 

def on_block(node)
  # We're looking for
  # (block
  #   (send nil :lambda)
  #   ...)
  block_method, = *node
  if block_method == TARGET
    selector = block_method.loc.selector.source
    lambda_length = lambda_length(node)
    if selector != '->' && lambda_length == 0
      convention(block_method, :expression, SINGLE_MSG)
    elsif selector == '->' && lambda_length > 0
      convention(block_method, :expression, MULTI_MSG)
    end
  end
end