Experimentation on what a code migration syntax might look like, ideally

code_migration_hypothetical.rb

module CodeMigrations
  # Number isn't important, vague syntax, and not all
  # of these are specifically Rails examples
  version from: 4.0, to: 5.0

  # The easiest case, literally A to B direct translation.
  class FilterToAction < Migration
    expect_translation(
      "after_filter" => "after_action",
      # ...snipped excess for example
      "skip_before_filter" => "skip_before_action",
      "skip_filter" => "skip_action_callback",
    )
  end

  # This one is harder, but very very doable
  class AssertBangToNot < Migration
    expect { assert !x }.to_become { assert_not x }
  end

  # This one is much harder, and requires more knowledge of Ruby syntax
  # 
  # `x.select { |v| v.even? }` becomes the AST:
  #   (begin
  #     (block (send (send nil :x) :select) (args (arg :v))
  #       (send (lvar :v) :even?))
  # 
  # `x.select(&:even?)` becomes the AST:
  #   (send (send nil :x) :select (block-pass (sym :even?))))
  # 
  # Meaning the wrapping elements are different, making it a
  # much harder conversion to infer without building in a lot of
  # Ruby knowledge of block vs block-pass shenanigans.
  class ToShorthandBlock
    expect { x.select { |v| v.even? } }.to_become { x.select(&:even?) }
  end
end