The changes about prevent leaking container_of globaly

solution-1.cr

# channel.cr
class Channel(T)
  struct Sender(T)
    @link : Crystal::StaticList
  end
  
  Crystal::StaticList.container_of(ptr, Channel::Sender(T), @link)
end

# crystal/static_list.cr
struct Crystal::StaticList
  macro container_of(ptr, type, member)
  end
end

solution-2.cr

# channel.cr
class Channel(T)
  include Crystal::StaticList::Reflect
  private struct Sender(T)
    @link : Crystal::StaticList
  end
  
  container_of(ptr, Sender(T), @link)
end

# crystal/static_list.cr
struct Crystal::StaticList
  module Reflect
    private macro container_of(ptr, type, member)
    end
  end
end

solution-3.cr

# channel.cr
class Channel(T)
  @[Crystal::StaticList::Reflect::Fields(:link)]
  private struct Sender(T)
    include Crystal::StaticList::Reflect
    @link : Crystal::StaticList
  end
  
  Sender(T).from_link(ptr)
end

# crystal/static_list.cr
struct Crystal::StaticList
  module Reflect
    annotation Fields; end

    macro included
      {% for member in @type.annotation(Crystal::StaticList::Reflect::Fields) %}
        @[AlwaysInline]
        def self.from_{{ member.id }}(ptr : Crystal::StaticList*)
        end
      {% end %}
    end
  end
end