An example code action for null-ls.nvim

custom_code_actions.lua

local frozen_string_actions = {
    method = null_ls.methods.CODE_ACTION,
    filetypes = {"ruby"},
    generator = {
        fn = function(context)
            frozen_string_literal_comment = "# frozen_string_literal: true"

            first_line = context.content[1]

            if first_line ~= frozen_string_literal_comment then
                return {
                    {
                        title = "🥶Add frozen string literal comment",
                        action = function()
                            lines = {
                                frozen_string_literal_comment, "", first_line
                            }

                            vim.api.nvim_buf_set_lines(context.bufnr, 0, 1,
                                                       false, lines)
                        end
                    }
                }
            end
        end
    }
}
null_ls.register(frozen_string_actions)