cmp.setup {
  mapping = {
    ['<Tab>'] = function(fallback)
      if vim.fn.pumvisible() == 1 then
        vim.api.nvim_feedkeys(vim.api.nvim_replace_termcodes('<C-n>', true, true, true), true)
      else
        fallback()
      end
    end
  }
}

cmp.setup {
  mapping = {
    ['<Tab>'] = function(fallback)
      if cmp.visible() then
        cmp.select_next_item()
      else
        fallback()
      end
    end
  }
}

cmp.setup {
  sources = {
    {
      name = 'buffer',
      opts = {
        get_bufnrs = function() return { vim.api.nvim_get_current_buf() } end
      }
    }
  }
}

cmp.setup {
  sources = {
    {
      name = 'buffer',
      option = {
        get_bufnrs = function() return { vim.api.nvim_get_current_buf() } end
      }
    }
  }
}

cmp.setup {
  mapping = {
    ['<C-x><C-s>'] = cmp.mapping.complete({
      sources = {
        { name = 'vsnip' }
      }
    }),
  }
}

cmp.setup {
  mapping = {
    ['<C-x><C-s>'] = cmp.mapping.complete({
      config = {
        sources = {
          { name = 'vsnip' }
        }
      }
    }),
  }
}

cmp.setup {
  ...
  mapping = cmp.mapping.preset.insert({
    -- Your configuration here.
  })
  ...
}

cmp.setup.cmdline {
  ...
  mapping = cmp.mapping.preset.cmdline({
    -- Your configuration here.
  })
  ...

}