Example mappings

You can use the following examples.

Super-Tab like mapping

vim-vsnip

local has_words_before = function()
  local line, col = unpack(vim.api.nvim_win_get_cursor(0))
  return col ~= 0 and vim.api.nvim_buf_get_lines(0, line - 1, line, true)[1]:sub(col, col):match("%s") == nil
end

local feedkey = function(key, mode)
  vim.api.nvim_feedkeys(vim.api.nvim_replace_termcodes(key, true, true, true), mode, true)
end

local cmp = require('cmp')
cmp.setup {

  -- ... Your other configuration ...

  mapping = {

    -- ... Your other mappings ...

    ["<Tab>"] = cmp.mapping(function(fallback)
      if cmp.visible() then
        cmp.select_next_item()
      elseif vim.fn["vsnip#available"](1) == 1 then
        feedkey("<Plug>(vsnip-expand-or-jump)", "")
      elseif has_words_before() then
        cmp.complete()
      else
        fallback() -- The fallback function sends a already mapped key. In this case, it's probably `<Tab>`.
      end
    end, { "i", "s" }),

    ["<S-Tab>"] = cmp.mapping(function()
      if cmp.visible() then
        cmp.select_prev_item()
      elseif vim.fn["vsnip#jumpable"](-1) == 1 then
        feedkey("<Plug>(vsnip-jump-prev)", "")
      end
    end, { "i", "s" }),

    -- ... Your other mappings ...

  }

  -- ... Your other configuration ...

}

luasnip

local has_words_before = function()
  local line, col = unpack(vim.api.nvim_win_get_cursor(0))
  return col ~= 0 and vim.api.nvim_buf_get_lines(0, line - 1, line, true)[1]:sub(col, col):match("%s") == nil
end

local luasnip = require("luasnip")
local cmp = require("cmp")

cmp.setup({

  -- ... Your other configuration ...

  mapping = {

    -- ... Your other mappings ...

    ["<Tab>"] = cmp.mapping(function(fallback)
      if cmp.visible() then
        cmp.select_next_item()
      elseif luasnip.expand_or_jumpable() then
        luasnip.expand_or_jump()
      elseif has_words_before() then
        cmp.complete()
      else
        fallback()
      end
    end, { "i", "s" }),

    ["<S-Tab>"] = cmp.mapping(function(fallback)
      if cmp.visible() then
        cmp.select_prev_item()
      elseif luasnip.jumpable(-1) then
        luasnip.jump(-1)
      else
        fallback()
      end
    end, { "i", "s" }),

    -- ... Your other mappings ...
  },

  -- ... Your other configuration ...
})

No snippet plugin

Snippets don't expand with <Tab> in this config. Use <C-Space>.

local has_words_before = function()
  local line, col = unpack(vim.api.nvim_win_get_cursor(0))
  return col ~= 0 and vim.api.nvim_buf_get_lines(0, line - 1, line, true)[1]:sub(col, col):match('%s') == nil
end

local cmp = require('cmp')
cmp.setup {
  -- ... Your other configuration ...

  mapping = {

    -- ... Your other configuration ...

    ['<C-Space>'] = cmp.mapping.confirm {
      behavior = cmp.ConfirmBehavior.Insert,
      select = true,
    },

    ['<Tab>'] = function(fallback)
      if not cmp.select_next_item() then
        if vim.bo.buftype ~= 'prompt' and has_words_before() then
          cmp.complete()
        else
          fallback()
        end
      end
    end,

    ['<S-Tab>'] = function(fallback)
      if not cmp.select_prev_item() then
        if vim.bo.buftype ~= 'prompt' and has_words_before() then
          cmp.complete()
        else
          fallback()
        end
      end
    end,
  },

  snippet = {
    -- We recommend using *actual* snippet engine.
    -- It's a simple implementation so it might not work in some of the cases.
    expand = function(args)
      local line_num, col = unpack(vim.api.nvim_win_get_cursor(0))
      local line_text = vim.api.nvim_buf_get_lines(0, line_num - 1, line_num, true)[1]
      local indent = string.match(line_text, '^%s*')
      local replace = vim.split(args.body, '\n', true)
      local surround = string.match(line_text, '%S.*') or ''
      local surround_end = surround:sub(col)

      replace[1] = surround:sub(0, col - 1)..replace[1]
      replace[#replace] = replace[#replace]..(#surround_end > 1 and ' ' or '')..surround_end
      if indent ~= '' then
        for i, line in ipairs(replace) do
          replace[i] = indent..line
        end
      end

      vim.api.nvim_buf_set_lines(0, line_num - 1, line_num, true, replace)
    end,
  },

  -- ... Your other configuration ...
}