Setup for unobtrusive shell-like completion behaviour

[powerman]

• No popup menu jumping on you all of time and hiding your code in nearest lines. 😫
• No fuzzy search - only exact prefix match, we love predictability and fewer completion items! 😌
• Use <Tab> Force, Luke!

NOTE: This is not a "Super-Tab" setup. It does expand snippets by <Tab> but jumping between snippet placeholders should be mapped to some other keys instead of <Tab> and <S-Tab>.

May worth adding to the wiki, but I'll leave this decision to someone else.

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

cmp.setup {
    completion = {
        -- Show menu only when I ask for it!
        autocomplete = false,
    },

    matching = {
        -- Allow only prefix matching!
        disallow_fuzzy_matching = true,
        disallow_fullfuzzy_matching = true,
        disallow_partial_fuzzy_matching = true,
        disallow_partial_matching = true,
        disallow_prefix_unmatching = true,
    },

    -- HACK: Work around https://github.com/hrsh7th/nvim-cmp/issues/1809.
    preselect = 'None',

    mapping = {
        -- Smart unobtrusive completion in a shell-like way.
        --
        --  - Menu is shown only when user explicitly asks for it and there are
        --    multiple possible completions/snippets available.
        --  - Normal editing keys works as usually even if menu is shown:
        --    - Completion menu might be shown only on <Tab> after word or <S-Tab>.
        --    - Item in menu can be selected only by extra <Tab> or <S-Tab>.
        --    - Behaviour of these keys changes only when menu item is selected
        --      (i.e. after at least two <Tab> or <S-Tab>, meaning active
        --      interaction with menu):
        --      - <CR> completes/expands current item.
        --      - <C-Down>, <C-Up> scroll current item docs.
        --  - <Tab> action for a current word, with or without menu opened:
        --    - If there only one possible snippet then expand it.
        --    - If there only one possible completion then complete it.
        --    - If there only one possible partial completion then complete it.
        --    - If there are multiple possible completions then:
        --      - If menu isn't opened then open menu (without selecting anything,
        --        so it serves as a hint what you can type next).
        --      - If menu is opened then select next item.
        --  - <S-Tab> action:
        --    - If menu is opened then select previous item.
        --    - If menu is not opened then open menu (useful on empty line).
        ['<Tab>'] = cmp.mapping(function(fallback)
            if cmp.visible() then
                if cmp.complete_common_string() then
                    return
                elseif #cmp.get_entries() == 1 then
                    cmp.confirm { select = true }
                else
                    cmp.select_next_item {
                        behavior = types.cmp.SelectBehavior.Select,
                    }
                end
            elseif has_words_before() then
                cmp.complete()
                if cmp.complete_common_string() then
                    cmp.close()
                elseif #cmp.get_entries() == 1 then
                    cmp.confirm {
                        select = true,
                    }
                end
            else
                fallback()
            end
        end, { 'i', 'c' }),
        ['<S-Tab>'] = cmp.mapping(function(_)
            if cmp.visible() then
                cmp.select_prev_item {
                    behavior = types.cmp.SelectBehavior.Select,
                }
            else
                cmp.complete()
            end
        end, { 'i', 'c' }),
        ['<CR>'] = cmp.mapping(function(fallback)
            if not cmp.confirm { select = false } then
                fallback()
            end
        end, { 'i', 'c' }),
        ['<C-e>'] = cmp.mapping(function(fallback)
            if not cmp.abort() then
                fallback()
            end
        end, { 'i', 'c' }),

        -- ... other mappings: scroll docs, snippet jumps, ...
    },

    -- ... other settings ...
}

cmp.setup.cmdline({'/', '?'}, {
    -- ... setup what you need except `mapping` ...
})

cmp.setup.cmdline(':', {
    -- ... setup what you need except `mapping` ...
})

Let me know if there are any issues with this setup.