---Utility for keymap creation.
    ---@param lhs string
    ---@param rhs string|function
    ---@param opts string|table
    ---@param mode? string|string[]
    local function keymap(lhs, rhs, opts, mode)
        opts = type(opts) == 'string' and { desc = opts }
            or vim.tbl_extend('error', opts --[[@as table]], { buffer = bufnr })
        mode = mode or 'n'
        vim.keymap.set(mode, lhs, rhs, opts)
    end

    ---For replacing certain <C-x>... keymaps.
    ---@param keys string
    local function feedkeys(keys)
        vim.api.nvim_feedkeys(vim.api.nvim_replace_termcodes(keys, true, false, true), 'n', true)
    end

    ---Is the completion menu open?
    local function pumvisible()
        return tonumber(vim.fn.pumvisible()) ~= 0
    end

    -- Enable completion and configure keybindings.
    if client.supports_method(methods.textDocument_completion) then
        vim.lsp.completion.enable(true, client.id, bufnr, { autotrigger = true })
        
        -- Use enter to accept completions.
        keymap('<cr>', function()
            return pumvisible() and '<C-y>' or '<cr>'
        end, { expr = true }, 'i')
        
        -- Use slash to dismiss the completion menu.
        keymap('/', function()
            return pumvisible() and '<C-e>' or '/'
        end, { expr = true }, 'i')
    
        -- Use <C-n> to navigate to the next completion or:
        -- - Trigger LSP completion.
        -- - If there's no one, fallback to vanilla omnifunc.
        keymap('<C-n>', function()
            if pumvisible() then
                feedkeys '<C-n>'
            else
                if next(vim.lsp.get_clients { bufnr = 0 }) then
                    vim.lsp.completion.trigger()
                else
                    if vim.bo.omnifunc == '' then
                        feedkeys '<C-x><C-n>'
                    else
                        feedkeys '<C-x><C-o>'
                    end
                end
            end
        end, 'Trigger/select next completion', 'i')
    
        -- Buffer completions.
        keymap('<C-u>', '<C-x><C-n>', { desc = 'Buffer completions' }, 'i')
    
        -- Use <Tab> to accept a Copilot suggestion, navigate between snippet tabstops,
        -- or select the next completion.
        -- Do something similar with <S-Tab>.
        keymap('<Tab>', function()
            local copilot = require 'copilot.suggestion'

            if copilot.is_visible() then
                copilot.accept()
            elseif pumvisible() then
                feedkeys '<C-n>'
            elseif vim.snippet.active { direction = 1 } then
                vim.snippet.jump(1)
            else
                feedkeys '<Tab>'
            end
        end, {}, { 'i', 's' })
        keymap('<S-Tab>', function()
            if pumvisible() then
                feedkeys '<C-p>'
            elseif vim.snippet.active { direction = -1 } then
                vim.snippet.jump(-1)
            else
                feedkeys '<S-Tab>'
            end
        end, {}, { 'i', 's' })

        -- Inside a snippet, use backspace to remove the placeholder.
        keymap('<BS>', '<C-o>s', {}, 's')
    end