Avoiding LSP formatting conflicts

Neovim 0.8

The recommended approach is to use the new vim.lsp.buf.format API, which makes it easy to select which server you want to use for formatting:

local lsp_formatting = function(bufnr)
    vim.lsp.buf.format({
        filter = function(client)
            -- apply whatever logic you want (in this example, we'll only use null-ls)
            return client.name == "null-ls"
        end,
        bufnr = bufnr,
    })
end

-- if you want to set up formatting on save, you can use this as a callback
local augroup = vim.api.nvim_create_augroup("LspFormatting", {})

-- add to your shared on_attach callback
local on_attach = function(client, bufnr)
    if client.supports_method("textDocument/formatting") then
        vim.api.nvim_clear_autocmds({ group = augroup, buffer = bufnr })
        vim.api.nvim_create_autocmd("BufWritePre", {
            group = augroup,
            buffer = bufnr,
            callback = function()
                lsp_formatting(bufnr)
            end,
        })
    end
end

Another way is to block a client's formatting capabilities in your on_attach callback. This approach is not recommended versus the other approaches mentioned here:

local on_attach = function(client, bufnr)
    if client.name == "tsserver" then                                                                                                   
        client.resolved_capabilities.document_formatting = false -- 0.7 and earlier
        client.server_capabilities.documentFormattingProvider = false -- 0.8 and later
    end
    -- rest of the initialization
end

Neovim 0.7 and earlier

If a buffer is attached to more than one language server with formatting capabilities, Neovim's default formatting handler will ask you which server you want to use whenever you run vim.lsp.buf.formatting() or vim.lsp.buf.formatting_sync(). If you only want to use null-ls formatting, you need to enable formatting only for the language server you choose to.

Avoid breaking formatexpr (i.e. gq)

See the discussion

Neovim automatically sets formatexpr (if unset) to vim.lsp.formatexpr() if the LSP server advertises the documentRangeFormattingProvider capability. Because there is no way to advertise this separately per-provider, Null-LS advertises all capabilities it might potentially support.

This means formatexpr will be set whenever null-ls attaches to the buffer, whether or not any of the active sources provide a formatting method. gq will no-op.

A workaround, provided by @Furkanzmc, sets formatexpr appropriately depending on the available null-ls sources:

local function is_null_ls_formatting_enabled(bufnr)
    local file_type = vim.api.nvim_buf_get_option(bufnr, "filetype")
    local generators = require("null-ls.generators").get_available(
        file_type,
        require("null-ls.methods").internal.FORMATTING
    )
    return #generators > 0
end

function on_attach(client, bufnr)
    if client.server_capabilities.documentFormattingProvider then
        if
            client.name == "null-ls" and is_null_ls_formatting_enabled(bufnr)
            or client.name ~= "null-ls"
        then
            vim.bo[bufnr].formatexpr = "v:lua.vim.lsp.formatexpr()"
            vim.keymap.set("n", "<leader>gq", "<cmd>lua vim.lsp.buf.format({ async = true })<CR>", opts)
        else
            vim.bo[bufnr].formatexpr = nil
        end
    end
end