Use LSP to format only modified text in Neovim (like VSCode's format modifications feature).

Many projects suggest contributors use an autoformatter to keep code style consistent.

However, in larger projects with legacy code, there can be plenty of places where the style deviates from the autoformatter's preference.

In that case, you don't want to format entire files — you'll be left with very noisy diffs!

You also don't want to comb through all of the changes you've made, manually applying the autoformatter before you commit. We are too lazy for that!

One reasonable solution is to run the autoformatter over the changed lines as defined by the revision control system. VSCode can do this — and this is something that I didn't find an analog for in Neovim. Hence, this plugin!

• Neovim ≥0.8 is required. Neovim 0.8 has a more flexible API for LSP formatting, which this plugin leverages.
• Any LSP server that you want to use with this plugin must support the DocumentRangeFormattingProvider server capability — lsp-format-modifications.nvim will warn if an unsupported LSP server is used.
• nvim-lua/plenary.nvim is required (hint: you are probably already using this).

Install with your favourite plugin manager — for example, with junegunn/vim-plug:

Plug 'nvim-lua/plenary.nvim'
Plug 'joechrisellis/lsp-format-modifications.nvim'

In terms of basic usage:

require"lsp-format-modifications".format_modifications(<lsp-client>, <bufnr>, <config>)

... will format the modifications in the buffer with ID <bufnr> using LSP client <lsp-client>, according to the options specified in <config>.

For convenience, you may choose to create a user command that does this for you. For example, in your LSP on_attach function:

local on_attach = function(client, bufnr)
  -- your usual configuration — options, keymaps, etc
  -- ...

  vim.api.nvim_buf_create_user_command(
    bufnr,
    "FormatModifications",
    function()
      local lsp_format_modifications = require"lsp-format-modifications"
      lsp_format_modifications.format_modifications(client, bufnr)
    end,
    {}
  )
end

If you'd prefer to instead format-on-save, try:

local on_attach = function(client, bufnr)
  -- your usual configuration — options, keymaps, etc
  -- ...

  local augroup_id = vim.api.nvim_create_augroup(
    "FormatModificationsDocumentFormattingGroup",
    { clear = false }
  )
  vim.api.nvim_clear_autocmds({ group = augroup_id, buffer = bufnr })

  vim.api.nvim_create_autocmd(
    { "BufWritePre" },
    {
      group = augroup_id,
      buffer = bufnr,
      callback = function()
        local lsp_format_modifications = require"lsp-format-modifications"
        lsp_format_modifications.format_modifications(client, bufnr)
      end,
    }
  )
end

A complete configuration table is below:

local config = {
  -- The callback that is invoked to compute a diff so that we know what to
  -- format. This defaults to vim.diff with some sensible defaults.
  diff_callback = function(compareee_content, buf_content)
    return vim.diff(compareee_content, buf_content, {...})
  end,

  -- The callback that is invoked to actually do the formatting on the changed
  -- hunks. Defaults to vim.lsp.buf.format (requires Neovim ≥ 0.8).
  format_callback = vim.lsp.buf.format,

  -- The VCS to use. Possible options are: "git", "hg". Defaults to "git".
  vcs = "git",

  -- EXPERIMENTAL: when true, do not attempt to format the outermost empty
  -- lines in diff hunks, and do not touch hunks consisting of entirely empty
  -- lines. For some LSP servers, this can result in more intuitive behaviour.
  experimental_empty_line_handling = false
}

Please raise an issue if something is wrong — but read this section first.

In my experience with LSP, I've found that most language servers have imperfect support for range formatting. A lot of the time, selecting and formatting a range results in the formatter also capturing some of the surrounding text. I'm not totally sure why this is, but the upshot is that for some language servers, :FormatModifications might capture more than just the lines in the hunk. This is usually not a big deal.

A good way to test whether lsp-format-modifications.nvim is playing up, or whether it's just your language server, is to visually select the changed range and hit gq (invoking formatexpr). If you see the same problem, it's more likely to be problem with your language server.

Adding support for a new VCS is fairly simple (see this PR for an example) — pull requests are very welcome.