Feature request: Formatting on changed lines only

[diogox]

This may not be the right place to ask, but I was wondering: Is it possible to run the formatters automatically, but only for the new changes as per the git diff?

I would like to use Neovim for work, but I find that it changes code that I didn’t touch and makes my PRs unnecessarily long 😅

[stevearc]

conform supports range formatting, so you could probably re-use a lot of the code in the injected formatter and pass in git diff ranges instead of treesitter ranges

[diogox]

I see!

I would only add that - if this feature gets implemented - it may make sense to enable this on a per-formatter basis.

For eg. I would only want golines to take effect in lines I've changed, but it would make sense to have goimports trigger every time I change a file as for the order of the imports to be correct, the whole context (all of the imports) need to be passed into the formatter - not just the import lines that were changed.

Thanks for taking this feature request into consideration :)

[ribru17]

Here's a command I made for this, maybe it could be used a reference for a future PR:

vim.api.nvim_create_user_command('DiffFormat', function()
  local lines = vim.fn.system('git diff --unified=0'):gmatch('[^\n\r]+')
  local ranges = {}
  for line in lines do
    if line:find('^@@') then
      local line_nums = line:match('%+.- ')
      if line_nums:find(',') then
        local _, _, first, second = line_nums:find('(%d+),(%d+)')
        table.insert(ranges, {
          start = { tonumber(first), 0 },
          ['end'] = { tonumber(first) + tonumber(second), 0 },
        })
      else
        local first = tonumber(line_nums:match('%d+'))
        table.insert(ranges, {
          start = { first, 0 },
          ['end'] = { first + 1, 0 },
        })
      end
    end
  end
  local format = require('conform').format
  for _, range in pairs(ranges) do
    format {
      range = range,
    }
  end
end, { desc = 'Format changed lines' })

[linusboehm]

In case someone is interested. Similar to the above command, but using gitsigns to to get the hunks:

function()
  local hunks = require("gitsigns").get_hunks()
  local format = require("conform").format
  for i = #hunks, 1, -1 do
    local hunk = hunks[i]
    if hunk ~= nil and hunk.type ~= "delete" then
      local start = hunk.added.start
      local last = start + hunk.added.count
      -- nvim_buf_get_lines uses zero-based indexing -> subtract from last
      local last_hunk_line = vim.api.nvim_buf_get_lines(0, last - 2, last - 1, true)[1]
      local range = { start = { start, 0 }, ["end"] = { last - 1, last_hunk_line:len() } }
      format({ range = range })
    end
  end
end

[linusboehm]

another update: in the previous snipped, the command doesn't return until everything is reformatted, which sometimes takes a while. The following snipped does it asynchronously.
I only tested it with clang-format (C++), and ruff_lsp (python). The lua range-formatting doesn't seem to be working properly.

function()

  local ignore_filetypes = { "lua" }
  if vim.tbl_contains(ignore_filetypes, vim.bo.filetype) then
    vim.notify("range formatting for " .. vim.bo.filetype .. " not working properly.")
    return
  end

  local hunks = require("gitsigns").get_hunks()
  if hunks == nil then
    return
  end

  local format = require("conform").format

  local function format_range()
    if next(hunks) == nil then
      vim.notify("done formatting git hunks", "info", { title = "formatting" })
      return
    end
    local hunk = nil
    while next(hunks) ~= nil and (hunk == nil or hunk.type == "delete") do
      hunk = table.remove(hunks)
    end

    if hunk ~= nil and hunk.type ~= "delete" then
      local start = hunk.added.start
      local last = start + hunk.added.count
      -- nvim_buf_get_lines uses zero-based indexing -> subtract from last
      local last_hunk_line = vim.api.nvim_buf_get_lines(0, last - 2, last - 1, true)[1]
      local range = { start = { start, 0 }, ["end"] = { last - 1, last_hunk_line:len() } }
      format({ range = range, async = true, lsp_fallback = true }, function()
        vim.defer_fn(function()
          format_range()
        end, 1)
      end)
    end
  end

  format_range()
end

NOTE: this hasn't been tested thoroughly and editing the file, while it is being formatted asynchronously in the background might mess things up. Also note: it is important to go through the hunks in reverse order to make sure that the formatting does not affect line numbers of subsequent hunks,

[wenjinnn]

Here's another implementation with mini.diff, you can use it with mini.git or gitsigns as diff source.

      local range_ignore_filetypes = { "lua" }
      local diff_format = function()
        local data = MiniDiff.get_buf_data()
        if not data or not data.hunks then
          vim.notify("No hunks in this buffer")
          return
        end
        local format = require("conform").format
        -- stylua range format mass up indent, so use full format for now
        if vim.tbl_contains(range_ignore_filetypes, vim.bo.filetype) then
          format({ lsp_fallback = true, timeout_ms = 500 })
          return
        end
        local ranges = {}
        for _, hunk in pairs(data.hunks) do
          if hunk.type ~= "delete" then
            -- always insert to index 1 so format below could start from last hunk, which this sort didn't mess up range
            table.insert(ranges, 1, {
              start = { hunk.buf_start, 0 },
              ["end"] = { hunk.buf_start + hunk.buf_count, 0 },
            })
          end
        end
        for _, range in pairs(ranges) do
          format({ lsp_fallback = true, timeout_ms = 500, range = range })
        end
      end

NOTE: this hasn't been tested thoroughly and editing the file, while it is being formatted asynchronously in the background might mess things up. Also note: it is important to go through the hunks in reverse order to make sure that the formatting does not affect line numbers of subsequent hunks,

And I tested this code with async = true, it does fail sometime, so I'm using sync format in here.

[ArMot]

Thanks everyone.
I'm kinda new in neovim and conform .
I wanted to make this default behavior of format_on_save in my environment, like the way in vscode:

{
  "editor.formatOnSaveMode": "modificationsIfAvailable",
}

I want conform to check if there is git hunks there. and format the code based on that. but if not using git for file. format the whole document.
another thing is, consider python i use isort and ruff, i want Isort format the imports no matter what but ruff checks for git hunks first.
the above code snippets works and created a DiffFormat command . but I don't know how I can make it the default behavior of conform format_on_save and specific formatters .
can someone please help me with this ?

[YuYuCong]

Thanks everyone. I'm kinda new in neovim and conform . I wanted to make this default behavior of format_on_save in my environment, like the way in vscode:

{
  "editor.formatOnSaveMode": "modificationsIfAvailable",
}

I want conform to check if there is git hunks there. and format the code based on that. but if not using git for file. format the whole document. another thing is, consider python i use isort and ruff, i want Isort format the imports no matter what but ruff checks for git hunks first. the above code snippets works and created a DiffFormat command . but I don't know how I can make it the default behavior of conform format_on_save and specific formatters . can someone please help me with this ?

I wanted the same feature as you did, and I got one solution: https://github.com/joechrisellis/lsp-format-modifications.nvim

[pgzm29]

Thanks everyone. I'm kinda new in neovim and conform . I wanted to make this default behavior of format_on_save in my environment, like the way in vscode:

{
  "editor.formatOnSaveMode": "modificationsIfAvailable",
}

I want conform to check if there is git hunks there. and format the code based on that. but if not using git for file. format the whole document. another thing is, consider python i use isort and ruff, i want Isort format the imports no matter what but ruff checks for git hunks first. the above code snippets works and created a DiffFormat command . but I don't know how I can make it the default behavior of conform format_on_save and specific formatters . can someone please help me with this ?

For anyone that is interested in this kind of behavior, I got it working by using the function provided by @linusboehm and then adding this to the format_on_save option:

local function format_hunks()
  -- Logic to format only the hunks in the file
end

require("conform").setup({
  -- options,
  format_on_save = function()
    -- Prefer to format git hunks instead of the entire file
    format_hunks()
  end,
})

I'm still testing this but so far, I haven't had any issues.

And in case you still want to format the entire file, you can create a keymap to do so:

vim.keymap.set({ "n", "v" }, "<leader>cf", function()
  conform.format(default_format_options)
end, { desc = "Format File or Range (visual mode)" })

Edit: In order to make it work with files that do not support range formatting, I added this to the function:

if vim.tbl_contains(range_ignore_filetypes, vim.bo.filetype) then
  format({ lsp_fallback = true, timeout_ms = 500 })
  return
end

This was taken from the function provided by @wenjinnn. By doing this, it will work as the modificationsIfAvailable mode from VS Code.

[kubahasek]

Hey @pgzm29! Thanks for this, it works great for formatting changes, but when I create a new file, it does not get formatted. Did you change anything in the function for that? Thanks!