In Neovim, we can use the virtual text feature to place virtual text anywhere in the current buffer, which is not possible with normal text.

A bit of history

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Initially, the Nvim core dev bfredl introduced nvim_buf_set_virtual_text() to add virtual text to a buffer in this PR.

Over the time, the code is refactored and deprecated, in favor of the new API function nvim_buf_set_extmark()¹.

Plugins utilizing virtual text

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Plugins have been using virtual text feature to do various things. Here is a non-exhaustive list:

• IndentBlankLine.nvim: Show indent guides even on blank lines. Previously, it is not possible to place text on blank lines. So there will be a gap in indent lines when there is a blank line, see this issue.
• nvim-cmp: show function signature using virtual text.
• nvim-hlslens: Show search count and index at EOL, an excellent alternative to vim-anzu and the likes.
• blamer.nvim: Show line blame messages at EOL².
• Nvim-lspconfig: Show diagnostics as virtual text in the end of a line.
• virt-column.nvim: replace option colorcolumn using virtual text.
• virtual-type.nvim: show variable type using virtual text.
• Extmark-toy.nvim: Showing cool visual effects.

Using extmarks

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The nvim_buf_set_extmark() function has a lot of parameters, and some of them can be confusing for users due to the conciseness of its documentation.

Here is a working example to demonstrate its usage:

local api = vim.api

local bnr = vim.fn.bufnr('%')
local ns_id = api.nvim_create_namespace('demo')

local line_num = 5
local col_num = 5

local opts = {
  end_line = 10,
  id = 1,
  virt_text = {{"demo", "IncSearch"}},
  virt_text_pos = 'overlay',
  -- virt_text_win_col = 20,
}

local mark_id = api.nvim_buf_set_extmark(bnr, ns_id, line_num, col_num, opts)

In the above, line_num specifies which line we want to put the virtual text. col_num specifies which column we want to put virtual text (but this is actually only half true, since we can put virtual text at any position). col_num shouldn’t exceed the actual column number of the line indicated by line_num. Otherwise, we get the following error message:

E5113: Error while calling lua chunk: test.lua:17: col value outside range

In the opts parameter, there are more options to change the behavior of this function. virt_text is a table of table, where each sub-table contains some text and the highlight group for it.

The option virt_text_pos has three possible values:

• overlay: overlay the virtual text on the column specified with col_num, this is actually the only case where col_num really works.
• eol: virtual text is placed at the end of the line (col_num does not take effect).
• right_align: virtual text is placed on the right of current line (col_num does not take effect).

Check the title image for a comparison between the three position method.

To actually put virtual text on any column, we should use virt_text_win_col in the opts table. If we use virt_text_win_col, virt_text_pos does not take effect any more.

To delete a virtual text, use API function nvim_buf_del_extmark(bnr, ns_id, id), where id is the id used in opts.

References

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• https://www.reddit.com/r/neovim/comments/lppnvn/indent_blankline_can_now_display_indent_guides_on/