feat(treesitter): table of contents for checkhealth, markdown (#32282)

Problem: It's difficult to navigate large structured text files (vim
help, checkhealth, Markdown).

Solution: Support `gO` for table of contents and `]]`/`[[` for moving
between headings for all these filetypes using treesitter queries.

Refactor: colorization of highlight groups is moved to the `help` ftplugin
while headings-related functionality is implemented in a private
`vim.treesitter` module for possible future use for other filetypes.

runtime/lua/vim/treesitter/_headings.lua

@@ -0,0 +1,144 @@
+local ts = vim.treesitter
+local api = vim.api
+
+--- Treesitter-based navigation functions for headings
+local M = {}
+
+-- TODO(clason): use runtimepath queries (for other languages)
+local heading_queries = {
+  vimdoc = [[
+    (h1 (heading) @h1)
+    (h2 (heading) @h2)
+    (h3 (heading) @h3)
+    (column_heading (heading) @h4)
+  ]],
+  markdown = [[
+    (setext_heading
+      heading_content: (_) @h1
+      (setext_h1_underline))
+    (setext_heading
+      heading_content: (_) @h2
+      (setext_h2_underline))
+    (atx_heading
+      (atx_h1_marker)
+      heading_content: (_) @h1)
+    (atx_heading
+      (atx_h2_marker)
+      heading_content: (_) @h2)
+    (atx_heading
+      (atx_h3_marker)
+      heading_content: (_) @h3)
+    (atx_heading
+      (atx_h4_marker)
+      heading_content: (_) @h4)
+    (atx_heading
+      (atx_h5_marker)
+      heading_content: (_) @h5)
+    (atx_heading
+      (atx_h6_marker)
+      heading_content: (_) @h6)
+  ]],
+}
+
+local function hash_tick(bufnr)
+  return tostring(vim.b[bufnr].changedtick)
+end
+
+---@class TS.Heading
+---@field bufnr integer
+---@field lnum integer
+---@field text string
+---@field level integer
+
+--- Extract headings from buffer
+--- @param bufnr integer buffer to extract headings from
+--- @return TS.Heading[]
+local get_headings = vim.func._memoize(hash_tick, function(bufnr)
+  local lang = ts.language.get_lang(vim.bo[bufnr].filetype)
+  if not lang then
+    return {}
+  end
+  local parser = assert(ts.get_parser(bufnr, lang, { error = false }))
+  local query = ts.query.parse(lang, heading_queries[lang])
+  local root = parser:parse()[1]:root()
+  local headings = {}
+  for id, node, _, _ in query:iter_captures(root, bufnr) do
+    local text = ts.get_node_text(node, bufnr)
+    local row, col = node:start()
+    --- why can't you just be normal?!
+    local skip ---@type boolean|integer
+    if lang == 'vimdoc' then
+      -- only column_headings at col 1 are headings, otherwise it's code examples
+      skip = (id == 4 and col > 0)
+        -- ignore tabular material
+        or (id == 4 and (text:find('\t') or text:find('  ')))
+        -- ignore tag-only headings
+        or (node:child_count() == 1 and node:child(0):type() == 'tag')
+    end
+    if not skip then
+      table.insert(headings, {
+        bufnr = bufnr,
+        lnum = row + 1,
+        text = text,
+        level = id,
+      })
+    end
+  end
+  return headings
+end)
+
+--- Show a table of contents for the help buffer in a loclist
+function M.show_toc()
+  local bufnr = api.nvim_get_current_buf()
+  local headings = get_headings(bufnr)
+  if #headings == 0 then
+    return
+  end
+  -- add indentation for nicer list formatting
+  for _, heading in pairs(headings) do
+    if heading.level > 2 then
+      heading.text = '  ' .. heading.text
+    end
+    if heading.level > 4 then
+      heading.text = '  ' .. heading.text
+    end
+  end
+  vim.fn.setloclist(0, headings, ' ')
+  vim.fn.setloclist(0, {}, 'a', { title = 'Help TOC' })
+  vim.cmd.lopen()
+end
+
+--- Jump to section
+--- @param opts table jump options
+---  - count integer direction to jump (>0 forward, <0 backward)
+---  - level integer only consider headings up to level
+--- todo(clason): support count
+function M.jump(opts)
+  local bufnr = api.nvim_get_current_buf()
+  local headings = get_headings(bufnr)
+  if #headings == 0 then
+    return
+  end
+
+  local winid = api.nvim_get_current_win()
+  local curpos = vim.fn.getcurpos(winid)[2] --[[@as integer]]
+  local maxlevel = opts.level or 6
+
+  if opts.count > 0 then
+    for _, heading in ipairs(headings) do
+      if heading.lnum > curpos and heading.level <= maxlevel then
+        api.nvim_win_set_cursor(winid, { heading.lnum, 0 })
+        return
+      end
+    end
+  elseif opts.count < 0 then
+    for i = #headings, 1, -1 do
+      if headings[i].lnum < curpos and headings[i].level <= maxlevel then
+        api.nvim_win_set_cursor(winid, { headings[i].lnum, 0 })
+        return
+      end
+    end
+  end
+end
+
+return M

runtime/lua/vim/treesitter/language.lua

@@ -5,6 +5,7 @@ local M = {}
 ---@type table<string,string>
 local ft_to_lang = {
   help = 'vimdoc',
+  checkhealth = 'vimdoc',
 }
 
 --- Returns the filetypes for which a parser named {lang} is used.