feat(treesitter): start moving get_parser to return nil #30313

**Problem:** `vim.treesitter.get_parser` will throw an error if no parser
can be found.

- This means the caller is responsible for wrapping it in a `pcall`,
  which is easy to forget
- It also makes it slightly harder to potentially memoize `get_parser`
  in the future
- It's a bit unintuitive since many other `get_*` style functions
  conventionally return `nil` if no object is found (e.g. `get_node`,
  `get_lang`, `query.get`, etc.)

**Solution:** Return `nil` if no parser can be found or created

- This requires a function signature change, and some new assertions in
  places where the parser will always (or should always) be found.
- This commit starts by making this change internally, since it is
  breaking. Eventually it will be rolled out to the public API.

6 files changed, 50 insertions, 28 deletions

diff --git a/runtime/lua/vim/_comment.lua b/runtime/lua/vim/_comment.lua
index efe289b3e1..c821f01257 100644
--- a/runtime/lua/vim/_comment.lua
+++ b/runtime/lua/vim/_comment.lua
@@ -9,8 +9,8 @@
 local function get_commentstring(ref_position)
   local buf_cs = vim.bo.commentstring
 
-  local has_ts_parser, ts_parser = pcall(vim.treesitter.get_parser)
-  if not has_ts_parser then
+  local ts_parser = vim.treesitter._get_parser()
+  if not ts_parser then
     return buf_cs
   end
 
diff --git a/runtime/lua/vim/treesitter.lua b/runtime/lua/vim/treesitter.lua
index 4629203138..809ea59b94 100644
--- a/runtime/lua/vim/treesitter.lua
+++ b/runtime/lua/vim/treesitter.lua
@@ -74,14 +74,14 @@ end
 
 --- Returns the parser for a specific buffer and attaches it to the buffer
 ---
---- If needed, this will create the parser.
+--- If needed, this will create the parser. If no parser can be found or created, returns `nil`.
 ---
 ---@param bufnr (integer|nil) Buffer the parser should be tied to (default: current buffer)
 ---@param lang (string|nil) Language of this parser (default: from buffer filetype)
 ---@param opts (table|nil) Options to pass to the created language tree
 ---
----@return vim.treesitter.LanguageTree object to use for parsing
-function M.get_parser(bufnr, lang, opts)
+---@return vim.treesitter.LanguageTree? object to use for parsing, or `nil` if not found
+function M._get_parser(bufnr, lang, opts)
   opts = opts or {}
 
   if bufnr == nil or bufnr == 0 then
@@ -94,18 +94,14 @@ function M.get_parser(bufnr, lang, opts)
 
   if not valid_lang(lang) then
     if not parsers[bufnr] then
-      error(
-        string.format(
-          'There is no parser available for buffer %d and one could not be'
-            .. ' created because lang could not be determined. Either pass lang'
-            .. ' or set the buffer filetype',
-          bufnr
-        )
-      )
+      return nil
     end
   elseif parsers[bufnr] == nil or parsers[bufnr]:lang() ~= lang then
-    assert(lang, 'lang should be valid')
-    parsers[bufnr] = M._create_parser(bufnr, lang, opts)
+    local parser = vim.F.npcall(M._create_parser, bufnr, lang, opts)
+    if not parser then
+      return nil
+    end
+    parsers[bufnr] = parser
   end
 
   parsers[bufnr]:register_cbs(opts.buf_attach_cbs)
@@ -113,6 +109,29 @@ function M.get_parser(bufnr, lang, opts)
   return parsers[bufnr]
 end
 
+--- Returns the parser for a specific buffer and attaches it to the buffer
+---
+--- If needed, this will create the parser.
+---
+---@param bufnr (integer|nil) Buffer the parser should be tied to (default: current buffer)
+---@param lang (string|nil) Language of this parser (default: from buffer filetype)
+---@param opts (table|nil) Options to pass to the created language tree
+---
+---@return vim.treesitter.LanguageTree object to use for parsing
+function M.get_parser(bufnr, lang, opts)
+  -- TODO(ribru17): Remove _get_parser and move that logic back here once the breaking function
+  -- signature change is acceptable.
+  local parser = M._get_parser(bufnr, lang, opts)
+  if not parser then
+    vim.notify_once(
+      'WARNING: vim.treesitter.get_parser will return nil instead of raising an error in Neovim 0.12',
+      vim.log.levels.WARN
+    )
+    error('Parser not found.')
+  end
+  return parser
+end
+
 --- Returns a string parser
 ---
 ---@param str string Text to parse
@@ -386,7 +405,7 @@ function M.get_node(opts)
 
   local ts_range = { row, col, row, col }
 
-  local root_lang_tree = M.get_parser(bufnr, opts.lang)
+  local root_lang_tree = M._get_parser(bufnr, opts.lang)
   if not root_lang_tree then
     return
   end
@@ -419,7 +438,11 @@ end
 ---@param lang (string|nil) Language of the parser (default: from buffer filetype)
 function M.start(bufnr, lang)
   bufnr = bufnr or api.nvim_get_current_buf()
-  local parser = M.get_parser(bufnr, lang)
+  local parser = M._get_parser(bufnr, lang)
+  if not parser then
+    vim.notify('No parser for the given buffer.', vim.log.levels.WARN)
+    return
+  end
   M.highlighter.new(parser)
 end
 
diff --git a/runtime/lua/vim/treesitter/_fold.lua b/runtime/lua/vim/treesitter/_fold.lua
index 7375beaf9d..49792c3891 100644
--- a/runtime/lua/vim/treesitter/_fold.lua
+++ b/runtime/lua/vim/treesitter/_fold.lua
@@ -114,7 +114,7 @@ local function compute_folds_levels(bufnr, info, srow, erow, parse_injections)
   srow = srow or 0
   erow = erow or api.nvim_buf_line_count(bufnr)
 
-  local parser = ts.get_parser(bufnr)
+  local parser = assert(ts._get_parser(bufnr))
 
   parser:parse(parse_injections and { srow, erow } or nil)
 
@@ -392,7 +392,7 @@ function M.foldexpr(lnum)
   lnum = lnum or vim.v.lnum
   local bufnr = api.nvim_get_current_buf()
 
-  local parser = vim.F.npcall(ts.get_parser, bufnr)
+  local parser = ts._get_parser(bufnr)
   if not parser then
     return '0'
   end
diff --git a/runtime/lua/vim/treesitter/_query_linter.lua b/runtime/lua/vim/treesitter/_query_linter.lua
index ea1ae5416a..30acef559e 100644
--- a/runtime/lua/vim/treesitter/_query_linter.lua
+++ b/runtime/lua/vim/treesitter/_query_linter.lua
@@ -172,7 +172,7 @@ function M.lint(buf, opts)
     --- @type (table|nil)
     local parser_info = vim.F.npcall(vim.treesitter.language.inspect, lang)
 
-    local parser = vim.treesitter.get_parser(buf)
+    local parser = assert(vim.treesitter._get_parser(buf), 'query parser not found.')
     parser:parse()
     parser:for_each_tree(function(tree, ltree)
       if ltree:lang() == 'query' then
diff --git a/runtime/lua/vim/treesitter/dev.lua b/runtime/lua/vim/treesitter/dev.lua
index e89aea2b85..606d5b27b9 100644
--- a/runtime/lua/vim/treesitter/dev.lua
+++ b/runtime/lua/vim/treesitter/dev.lua
@@ -76,10 +76,9 @@ end
 ---
 ---@package
 function TSTreeView:new(bufnr, lang)
-  local ok, parser = pcall(vim.treesitter.get_parser, bufnr or 0, lang)
-  if not ok then
-    local err = parser --[[ @as string ]]
-    return nil, 'No parser available for the given buffer:\n' .. err
+  local parser = vim.treesitter._get_parser(bufnr or 0, lang)
+  if not parser then
+    return nil, 'No parser available for the given buffer.'
   end
 
   -- For each child tree (injected language), find the root of the tree and locate the node within
@@ -539,7 +538,7 @@ local edit_ns = api.nvim_create_namespace('treesitter/dev-edit')
 local function update_editor_highlights(query_win, base_win, lang)
   local base_buf = api.nvim_win_get_buf(base_win)
   local query_buf = api.nvim_win_get_buf(query_win)
-  local parser = vim.treesitter.get_parser(base_buf, lang)
+  local parser = assert(vim.treesitter._get_parser(base_buf, lang))
   api.nvim_buf_clear_namespace(base_buf, edit_ns, 0, -1)
   local query_content = table.concat(api.nvim_buf_get_lines(query_buf, 0, -1, false), '\n')
 
@@ -596,8 +595,8 @@ function M.edit_query(lang)
   end
   vim.cmd(cmd)
 
-  local ok, parser = pcall(vim.treesitter.get_parser, buf, lang)
-  if not ok then
+  local parser = vim.treesitter._get_parser(buf, lang)
+  if not parser then
     return nil, 'No parser available for the given buffer'
   end
   lang = parser:lang()
diff --git a/runtime/lua/vim/vimhelp.lua b/runtime/lua/vim/vimhelp.lua
index 33324602c9..01af5425c3 100644
--- a/runtime/lua/vim/vimhelp.lua
+++ b/runtime/lua/vim/vimhelp.lua
@@ -33,7 +33,7 @@ end
 --- Show a table of contents for the help buffer in a loclist
 function M.show_toc()
   local bufnr = vim.api.nvim_get_current_buf()
-  local parser = vim.treesitter.get_parser(bufnr, 'vimdoc')
+  local parser = assert(vim.treesitter._get_parser(bufnr, 'vimdoc'), 'vimdoc parser not found.')
   local query = vim.treesitter.query.parse(
     parser:lang(),
     [[