fix(treesitter)!: enforce buffer is loaded when creating parser

Problem: `vim.treesitter._create_parser()` silently loads the buffer,
          bypassing the swapfile prompt.
Solution: Error for an unloaded buffer, ensure buffer is loaded in
          `vim.treesitter.start()` instead.

runtime/doc/news.txt

@@ -175,6 +175,7 @@ TREESITTER
   the tree before returning. Scripts must call |LanguageTree:parse()| explicitly. >lua
     local p = vim.treesitter.get_parser(0, 'c')
     p:parse()
+• |vim.treesitter.get_parser()| expects its buffer to be loaded.
 <
 
 TUI

runtime/lua/vim/treesitter.lua

@@ -34,8 +34,6 @@ M.minimum_language_version = vim._ts_get_minimum_language_version()
 function M._create_parser(bufnr, lang, opts)
   bufnr = vim._resolve_bufnr(bufnr)
 
-  vim.fn.bufload(bufnr)
-
   local self = LanguageTree.new(bufnr, lang, opts)
 
   local function bytes_cb(_, ...)
@@ -102,6 +100,9 @@ function M.get_parser(bufnr, lang, opts)
       return nil, err_msg
     end
   elseif parsers[bufnr] == nil or parsers[bufnr]:lang() ~= lang then
+    if not api.nvim_buf_is_loaded(bufnr) then
+      error(('Buffer %s must be loaded to create parser'):format(bufnr))
+    end
     local parser = vim.F.npcall(M._create_parser, bufnr, lang, opts)
     if not parser then
       local err_msg =
@@ -415,6 +416,14 @@ end
 ---@param lang string? Language of the parser (default: from buffer filetype)
 function M.start(bufnr, lang)
   bufnr = vim._resolve_bufnr(bufnr)
+  -- Ensure buffer is loaded. `:edit` over `bufload()` to show swapfile prompt.
+  if not api.nvim_buf_is_loaded(bufnr) then
+    if api.nvim_buf_get_name(bufnr) ~= '' then
+      pcall(api.nvim_buf_call, bufnr, vim.cmd.edit)
+    else
+      vim.fn.bufload(bufnr)
+    end
+  end
   local parser = assert(M.get_parser(bufnr, lang, { error = false }))
   M.highlighter.new(parser)
 end