fix(defaults): visual mode star (*,#) is fragile

Problem:
Visual mode "*", "#" mappings don't work on text with "/", "\", "?", and
newlines.

Solution:
Get the visual selection and escape it as a search pattern.
Add functions vim.get_visual_selection and _search_for_visual_selection.

Fix #21676

runtime/doc/lua.txt

@@ -1410,6 +1410,25 @@ deprecate({name}, {alternative}, {version}, {plugin}, {backtrace})
     Return: ~
         Deprecated message, or nil if no message was shown.
 
+                                                  *vim.get_visual_selection()*
+get_visual_selection({list}, {append_empty})
+    Gets the content of the visual selection.
+
+    The result is either a string or, if {list} is `true`, a list of strings.
+    If not in any |visual-mode|, `nil` is returned.
+
+    Parameters: ~
+      • {list}          boolean|nil Return a list of strings instead of a
+                        string. See |getreg()|. Defaults to `false`.
+      • {append_empty}  boolean|nil Append an empty string to the result when
+                        in |linewise-visual| mode and {list} is `true`. This
+                        will preserve the trailing newline of the selection
+                        when the result is concatenated with `"\n"`. Defaults
+                        to `false`.
+
+    Return: ~
+        string|table
+
 inspect({object}, {options})                                   *vim.inspect()*
     Gets a human-readable representation of the given object.
 

runtime/doc/vim_diff.txt

@@ -119,10 +119,11 @@ of these in your config by simply removing the mapping, e.g. ":unmap Y".
 	nnoremap <C-L> <Cmd>nohlsearch<Bar>diffupdate<Bar>normal! <C-L><CR>
 	inoremap <C-U> <C-G>u<C-U>
 	inoremap <C-W> <C-G>u<C-W>
-	xnoremap * y/\V<C-R>"<CR>
-	xnoremap # y?\V<C-R>"<CR>
 	nnoremap & :&&<CR>
 <
+Default mappings composed of Lua code are not listed above. Use ":map" to see
+the |map-listing|.
+
 DEFAULT AUTOCOMMANDS
 							*default-autocmds*
 Default autocommands exist in the following groups. Use ":autocmd! {group}" to

runtime/lua/vim/_editor.lua

@@ -435,7 +435,7 @@ vim.cmd = setmetatable({}, {
 do
   local validate = vim.validate
 
-  --@private
+  ---@private
   local function make_dict_accessor(scope, handle)
     validate({
       scope = { scope, 's' },
@@ -535,6 +535,55 @@ function vim.region(bufnr, pos1, pos2, regtype, inclusive)
   return region
 end
 
+--- Gets the content of the visual selection.
+---
+--- The result is either a string or, if {list} is `true`, a list of strings.
+--- If not in any |visual-mode|, `nil` is returned.
+---
+--- @param list boolean|nil
+---        Return a list of strings instead of a string.  See |getreg()|.
+---        Defaults to `false`.
+--- @param append_empty boolean|nil
+---        Append an empty string to the result when in |linewise-visual| mode and {list} is `true`.
+---        This will preserve the trailing newline of the selection when the result is concatenated with `"\n"`.
+---        Defaults to `false`.
+--- @return string|table
+function vim.get_visual_selection(list, append_empty)
+  list = list or false
+  append_empty = append_empty or false
+
+  local mode = vim.api.nvim_get_mode().mode
+  if mode ~= 'v' and mode ~= 'V' and mode:byte() ~= 22 then
+    return nil
+  end
+
+  local reg_name_unnamed = '"'
+  local reg_name_yank = '0'
+
+  local reg_info_unnamed = vim.fn.getreginfo(reg_name_unnamed)
+  local reg_info_yank = vim.fn.getreginfo(reg_name_yank)
+  local opt_clipboard = vim.o.clipboard
+  local opt_report = vim.o.report
+
+  vim.o.clipboard = ''
+  vim.o.report = vim.v.maxcol
+
+  vim.api.nvim_feedkeys('y', 'nx', false)
+
+  local yanked = vim.fn.getreg(reg_name_yank, 1, list)
+
+  vim.fn.setreg(reg_name_unnamed, reg_info_unnamed)
+  vim.fn.setreg(reg_name_yank, reg_info_yank)
+  vim.o.clipboard = opt_clipboard
+  vim.o.report = opt_report
+
+  if list and append_empty and mode == 'V' then
+    table.insert(yanked, '')
+  end
+
+  return yanked
+end
+
 --- Defers calling {fn} until {timeout} ms passes.
 ---
 --- Use to do a one-shot timer that calls {fn}
@@ -985,7 +1034,7 @@ end
 ---                              Defaults to "Nvim".
 ---@param backtrace   boolean|nil Prints backtrace. Defaults to true.
 ---
----@returns Deprecated message, or nil if no message was shown.
+---@return Deprecated message, or nil if no message was shown.
 function vim.deprecate(name, alternative, version, plugin, backtrace)
   local msg = ('%s is deprecated'):format(name)
   plugin = plugin or 'Nvim'
@@ -1010,9 +1059,28 @@ end
 function vim._init_default_mappings()
   -- mappings
 
-  --@private
+  ---@private
+  local function _search_for_visual_selection(search_prefix)
+    if search_prefix ~= '/' and search_prefix ~= '?' then
+      return
+    end
+    -- Escape these characters
+    local replacements = {
+      [search_prefix] = [[\]] .. search_prefix,
+      [ [[\]] ] = [[\\]],
+      ['\t'] = [[\t]],
+      ['\n'] = [[\n]],
+    }
+    local pattern = '[' .. table.concat(vim.tbl_keys(replacements), '') .. ']'
+    local visual_selection = vim.get_visual_selection(false)
+    local escaped_visual_selection = string.gsub(visual_selection, pattern, replacements)
+    local search_cmd = search_prefix .. [[\V]] .. escaped_visual_selection .. '\n'
+    vim.api.nvim_feedkeys(search_cmd, 'nx', true)
+  end
+
+  ---@private
   local function map(mode, lhs, rhs)
-    vim.api.nvim_set_keymap(mode, lhs, rhs, { noremap = true, desc = 'Nvim builtin' })
+    vim.keymap.set(mode, lhs, rhs, { noremap = true, desc = 'Nvim builtin' })
   end
 
   map('n', 'Y', 'y$')
@@ -1020,8 +1088,12 @@ function vim._init_default_mappings()
   map('n', '<C-L>', '<Cmd>nohlsearch<Bar>diffupdate<Bar>normal! <C-L><CR>')
   map('i', '<C-U>', '<C-G>u<C-U>')
   map('i', '<C-W>', '<C-G>u<C-W>')
-  map('x', '*', 'y/\\V<C-R>"<CR>')
+  map('x', '*', function()
-  map('x', '#', 'y?\\V<C-R>"<CR>')
+    _search_for_visual_selection('/')
+  end)
+  map('x', '#', function()
+    _search_for_visual_selection('?')
+  end)
   -- Use : instead of <Cmd> so that ranges are supported. #19365
   map('n', '&', ':&&<CR>')