query-replace-interactive

query-replace-interactive is a variable defined in `replace.el'.
Its value is

nil

• This variable is obsolete since 24.3;
  use `M-n' to pull the last incremental search string to the minibuffer that reads the string to replace, or invoke replacements from Isearch by using a key sequence like `C-s C-s M-%'.

Documentation:
Non-nil means `query-replace' uses the last search string.
That becomes the "string to replace".