Which Function mode is a minor mode that displays the current function name in the mode line, updating it as you move around in a buffer.
To enable (or disable) Which Function mode, use the command M-x
which-function-mode. This command is global; it applies to all
buffers, both existing ones and those yet to be created. However, it
takes effect only in certain major modes, those listed in the value of
which-func-modes. If the value is t, then Which Function
mode applies to all major modes that know how to support it—in other
words, all the major modes that support Imenu.