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 either 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.