ido-cannot-complete-command is a variable defined in `ido.el'.
Its value is
ido-completion-help
This variable may be risky if used as a file-local variable.
Documentation:
Command run when `ido-complete' can't complete any more.
The most useful values are `ido-completion-help', which pops up a
window with completion alternatives, or `ido-next-match' or
`ido-prev-match', which cycle the buffer list.
You can customize this variable.