completion-setup-hook is a variable defined in `minibuffer.el'.
Its value is

  • This variable may be risky if used as a file-local variable.

Normal hook run at the end of setting up a completion list buffer.
When this hook is run, the current buffer is the one in which the
command to display the completion list buffer was run.
The completion list buffer is available as the value of `standard-output'.
See also `display-completion-list'.