/usr/share/bash-completion/completions/pivot_root is in util-linux 2.27.1-6ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21  | _pivot_root_module()
{
	local cur prev
	COMPREPLY=()
	cur="${COMP_WORDS[COMP_CWORD]}"
	prev="${COMP_WORDS[COMP_CWORD-1]}"
	case $prev in
		'-h'|'--help'|'-V'|'--version')
			return 0
			;;
	esac
	case $COMP_CWORD in
		1|2)
			local IFS=$'\n'
			compopt -o filenames
			COMPREPLY=( $(compgen -o dirnames -- ${cur:-"/"}) )
			;;
	esac
	return 0
}
complete -F _pivot_root_module pivot_root
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