/usr/share/bash-completion/completions/losetup is in mount 2.25.2-6.
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 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 | _losetup_module()
{
	local cur prev OPTS ARG
	COMPREPLY=()
	cur="${COMP_WORDS[COMP_CWORD]}"
	prev="${COMP_WORDS[COMP_CWORD-1]}"
	case $prev in
		'-d'|'--detach')
			ARG="$(losetup --output NAME | awk '{if (1 < NR) {print}}')"
			COMPREPLY=( $(compgen -W "$ARG" -- $cur) )
			return 0
			;;
		'-j'|'--associated')
			ARG="$(losetup --output BACK-FILE | awk '{if (1 < NR) {print}}')"
			COMPREPLY=( $(compgen -W "$ARG" -- $cur) )
			return 0
			;;
		'-c'|'--set-capacity')
			ARG="$(for I in /dev/loop[0-9]*; do if [ -e $I ]; then echo $I; fi; done)"
			COMPREPLY=( $(compgen -W "$ARG" -- $cur) )
			return 0
			;;
		'-o'|'--offset'|'--sizelimit')
			COMPREPLY=( $(compgen -W "number" -- $cur) )
			return 0
			;;
		'-O'|'--output')
			# FIXME: how to append to a string with compgen?
			local OUTPUT
			OUTPUT="NAME AUTOCLEAR BACK-FILE BACK-INO
				BACK-MAJ:MIN MAJ:MIN OFFSET PARTSCAN RO
				SIZELIMIT"
			compopt -o nospace
			COMPREPLY=( $(compgen -W "$OUTPUT" -S ',' -- $cur) )
			return 0
			;;
		'-h'|'--help'|'-V'|'--version')
			return 0
			;;
	esac
	case $cur in
		-*)
			OPTS="--all
				--detach
				--detach-all
				--find
				--set-capacity
				--associated
				--list
				--offset
				--output
				--sizelimit
				--partscan
				--read-only
				--show
				--verbose
				--help
				--version"
			COMPREPLY=( $(compgen -W "${OPTS[*]}" -- $cur) )
			return 0
			;;
	esac
	local IFS=$'\n'
	compopt -o filenames
	COMPREPLY=( $(compgen -f -- $cur) )
	return 0
}
complete -F _losetup_module losetup
 |