/usr/share/bash-completion/completions/btrfs is in btrfs-progs 4.15.1-1build1.
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 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 | # original by Alfredo Esteban <aedelatorre at xxxxxxxxx>
# (http://www.spinics.net/lists/linux-btrfs/msg15899.html)
# edited by Joseph Wang <joequant at gmail.com>
# (http://lists.alioth.debian.org/pipermail/bash-completion-devel/2013-June/004868.html)
# edited by John C F <john.ch.fr at gmail.com> on 2015-02-02
_btrfs_devs()
{
	local DEVS
	DEVS=''; while read dev; do DEVS+="$dev "; done < <(lsblk -pnro name)
	COMPREPLY+=( $( compgen -W "$DEVS" -- "$cur" ) )
}
_btrfs_mnts()
{
	local MNTS
	MNTS=''
	while read mnt; do MNTS+="$mnt "
	done < <(mount -t btrfs | awk '{print $3}')
	COMPREPLY+=( $( compgen -W "$MNTS" -- "$cur" ) )
}
_btrfs()
{
	local cur prev words cword
	_init_completion || return
	COMPREPLY=()
	local cmd=${words[1]}
	commands='subvolume filesystem balance device scrub check rescue restore inspect-internal property send receive quota qgroup replace help version'
	commands_subvolume='create delete list snapshot find-new get-default set-default show sync'
	commands_filesystem='defragment sync resize show df du label usage'
	commands_balance='start pause cancel resume status'
	commands_device='scan add delete remove ready stats usage'
	commands_scrub='start cancel resume status'
	commands_rescue='chunk-recover super-recover zero-log'
	commands_inspect_internal='inode-resolve logical-resolve subvolid-resolve rootid min-dev-size dump-tree dump-super tree-stats'
	commands_property='get set list'
	commands_quota='enable disable rescan'
	commands_qgroup='assign remove create destroy show limit'
	commands_replace='start status cancel'
	if [[ "$cur" == -* && $cword -le 3 && "$cmd" != "help" ]]; then
		COMPREPLY=( $( compgen -W '--help' -- "$cur" ) )
		return 0
	fi
	if [[ $cword -eq 1 ]]; then
		COMPREPLY=( $( compgen -W "$commands" -- "$cur" ) )
		return 0
	elif [[ $cword -eq 2 ]]; then
		case $cmd in
			subvolume)
				opts="$commands_subvolume"
				;;
			filesystem)
				opts="$commands_filesystem"
				;;
			balance)
				opts="$commands_balance"
				;;
			device)
				opts="$commands_device"
				;;
			scrub)
				opts="$commands_scrub"
				;;
			check)
				_btrfs_devs
				return 0
				;;
			rescue)
				opts="$commands_rescue"
				;;
			restore)
				_btrfs_devs
				return 0
				;;
			inspect-internal)
				opts="$commands_inspect_internal"
				;;
			property)
				opts="$commands_property"
				;;
			send|receive)
				_filedir -d
				return 0
				;;
			quota)
				opts="$commands_quota"
				;;
			qgroup)
				opts="$commands_qgroup"
				;;
			replace)
				opts="$commands_replace"
				;;
			help)
				opts="--full"
				;;
			version)
				return 0
				;;
		esac
		COMPREPLY=( $( compgen -W "$opts" -- "$cur" ) )
		return 0
	elif [[ $cword -eq 3 ]]; then
		case $cmd in
			filesystem)
				case $prev in
					defragment)
						_filedir
						return 0
						;;
					df|usage)
						_btrfs_mnts
						return 0
						;;
					label)
						_btrfs_mnts
						_btrfs_devs
						return 0
						;;
				esac
				;;
			device|rescue)
				_btrfs_devs
				return 0
				;;
			inspect-internal)
				case $prev in
					min-dev-size)
						_btrfs_mnts
						return 0
						;;
					rootid)
						_filedir
						return 0
						;;
				esac
				;;
			receive)
				case $prev in
					-f)
						_filedir
						return 0
						;;
				esac
				;;
			replace)
				case $prev in
					status|cancel)
						_btrfs_mnts
						return 0
						;;
					start)
						_btrfs_devs
						return 0
						;;
				esac
				;;
			subvolume)
				case $prev in
					list)
						_btrfs_mnts
						return 0
						;;
				esac
				;;
		esac
	fi
	_filedir -d
	return 0
}
complete -F _btrfs btrfs
 |