#compdef su
local shell comp name usr base
[[ $words[2] != - ]]
(( base=$?+2 ))
if [[ CURRENT -eq base ]]; then
_users && return
usr=root
elif [[ CURRENT -ge base+1 ]]; then
usr=$words[base]
else
return
fi
shell="${${(M@)${(@f)$(</etc/passwd)}:#$usr*}##*:}"
compset -n $base
_dispatch $shell:t $shell $shell:t -default-