fish-shell/share/completions/cabal.fish

function __fish_complete_cabal
    if type -q -f cabal
        set cmd (commandline -poc)
        if test (count $cmd) -gt 1
            cabal $cmd[2..-1] --list-options
        else
            cabal --list-options
        end
    end
end

complete -c cabal -a '(__fish_complete_cabal)'