Odd behavior with various (q) array modifiers and non-printable characters (backspace, newline)

[Zach Riggle <zachriggle@gmail.com>]

Hello all!  It's me again, hopefully with a useful question.

TLDR: (q-) behaves unexpectedly for \010 aka "\b", and there are other
corner cases that may be unintended.

I love the (q-) modifier for array expansion, since it allows printing
out e.g. a command and then running it.

    run() {
        echo '$' "${(q-)@}"
        "$@"
    }

The docs (https://zsh.sourceforge.io/Doc/Release/Expansion.html,
search for "q-" to jump to the right area) state:

> If a q- is given (only a single q may appear), a minimal form of single quoting is used that only quotes the string if needed to protect special characters. Typically this form gives the most readable output.
> If a q+ is given, an extended form of minimal quoting is used that causes unprintable characters to be rendered using $’...’. This quoting is similar to that used by the output of values by the typeset family of commands.

However, it appears that half of the q modifiers handle 0x08 / \010 /
"\b" aka "backspace" in an unexpected manner.  Notably, (q) is
incorrect, (qq) (q+) and (q-) embed a literal backspace, which causes
unexpected output (it's there if you pipe to e.g. xxd, but it should
be escaped).

    $ mysed=( gsed "s/\ba\b/x/" )

    $ echo "aaaa a aaa" | "${mysed[@]}"
    aaaa x aaa

    $ echo ${(q)mysed}
    gsed s/\ba\b/x/

    $ echo ${(qq)mysed}
    'gsed' 's/x/'

    $ echo ${(qqq)mysed}
    "gsed" "s/\ba\b/x/"

    $ echo ${(qqqq)mysed}
    $'gsed' $'s/\ba\b/x/'

    $ echo ${(q+)mysed}
    gsed 's/x/'

    $ echo ${(q-)mysed}
    gsed 's/x/'

This issue also appears with newlines, but for these one could argue
it's entirely legitimate to have a *literal* newline that breaks the
quoted statement onto multiple lines.  Once again, the same culprits
are (qq), (q-), and (q+).  Interestingly, bare (q) is actually
incorrect.

    $ newline=( aa "b\nb" cc )

    $ echo ${(q)newline}
    aa b\nb cc

    $ echo ${(qq)newline}
    'aa' 'b
    b' 'cc'

    $ echo ${(qqq)newline}
    "aa" "b\nb" "cc"

    $ echo ${(qqqq)newline}
    $'aa' $'b\nb' $'cc'

    $ echo ${(q-)newline}
    aa 'b
    b' cc

    $ echo ${(q+)newline}
    aa 'b
    b' cc

IMHO both newline and backspace should be escaped at LEAST for (q-)
and (q+), and newline behavior for (q) is incorrect.  What's your
opinion?

Zach Riggle