[feature request] parameter transformation to safely add strings to PS1, &c.

[Emanuele Torre]

It would be nice to have a parameter transformation (e.g. "${par@p}")
that expands $par to a string that will not be expanded by PS1, PS2, &c.

example:

  tmp_var=$(blabla) # this variable will not exist when PS1 is expanded
  PS1="blabla ${tmp_var@p} blabla"

I think a valid way to achieve the same behaviour in the currenct
version of bash:

  tmp_var=$(blabla)
  tmp_var=${tmp_var//\\/\\\\}
  tmp_var=${tmp_var//\`/\\\`}
  tmp_var=${tmp_var//$/\\$}
  PS1="blabla $tmp_var blabla"

But a parameter transformation would be nicer.

This parameter transformation should also make sure to not cause an
expansion when concatenated.

  var='$'
  printf '%s\n' "${var@p}" # should not expand to `$'.
  PS1="${var@P}(date)" # because this should expand to `$(date)'
                       # and not to the output of `date'.

Cheers,
  emanuele6