[Help-bash] Expand a prompt string to calculate its length?

[Jesse Hathaway]

Is there anyway to expand a partial prompt string to calculate its length?

For instance expand:

  PS1='\h:\w$ '
  # to
  PS1='bubbles:~/tmp$'

For the purpose of determining its length, before displaying the prompt?

I would like to add a horizontal line to the first line of my PS1, but I need
to know the length of the PS1 prefix, in order to calculate the width of the
line.

Something like this:

  |bubbles:~/tmp-----------------------------------------|
  |$                                                     |