What's the problem this feature will solve?

I use a custom script (and I'm sure many others have similar scripts as well) for setting my prompt in Bash. It shows the name of the current virtualenv (if any) by querying the VIRTUAL_ENV environment variable, but if the virtualenv was created with a custom --prompt, it is unable to use this prompt prefix, as the activate script does not make this information available.

Describe the solution you'd like

The activate et al. scripts should set and export an environment variable named something like VIRTUAL_ENV_PROMPT_PREFIX that contains the prompt prefix (either custom or default) that virtualenv would prepend to the prompt. Ideally, this should be set even when VIRTUAL_ENV_DISABLE_PROMPT is set in case the user wants total control over their prompt.