Very often when searching, you would want to search for email addresses. Allow the user to use the configured address-book-cmd to complete email addresses. For maildir/imap :search and :filter it might be enough to add completions to the appropriate flages.

For notmuch it's a bit more complicated for two reasons:

• no space between search term and address, e.g :cf to:x@y.com
• complex logic, e.g. :cf to:(x@y.com OR z@hello.io)
• you can also supply regex to both to: and from:

One solution to this could be a configurable bind to trigger address completion, like the built-in vim completions. Another could be trying to be really smart to figure out when to complete. The former might be conceptually much easier, the latter might pave the way for further completion support for other notmuch search operators, e.g. mimetype, path and folder.