One of Google’s most underrated yet extremely useful search feature is auto-complete. Just type in the query and it shows auto-complete options. Gmail also has a search bar which is extremely useful for looking up old email exchanges you might want to look up.
Gmail also has auto-complete feature in its search bar but it only used to prompt with addresses or titles of email.
The new auto-complete as announced by Gmail, will be made more relevant to the email you are viewing currently.
Lets say you are reading a email from a friend about a book he recently bought, then when you search the auto-complete will show up more options which are related to books rather than just contacts. This will be data used from your emails in the past itself.
The change seems rather small and will be hardly noticed by most users except that their email search will get a lot more intuitive.
Scanning Emails is not really Evil!
This sort of auto-complete feature actually is a good example of how Google improves the email experience by crawling your emails.
Do drop in your comments.