Google Suggest is a very useful feature in Google Search, it suggests topics to you as you type words, however Gmail has been missing that feature for quite sometime, and the news that Gmail search would be getting a search Autocomplete feature definitely brought some joy to us.
This new feature is a part of Gmail Labs, so you will have to enable it by visiting Settings > Labs and selecting the enable option for Search Autocomplete.
Once you have enabled this feature, the search box will start suggesting results as you type.
Gmail also allows you to create complex geeky queries by providing autocomplete for them, for example, if you search for has in the search box, you will be shown suggestions searching emails which contain certain attachments or calendar events.
Selecting an option from the drop down list will convert it to a geeky query, this happens because, Gmail wants to give users the option to include additional filtering capabilities to the query, for example if you may also want to include GIF and BMP images, you can change the query to filename:(jpg OR jpeg OR png OR gif OR bmp), thus giving you more control over the search terms.
This is something we were definitely waiting for, go ahead and enable the labs feature to begin using search autocomplete in Gmail.
Gmail search made easier (and lazier) [Official Gmail Blog]