Gmail's Smart Compose wants to ghostwrite your witty subject lines
It'll suggest subjects based on the body of your email.

Sip your cappuccino, wordsmith, and let Gmail's Smart Compose feature do the writing.
Gmail's Smart Compose feature can now suggest subject lines for your emails, Google said Thursday. When you finish the body of your email and slip the cursor into the subject line, you'll see Smart Compose's suggestion.
"We've seen how Smart Compose suggestions can cut down on the effort it takes for you to write emails and replies," Google said in a blog post.
Gmail's Smart Compose makes subject suggestions based on the email's body.
Smart Compose came along last year. It uses artificial intelligence to guess what users are going to type before they type it and bases some of its suggestions on the recipient as well as on the sender's typical writing habits.
If you have Smart Compose switched off, you can fire it up by going to Settings and finding Experimental Access under the General tab.