An Avengers Infinity War teaser trailer was shown at D23. It was exclusive to the event and presented solely for those lucky enough to get into the panel.
There are verbal descriptions by youtubers online and there is a description of the teaser at CBR.com and many other places. There are some minor spoilers.
There is a lot of action and almost all the Marvel movie characters are given some cuts in the teaser.
The film arrives in theaters May 4, 2018.