The Scotsman reports that 20 minutes of Avengers: Infinity War is to be screened early in Edinburgh and London, featuring scenes filmed in Edinburgh. The movie is released in the US and the UK on the 26th April, with an LA premiere on the 23rd, but they state that this event will take place on the 10th April. And that the Scarlet Witch Elizabeth Olsen, the Vision Paul Bettany, and director Joe Russo will be on hand for the presentation followed by a post-screening cocktail party and dinner.
Bleeding Cool gets the word that there may be more to it. And take this with as much salt as you fancy, I can't verify sources but they certainly seem convincing.
Edinburgh Comic Con is on the 14th to 15th of April, and it is believed that both Olsen and Bettany intend to attend the show in full cosplay. It's not an official guest appearance. They're there to have fun, since they are in the city anyway. Though I'm sure the masks will come off at some point.
May I recommend Stormtrooper costumes? Always good to hide in a crowd…
And to confirm, this is the Edinburgh Comic Con in April and not the Scotland Comic Con in November. Just in case.