For some reason, today, when I logged into Mac Mail I discovered my Trash folder was missing. What the hell? I then went into the account settings and also discovered that under Accounts > iCloud > Mailbox Behaviors my Trash Mailbox was set to none. What the hell?

I then went to iCloud.com and sure enough my Trash folder was there, but it was empty. I tried to delete something and refresh Mac Mail and a new folder showed up called "Deleted Messages" but it wasn't the fun normal "Trash" Icon from before:

So after an hour of screwing around with this whole thing, it turns out all you have to do is in Accounts > iCloud > Mailbox Behaviors and select this "Deleted Messages" as the trash folder:

(By the way, it's not the All Trash one!)

Once you do that the "Deleted Messages" magically turns back into the "Trash" (with the trash icon) in your sidebar!

What...the....hell! lol Hope that helps someone.