Lan Wangji had never had call to perform the spell of silent communication before he and Wei Wuxian joined together to fight Xuanwu. He figured it was temporary. Spoiler alert: it wasn't. Or: the AU in which Lan Wangji and Wei Wuxian psychically bond in the cave, Wei Wuxian overhears Lan Wangji thinking about the fact that he actually likes him, and after several minutes of confusion, is fully on board with this.

Lan Wangji believes he and Wei Wuxian are essentially engaged. While they search for his missing betrothed, he accidentally reveals as much to Jiang Wanyin. Now everyone in the cultivation world knows about the imminent marriage, except for Wei Wuxian himself.