如梦一场空
[rú mèng yī chăng kōng]
It refers to a transient, dreamlike experience where everything felt real during its happening but ended up vanishing leaving no trace behind like it never existed, conveying a sense of emptiness and impermanence, much akin to the feeling after waking from a vivid dream only to realize nothing substantial remains in wake except ephemeral recollections dissipating quickly as time passes.