Cargo Features

mqtt_packet_3_5 has no features set by default.

[dependencies]
mqtt_packet_3_5 = { version = "0.2.2", features = ["serde_support"] }
serde_support = serde

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features. These features may have been created by mistake, and this functionality may be removed in the future.

serde serde_support?