Cargo Features

gmqtt-client = { version = "0.2.0", default-features = false, features = ["json"] }
default = json

The json feature is set by default whenever gmqtt-client is added without default-features = false somewhere in the dependency tree.

json default = serde, serde_json

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 json
serde_json json