a serde replacement, stub that does nothing
This can be useful if you have many items that derive serde Serialize/Deserialize but at some point you decided to disable this functionality and exclude serde crates from dependencies (or conditionally use them, e.g. using cargo features)
In this case instead of removing all Serialize Deserialize derives and possible field attributes (like #[serde(skip_serializing_if ...)]) you just use this crate which exposes all this features but they are empty.