fn present<'de, D, T>(de: D) -> Result<Option<T>, D::Error>where
D: Deserializer<'de>,
T: Deserialize<'de>,Expand description
Reads a field that may be present and null.
Option<Option<T>> with this on it distinguishes an absent key from one
written as null, which Option<T> alone cannot. putRecord declares
swapRecord nullable and an explicit null there means “assert this
record does not exist yet” – a different request from not asking at all,
and one this server has to be able to see in order to refuse it.