Function deep_merge 

pub fn deep_merge(dst: &Value, src: &Value) -> Value

Deep-merges src into dst (RFC 7386-ish): maps merge recursively, a handful of well-known list keys (additional_hostnames, lifecycle_hooks, dynamic_overrides) append rather than replace, other lists replace wholesale, and an explicit null in src deletes the key from the result at any depth. dst is not modified; the merged result is returned as a new value.