Specifying // +protobuf.nullable=true on a Go type that is an alias of a
map or slice will generate a synthetic protobuf message with the type
name that will serialize to the wire in a way that allows the difference
between empty and nil to be recorded.
For instance:
// +protobuf.nullable=true
types OptionalMap map[string]string
will create the following message:
message OptionalMap {
map<string, string> Items = 1
}
and generate marshallers that use the presence of OptionalMap to
determine whether the map is nil (rather than Items, which protobuf
provides no way to delineate between empty and nil).
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