jackson-databind

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Currently BeanDeserializer has _ignoreAllUnknown flag but it is only set if per-class annotation @JsonIgnoreProperties(ignoreUnknown = true) (or its ConfigOverride equivalent) is specified. This flag is used to optimize handling to skip buffering for unknown properties; it would make sense to make it also consider that if DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES is disabled, property value need not be buffered.

It'd be good to test detection as well to ensure we know how settings work; perhaps by checking how BeanDeserializerBuilder works.

Note that we might want to do dynamic check for the flag, instead of modifying _ignoreAllUnknown: this because DeserializationFeatures may be changed on per-call basis (and since it can, inevitably someone will :) ). And since the case of buffering seemingly unknown property value is presumably rare, minor overhead for feature-flag check is probably fine.