pub const VARY: HeaderName;
Determines how to match future requests with cached responses.
vary HTTP response header determines how to match future request headers to decide whether a cached response can be used rather than requesting a fresh one from the origin server. It is used by the server to indicate which headers it used when selecting a representation of a resource in a content negotiation algorithm.
vary header should be set on a 304 Not Modified response exactly like it would have been set on an equivalent 200 OK response.