If this is possible, it should work, as it ties the return value lifetime to the buf argument:
fn get<'b>(&mut self, buf: &'b std::vec::Vec<u8>) -> Option<&'b StringObject>
But I'm not sure that reflects what's actually implemented.
This would allow you to call cache.get more than once:
fn get(&mut self, buf: &std::vec::Vec<u8>) -> Option<&StringObject>
But the returned value will maintain exclusive the borrow of self until dropped. So you wouldn't be able to use the result of the first call after you made the second call.