Unable to create a polymorphic type because the trait cannot be made into an object

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The error comes from the fact that you can’t create trait objects for DnsRecordData due to the trait not being “object-safe”. This concept is explained in the trait objects section of The Rust Programming Language.

In your particular case, the trait contains a generic method. To create a trait object, the compiler has to synthesize a vtable for the trait, containing a function pointer for every method the trait has. But because the trait has a generic method, it effectively has as many methods as the method could be instantiated with, which is potentially infinite. Therefore, you cannot make a trait object for DnsRecordData.

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