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

Question

I've got this simplified Rust code:

use std::io::Result;

pub trait PacketBuffer {}

pub trait DnsRecordData {
    fn write<T: PacketBuffer>(&self, buffer: &mut T) -> Result<usize>;
}

pub struct DnsRecord<R: DnsRecordData + ?Sized> {
    pub data: Box<R>,
}

pub struct DnsPacket {
    pub answers: Vec<DnsRecord<DnsRecordData>>,
}

fn main() {}

The intention is that DnsRecord should be able to hold any struct implementing the DnsRecordData trait, with the different structs representing A, AAAA, CNAME etc.

This fails with the error:

error: the trait bound `DnsRecordData + 'static: DnsRecordData` is not satisfied [--explain E0277]
  --> src/main.rs:14:5
   |>
14 |>     pub answers: Vec<DnsRecord<DnsRecordData>>,
   |>     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: required by `DnsRecord`

error: the trait `DnsRecordData` cannot be made into an object [--explain E0038]
  --> src/main.rs:14:5
   |>
14 |>     pub answers: Vec<DnsRecord<DnsRecordData>>,
   |>     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: method `write` has generic type parameters

What's got me the most confused is that by removing the generics from DnsRecordData::write() , it compiles just fine:

use std::io::Result;

pub trait PacketBuffer {}

pub trait DnsRecordData {
    fn write(&self, buffer: &mut PacketBuffer) -> Result<usize>;
}

pub struct DnsRecord<R: DnsRecordData + ?Sized> {
    pub data: Box<R>,
}

pub struct DnsPacket {
    pub answers: Vec<DnsRecord<DnsRecordData>>,
}

fn main() {}

If anyone can explain what I'm missing, I'd very much appreciate it.

Answer 1

The intention is that DnsRecord should be able to hold any struct implementing the DnsRecordData trait

That's not what the code says.

Vec<DnsRecord<DnsRecordData>>

This is a vector of the struct DnsRecord containing the trait DnsRecordData . If you want "any struct implementing the DnsRecordData trait", you need a generic:

pub struct DnsPacket<D> 
    where D: DnsRecordData
{
    pub answers: Vec<DnsRecord<D>>,
}

Traits can be implemented , but they also have their own type. In order to create this type, the trait needs to be object-safe - Trait Object is not Object-safe error .

As the error message states, this trait cannot be a trait object because there are generic types on the method.

The first error states that DnsRecord requires that whatever type it is parameterized with must implement DnsRecordData . However, the type of the trait object doesn't actually implement that. Normally, you'd use a trait object via a reference ( &DnsRecordData ) or a box ( Box<DnsRecordData> ), both of which should implement the trait, preventing this error.

Answer 2

The second 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 .