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问题:
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.
回答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.
回答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.
