当 Hash + PartialEq 是 Supertraits 时，无法使特征 Object 安全

Question

I'm pretty new to Rust, so there is the possibility that this will be an easy Question.

I'm trying to create a small registry for Handlers that should return any struct that implements the TransferObject Trait:

pub trait TransferObject: Hash + PartialEq {}

Since I store the Handlers that are registered in a HashMap the Trait needs the Hash and PartialEq as the Supertraits:

pub struct RequestHandlerRegistry {
    handlers: HashMap<RequestMethod, HashMap<String, RequestHandler<dyn TransferObject>>>,
}

But in the Struct I get the Error, that TransferObject can't be made into an Object since PartialEq uses the parameter Self . I already tried to do something like this:

pub struct RequestHandlerRegistry {
    handlers: HashMap<RequestMethod, HashMap<String, RequestHandler<Box<dyn TransferObject>>>>,
}

But I still get the same Error.
Is there some way to get around this?
I also created a Playground for easy recreation of the Error.

Answer 1

This doesn't work because PartialEq by default means PartialEq<Self> . When using dynamic dispatch ( dyn ) there isn't enough information to know what Self is and therefore what type of reference the methods of PartialEq can accept.

However, your question belies a misunderstanding that got you into this mess:

Since I store the Handlers that are registered in a HashMap the Trait needs the Hash and PartialEq as the Supertraits

This is only true for things you use as keys . There are no such restrictions on values .

You can therefore drop the supertraits Hash and PartialEq from this trait; instead you need to add Hash , PartialEq , and Eq to the #[derive] macro for RequestMethod , since that's what you're using as a key.

Finally, you will indeed need Box to hold the dyn TransferObject values to give them a known size.