Make trait ChainHandle object-safe & clonable

[adizere]

Crate

relayer

Summary & problem definition

The ChainHandle trait is currently clonable thanks to its bound on Clone, which implies Sized, which in turn makes the ChainHandle non-object-safe.

https://github.com/informalsystems/ibc-rs/blob/65e4f268efc176d861d30949dc7bde049e0b3bb5/relayer/src/chain/handle.rs#L177

This means that we cannot make objects out of the ChainHandle trait. So the compiler will forbid something of the kind:

pub struct ForeignClient {
    /// The configuration of this client
    config: ForeignClientConfig,
    /// A handle to the chain hosting this client
    host_chain: dyn ChainHandle,
}

The precise error is as follows:

error[E0038]: the trait `ChainHandle` cannot be made into an object
   --> relayer/src/foreign_client.rs:59:17
    |
59  |     host_chain: Box<dyn ChainHandle>,
    |                 ^^^^^^^^^^^^^^^^^^^^ the trait `ChainHandle` cannot be made into an object
    |
   ::: relayer/src/chain/handle.rs:177:24
    |
177 | pub trait ChainHandle: Clone + Send + Sync {
    |           -----------  ----- ...because it requires `Self: Sized`
    |           |
    |           this trait cannot be made into an object...
error: aborting due to previous error
For more information about this error, try `rustc --explain E0038`.
error: could not compile `relayer`

See more details on object safety here:
https://rust-lang.github.io/rfcs/0255-object-safety.html#detailed-design

Proposal

At @romac's suggestion, we can replace the Clone bound with DynClone instead, and obtain object safety. https://github.com/kardeiz/objekt-clonable may also be an option. Then we can do this:

pub struct ForeignClient {
    /// The configuration of this client
    config: ForeignClientConfig,
    /// A handle to the chain hosting this client
    host_chain: Box<dyn ChainHandle>,
}

Alternative solutions include splitting the handle trait into the handle proper + a constructor, or adding specific where clauses cf. this. But the DynClone method seems the most robust and simple.

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