Implement ObjectStore for trait objects

[Kinrany]

Motivation

I'm writing an application that will use Amazon S3 for long-term storage. I want to also use local file system during development. I'd like to make this configurable: it should be possible to choose the kind of storage at runtime, based on user input.

AWS CLI is one well-known application that would clearly benefit from doing this.

Problem

Generics are common in Rust. object_store itself provides PrefixObjectStore that expects a T: ObjectStore value.

Unfortunately there's no (safe?) way to go back from a trait object to an owned value that implements the trait. That is, unless trait objects explicitly implement the trait.

Solution

impl ObjectStore for Box<dyn ObjectStore>

See #3866

Alternatives

Not sure. For wrappers provided by object_store itself it might be possible to have ObjectStore
provide combinators that return them.

Another approach that only really works for sealed traits is creating an enum that includes all possible trait implementations as variants.

[tustvold]

label_issue.py automatically added labels {'object-store'} from #3866