Avoid Double Arc for Arc<dyn> with T: ?Sized

[PeterUlb]

Expected Behavior

It should be possible to create web::Data from an Arc<dyn Xyz>.

Current Behavior

    // trait T{}, struct A{}, impl T for A{}
    // This works as of now
    let test: Arc<Box<dyn T>> = Arc::new(Box::new(A {}));
    let data = web::Data::from(test);
    // This should also work (see above with Sized)
    let test2: Arc<dyn T> = Arc::new(A {});
    let data2 = web::Data::from(test2);

=> the size for values of type dyn T cannot be known at compilation time

Possible Solution

Add Data<T: ?Sized>
Extensions.insert uses TypeId::of::<T>, where in this case T is Data<T: ?Sized>.
To extract we can use extensions.get::<Data<dyn T>>()

Steps to Reproduce (for bugs)

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=7f31f526ea15a6c03ae768a900d822d0

Compare <T: ?Sized> (new version) with <T> (current version)

Context

I want to dynamically pass a Trait object to actix. This allows me to swap e.g. Service implementations with mock objects for unit tests.

Your Environment

• Rust Version (I.e, output of rustc -V): rustc 1.46.0 (04488afe3 2020-08-24)
• Actix Web Version: 3.0

Question is: Would it break anything else when adding ?Sized. Are there places relying on a known size? Quick glance shows that even the Arc is additionally Boxed in the Extensions HashMap.

[robjtede]

I did some testing and this idea seems to have merit. I think it would be okay as long as it is emphasized in the docs that only one (of each T) explicit dyn T is possible. Would welcome a PR to ascertain any wider impact.