trait MyTrait {}
struct MyStruct;
impl MyTrait for MyStruct {}

fn compiles1() -> Box<dyn MyTrait> {
    Box::new(MyStruct)
}

fn compiles2() -> Box<dyn MyTrait> {
    let value = Box::new(MyStruct);
    value
}

struct MyWrapper<A>(A);

fn compiles3() -> MyWrapper<Box<dyn MyTrait>> {
    MyWrapper(Box::new(MyStruct))
}

fn does_not_compile() -> MyWrapper<Box<dyn MyTrait>> {
    let value = MyWrapper(Box::new(MyStruct));
    value
}

// after talking to the discord channel:

impl<T, U> CoerceUnsized<MyWrapper<U>> for MyWrapper<T> where T: CoerceUnsized<U> {}

fn now_it_compiles() -> MyWrapper<Box<dyn MyTrait>> {
    let value = MyWrapper(Box::new(MyStruct));
    value
}

// though this is not a good solution in practice, it's interesting