Is there any way to convert Box> to Box>?

Question

Consider such code:

trait Foo {
    fn foo(&self);
}

fn consume_func(b: Box>) {
    unimplemented!();
}

fn produce_func() -> Box<         


        

Answer 1

Here's one way to do it: dereference b ("un-boxing" it to a Box<Foo + Send>), then wrap it up immediately in another Box<T>, allowing the compiler to infer the correct T (in this case Box<Foo>).

consume_func(Box::new(*b));

This works because Box<Foo + Send> can be automatically coerced to Box<Foo>, but Box<Box<Foo + Send>> cannot be coerced to Box<Box<Foo>>.

Answer 2

Although you've stated you cannot change consume_func, others with similar issues can change it to accept a generic:

fn consume_func<F: Foo + ?Sized>(b: Box<Box<F>>) {
    unimplemented!();
}