Why does Rust not allow coercion to trait objects inside containers?

Question

I have a Vec> where T implements Foo. Why can I not coerce it to a Vec> even thou

Answer 1

Because Box<Bar> is a different size than Box<Foo>. The coercion is allowed on a single value, but here you'd have to resize the whole vector. The book goes into some detail on this in the section on Representation of Trait Objects. Short version: Box<Bar> is a pointer to a value. Box<Foo> is a pointer to a value and a pointer to a vtable.