Why does using Option::map to Box::new a trait object not work?

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Box::new only works with sized types; that is, it takes a value of a sized type T and returns Box<T>. In certain places a Box<T> can be coerced into a Box<U> (if T: Unsize<U>).

Such coercion does not happen in .map(Box::new), but does in Some(Box::new(s)); the latter is basically the same as Some(Box::new(s) as Box<FooTrait>).

You could create (in nightly) your own box constructor that returns boxes of unsized types like this:

#![feature(unsize)]

fn box_new_unsized<T, U>(v: T) -> Box<U>
where
    T: ::std::marker::Unsize<U>,
    U: ?Sized,
{
    Box::<T>::new(v)
}

and use it like .map(box_new_unsized). See Playground.

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