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

Question

trait FooTrait {}

struct FooStruct;

impl FooTrait for FooStruct {}

fn main() {
    let maybe_struct: Option

        

Answer 1

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

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).

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

#![feature(unsize)]

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

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