Is it possible to explicitly set a struct pointer to null?

[undefinedmaniac]

I am trying to make a struct field optional by using the null pointer state to represent that the value is not present (this is mentioned in the Cap'n Proto FAQ under "How do I make a field optional?").

Using this schema as an example:

struct Foo {}

struct Bar {
    inner @0 :Foo;
}

struct MyUnion {
    union {
        none @0 :Void;
        some @1 :Foo;
    }
}

Is there a way to create an instance of Bar with inner explicitly set to null? E.g.

let mut root = builder.init_root::<example_capnp::bar::Builder>();
root.set_inner(None); // does not work

If I omit all calls to init_inner/set_inner, the default value is null, however this creates a problem for union types. Imagine I want to create an instance of MyUnion with variant some and Foo set to null.

let mut root = builder.init_root::<example_capnp::my_union::Builder>();

// Required to set the union variant to 'some', but also initializes the pointer so that Foo is not null
let some = root.reborrow().init_some();

// I would like a way to call init_some() without making has_some() == true
assert!(root.has_some())

Ideally, on the receiver side I would be able to do something like:

if let Which::Some(r) = reader.reborrow().which().unwrap() {
    if reader.has_some() {
        // Struct is set to a value
    } else {
        // Struct is 'not present'
    }
}