Possible to give away the ownership of its field from a borrowed reference?

help
1

For example,

struct Foo {i: isize}
struct Bar {f: Foo}

fn main() {
  let foo = Foo{i: 0};
  let mut bar = Bar{f: foo};
  
  // #1: it seems fine to move its field if we have bar
  // let x = bar.f;
  
  let ref_bar = &mut bar;
  // #2: but it doesn't allow to move its field from a borrowed reference
  // let y = ref_bar.f;
}

At #1, the code is allowed to move a field if we have the struct. But if we only have a borrowed reference to the struct (#2), we can't move its field.

However, it seems pretty costly to keep a struct value all the time and pass it to functions (rustc needs to generate extra code to properly destroy it). And the field is not a Copy type, which means I cannot use Cell. What is the proper way to deal with this?

Thank you.

2

Since you've only borrowed it, Rust has to make sure that you leave the struct in good shape for the owner.

One possibility is to use Option<Foo> for that field. Then the &mut borrower can call .take() which leaves None in its place. This way the borrowed struct is still in a consistent state, but you've effectively taken ownership of the value you wanted.

3 Likes
3

That is exactly what I want. Thank you!