Hello everyone,
I would like to create a library that provides some access to another library. In other word, I create a Holder struct that contains a V struct on which I can apply some changes. I tried to keep the example below as simple as possible.
Since the V struct is external to my crate, I assume I cannot do any change to its code. In practice, V is a complex struct and cloning it comes at a large cost.
For the Holder struct, methods can only take references to Holder, not move objects. This restriction is very important.
Do you know if it is still possible to call V.add without actually moving other: &Holder inside the Holder.add method?
Thanks!
use std::rc::Rc;
/// Some example struct I cannot directly edit
/// And that does not implement the Copy trait
#[derive(Debug)]
struct V {
value: i32,
}
impl V {
fn add(&mut self, other: Self) {
self.value += other.value;
}
}
/// Some struct I write
/// to store an inner struct V
#[derive(Debug)]
struct Holder {
value: Rc<V>,
}
impl Holder {
/// Function that calls V.add
/// Here, I can only references to Holder
/// either &Self or &mut Self
fn add(&mut self, other: &mut Self) {
// How to call V.add without cloning
// Holder.value, since it can cost a lot of time?
let other: V = Rc::try_unwrap(other.value).unwrap();
self.value.add(other); // This does not work, obviously
}
}
fn main() {
let mut v1 = V { value: 1 };
let mut v2 = V { value: 2 };
let mut h1 = Holder { value: Rc::new(v1) };
let mut h2 = Holder { value: Rc::new(v2) };
h1.add(&mut h2);
println!("h1: {:?}", h1);
println!("h2: {:?}", h2);
}
Errors:
Compiling playground v0.0.1 (/playground)
error[E0507]: cannot move out of `other.value` which is behind a mutable reference
--> src/main.rs:30:39
|
30 | let other: V = Rc::try_unwrap(other.value).unwrap();
| ^^^^^^^^^^^ move occurs because `other.value` has type `Rc<V>`, which does not implement the `Copy` trait
error[E0596]: cannot borrow data in an `Rc` as mutable
--> src/main.rs:31:9
|
31 | self.value.add(other); // This does not work, obviously
| ^^^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable
|
= help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `Rc<V>`
warning: variable does not need to be mutable
--> src/main.rs:36:9
|
36 | let mut v1 = V { value: 1 };
| ----^^
| |
| help: remove this `mut`
|
= note: `#[warn(unused_mut)]` on by default
warning: variable does not need to be mutable
--> src/main.rs:37:9
|
37 | let mut v2 = V { value: 2 };
| ----^^
| |
| help: remove this `mut`
Some errors have detailed explanations: E0507, E0596.
For more information about an error, try `rustc --explain E0507`.
warning: `playground` (bin "playground") generated 2 warnings
error: could not compile `playground` due to 2 previous errors; 2 warnings emitted