rust - Borrowing references to attributes in a struct

Question

It seems that if you borrow a reference to a struct field, the whole struct is considered borrowed. I've managed to isolate and example of what I want to do. I just want to get a "read-only" reference to a field in B to obtain some data and then modify another field of B. Is there a idiomatic Rust way to do this?

struct A {
    i: i32,
}

struct B {
    j: i32,
    a: Box<A>,
}

impl B {
    fn get<'a>(&'a mut self) -> &'a A {
        &*self.a
    }
    fn set(&mut self, j: i32) {
        self.j = j
    }
}

fn foo(a: &A) -> i32 {
    a.i + 1
}

fn main() {
    let a = Box::new(A { i: 47 });
    let mut b = B { a: a, j: 1 };
    let a_ref = b.get();
    b.set(foo(a_ref));
}

error[E0499]: cannot borrow `b` as mutable more than once at a time
  --> src/main.rs:27:5
   |
26 |     let a_ref = b.get();
   |                 - first mutable borrow occurs here
27 |     b.set(foo(a_ref));
   |     ^ second mutable borrow occurs here
28 | }
   | - first borrow ends here

Answer 1

It's a feature of the language. From the compiler point of view, there is no way for it to know that it's safe to call your set() function while a is borrowed via get().

Your get() function borrows b mutably, and returns a reference, thus b will remain borrowed until this reference goes out of scope.

You have several way of handling this:

1. Separate your two fields into two different structs

2. Move the code which needs to access both attribute inside a method of B

3. Make your attributes public, you will thus be able to directly get references to them

4. Compute the new value before setting it, like this:

   fn main() {
       let a = Box::new(A { i: 47 });
       let mut b = B { a: a, j: 1 };
       let newval = {
           let a_ref = b.get();
           foo(a_ref)
       };
       b.set(newval);
   }