Error while trying to borrow 2 fields from a struc

Question

I have a struct which contains both data and a writer which will eventually be used to write the data. The struct is wrapped in a RefCell. Here's a small reproduction:

use std::cell::RefCell;
use std::io::Write;

struct Data {
    string: String,
}

struct S {
    data: Data,
    writer: Vec<u8>,
}

fn write(s: RefCell<S>) {
    let mut mut_s = s.borrow_mut();
    let str = &mut_s.data.string;
    mut_s.writer.write(str.as_bytes());
}

The compiler is angry:

error[E0502]: cannot borrow `mut_s` as mutable because it is also borrowed as immutable
  --> src\main.rs:16:5
   |
15 |     let str = &mut_s.data.string;
   |                ----- immutable borrow occurs here
16 |     mut_s.writer.write(str.as_bytes());
   |     ^^^^^ mutable borrow occurs here
17 | }
   | - immutable borrow ends here

Is there a different API I should use?

Answer 1

You can manually invoke DerefMut and then save the resulting reference:

fn write(s: RefCell<S>) {
    let mut mut_s = s.borrow_mut();
    let mut tmp = &mut *mut_s; // Here
    let str = &tmp.data.string;
    tmp.writer.write(str.as_bytes());
}

Or in one line:

fn write(s: RefCell<S>) {
    let mut_s = &mut *s.borrow_mut(); // Here
    let str = &mut_s.data.string;
    mut_s.writer.write(str.as_bytes());
}

---

The problem is that borrow_mut doesn't return your struct directly — it returns a RefMut. Normally, this is transparent as this struct implements Deref and DerefMut, so any methods called on it are passed to the underlying type. The pseudo-expanded code looks something like this:

use std::cell::RefMut;
use std::ops::{Deref, DerefMut};

fn write(s: RefCell<S>) {
    let mut mut_s: RefMut<S> = s.borrow_mut();
    let str = &Deref::deref(&mut_s).data.string;
    DerefMut::deref_mut(&mut mut_s).writer.write(str.as_bytes());
}

Rust doesn't track field-level borrows across function calls (even for Deref::deref or DerefMut::deref_mut). This causes your error, as the deref_mut method would need to be called during the outstanding borrow from the previous Deref::deref.

The expanded version with the explicit borrow looks like this, with a single call to Deref::deref_mut:

use std::cell::RefMut;
use std::ops::DerefMut;

fn write(s: RefCell<S>) {
    let mut mut_s: RefMut<S> = s.borrow_mut();
    let tmp: &mut S = DerefMut::deref_mut(&mut mut_s);
    let str = &tmp.data.string;
    tmp.writer.write(str.as_bytes());
}

The compiler can then track that the two borrows from that temporary value are disjoint.

---

Note that this problem isn't unique to RefCell! Any type that implements DerefMut can experience the same problem. Here's some from the standard library:

• Box
• MutexGuard (from Mutex)
• PeekMut (from BinaryHeap)
• RwLockWriteGuard (from RwLock)
• String
• Vec
• Pin