Fix E0753: Function Return Type Cannot Have Interior Mutability
Rust’s strict type system prevents certain patterns that would lead to undefined behavior. Error E0753 is one such case where the compiler rejects code that could create safety violations. This article dissects the error, explains why Rust blocks this pattern, and provides practical alternatives.
1. Symptoms
When you encounter E0753, the compiler produces an error message similar to the following:
error[E0753]: function return type cannot have an interior mutability type
–> src/main.rs:4:20
|
4 | fn create_cell() -> Cell {
| ^^^^^^^^^ interior mutability type
|
= note: Cell<T> allows modifying shared references, which requires interior mutability
= note: function return types with interior mutability are not currently supported
The key indicators are:
- The error code `E0753` is present
- The phrase "interior mutability type" appears in the note
- The problematic return type involves `Cell`, `RefCell`, `Mutex`, `RwLock`, or similar types
A slightly different variant may appear with `RefCell`:
error[E0753]: function return type cannot have an interior mutability type
–> src/main.rs:5:1
|
5 | fn get_refcell() -> RefCell {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ interior mutability type
|
= note: RefCell<T> has interior mutability
When using `Mutex`:
error[E0753]: function return type cannot have an interior mutability type
–> src/lib.rs:6:1
|
6 | fn get_mutex() -> Mutex<Vec> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ interior mutability type
|
= note: Mutex<T> has interior mutability
## 2. Root Cause
The error stems from how Rust handles function return types at the ABI (Application Binary Interface) level. When a function returns a value, the compiler must determine:
1. How much stack space to allocate for the return value
2. Who is responsible for cleaning up the value
3. How to pass ownership from the callee to the caller
Types with interior mutability—`Cell`, `RefCell`, `Mutex`, `RwLock`, `AtomicBool`, etc.—break these assumptions because they allow mutation through shared references. The compiler needs to know the layout and ownership semantics at compile time, but interior mutability types introduce runtime-dependent behavior.
Consider what happens with `Cell<i32>`:
```rust
use std::cell::Cell;
fn broken_function() -> Cell<i32> {
Cell::new(42)
}
The compiler cannot determine at compile time whether this Cell will be mutated later. Interior mutability allows mutation through a &T (shared reference), which violates Rust’s aliasing rules if returned by value and then shared.
The Rustonomicon explains this constraint:
The return type must be statically known to not contain interior mutability that could be observed through the return path.
In essence, returning a type with interior mutability by value would allow undefined behavior because the compiler cannot guarantee the aliasing rules will be upheld.
3. Step-by-Step Fix
There are several strategies to resolve E0753, depending on your use case.
Fix 1: Return a Reference Instead
If you only need to read or modify the inner value, return a reference:
Before:
use std::cell::RefCell;
struct Cache {
data: RefCell<String>,
}
impl Cache {
fn new() -> Self {
Cache {
data: RefCell::new(String::new()),
}
}
fn get_data(&self) -> RefCell<String> { // E0753 here
self.data.clone()
}
}
After:
use std::cell::RefCell;
struct Cache {
data: RefCell<String>,
}
impl Cache {
fn new() -> Self {
Cache {
data: RefCell::new(String::new()),
}
}
fn get_data(&self) -> Ref<String> {
self.data.borrow()
}
}
The method now returns Ref<String> instead of RefCell<String>, which the compiler can handle because the borrow is tied to self.
Fix 2: Wrap in Arc for Shared Ownership
If multiple owners need access, use Arc:
Before:
use std::cell::Cell;
fn create_counter() -> Cell<i32> { // E0753
Cell::new(0)
}
After:
use std::sync::Arc;
use std::cell::Cell;
fn create_counter() -> Arc<Cell<i32>> {
Arc::new(Cell::new(0))
}
fn main() {
let counter = create_counter();
counter.set(42);
println!("Value: {}", counter.get());
}
The Arc<Cell<i32>> can be cloned and shared across threads (if Cell<i32> is Send + Sync, which it is for primitive types).
Fix 3: Use Mutex with Arc for Thread-Safe Scenarios
For thread-safe interior mutability:
Before:
use std::sync::Mutex;
fn create_lock() -> Mutex<Vec<u8>> { // E0753
Mutex::new(Vec::new())
}
After:
use std::sync::{Arc, Mutex};
fn create_lock() -> Arc<Mutex<Vec<u8>>> {
Arc::new(Mutex::new(Vec::new()))
}
fn main() {
let data = create_lock();
let mut guard = data.lock().unwrap();
guard.push(1);
println!("Data: {:?}", guard);
}
Fix 4: Return the Inner Value Directly
If you don’t need interior mutability, return the value directly:
Before:
use std::cell::RefCell;
fn get_value() -> RefCell<i32> { // E0753
RefCell::new(10)
}
After:
fn get_value() -> i32 {
10
}
fn main() {
let val = get_value();
println!("Value: {}", val);
}
If you need mutability at the call site, use a mutable binding:
fn get_value_mut() -> i32 {
42
}
fn main() {
let mut val = get_value_mut();
val += 1;
println!("Mutated value: {}", val);
}
Fix 5: Store as Field Instead of Returning
Encapsulate the interior mutability type within a struct:
Before:
use std::cell::Cell;
fn create_state() -> Cell<u64> { // E0753
Cell::new(0)
}
After:
use std::cell::Cell;
struct State {
counter: Cell<u64>,
}
impl State {
fn new() -> Self {
State {
counter: Cell::new(0),
}
}
fn increment(&self) {
self.counter.set(self.counter.get() + 1);
}
fn get(&self) -> u64 {
self.counter.get()
}
}
fn main() {
let state = State::new();
state.increment();
state.increment();
println!("Count: {}", state.get()); // Prints: 2
}
4. Verification
After applying a fix, verify it works by compiling your code:
cargo build
A successful build produces:
Compiling my-project v0.1.0
Finished dev [unoptimized + debuginfo] target(s) in 0.45s
Run your tests to ensure the behavior is correct:
cargo test
Example test verification:
use std::sync::Arc;
use std::cell::Cell;
fn create_counter() -> Arc<Cell<i32>> {
Arc::new(Cell::new(0))
}
#[test]
fn test_counter() {
let counter = create_counter();
counter.set(100);
assert_eq!(counter.get(), 100);
}
Run the test:
cargo test test_counter
Output:
running 1 test
test test_counter ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
For the struct-based approach:
#[test]
fn test_state_increment() {
let state = State::new();
state.increment();
state.increment();
state.increment();
assert_eq!(state.get(), 3);
}
Test output:
running 1 test
test test_state_increment ... ok
5. Common Pitfalls
Pitfall 1: Confusing Cell with Rc
Beginners sometimes try Rc<Cell<T>> instead of Arc<Cell<T>> for interior mutability that needs sharing. Rc is not Send + Sync, so it cannot be shared across thread boundaries:
use std::rc::Rc;
use std::cell::Cell;
fn create() -> Rc<Cell<i32>> { // Works in single-threaded contexts
Rc::new(Cell::new(0))
}
This compiles but is limited to single-threaded contexts. If you need thread safety, use Arc<Cell<T>>.
Pitfall 2: Forgetting to Unlock Mutex
When using Mutex, ensure you properly handle the Result<Guard, PoisonError>:
use std::sync::{Arc, Mutex};
let data = Arc::new(Mutex::new(vec![1, 2, 3]));
// Correct: handle the Result
if let Ok(mut guard) = data.lock() {
guard.push(4);
}
// Incorrect: ignoring the Result might cause panics on poison
// let mut guard = data.lock().unwrap(); // Panics if poisoned
Pitfall 3: Nested Interior Mutability
Avoid returning nested interior mutability types directly:
use std::cell::RefCell;
struct Wrapper {
inner: RefCell<Vec<RefCell<i32>>>,
}
impl Wrapper {
fn get_inners(&self) -> Vec<RefCell<i32>> { // E0753
self.inner.borrow().clone()
}
}
Instead, return references or a safer abstraction:
impl Wrapper {
fn get(&self, index: usize) -> Option<i32> {
self.inner.borrow().get(index).map(|r| r.get())
}
}
Pitfall 4: Using Cell in Async Contexts
Be cautious with Cell in async code. Cell is not Sync, so you cannot use it in Send futures:
use std::cell::Cell;
// This will cause issues in async contexts that need Send
async fn problematic() {
let counter = Cell::new(0u64); // Cell is not Sync
}
Use AtomicU64 or Arc<Mutex<u64>> instead:
use std::sync::Arc;
use std::sync::atomic::{AtomicU64, Ordering};
async fn correct() {
let counter = Arc::new(AtomicU64::new(0));
counter.fetch_add(1, Ordering::SeqCst);
}
Pitfall 5: Ignoring the Owned Value Semantics
When you return Arc<T>, remember that cloning increases the reference count. Monitor reference counts in long-running applications:
use std::sync::Arc;
use std::cell::Cell;
fn create_shared() -> Arc<Cell<i32>> {
Arc::new(Cell::new(42))
}
fn main() {
let original = create_shared();
let clone1 = original.clone();
let clone2 = original.clone();
// All point to the same Cell
original.set(100);
assert_eq!(clone1.get(), 100);
assert_eq!(clone2.get(), 100);
// Arc automatically cleans up when last reference drops
}
6. Related Errors
Understanding E0753 becomes easier when you know these related errors:
E0507 - Cannot Move Out of a Reference
When you try to move a value out of a borrowed context:
use std::cell::RefCell;
let r = RefCell::new(String::new());
let s = *r.borrow(); // E0507: cannot move out
E0277 - Trait Bound Not Satisfied
Occurs when your type doesn’t implement required traits:
use std::cell::Cell;
fn takes_send<T: Send>(val: T) {}
fn main() {
let cell = Cell::new(vec![1, 2, 3]);
takes_send(cell); // E0277: Cell<Vec> is not Send
}
E0505 - Cannot Move Out of Because it is Borrowed
When you try to move a value while it’s borrowed:
let v = vec![1, 2, 3];
let r = &v;
let v2 = v; // E0505: cannot move out of borrowed content
E0594 - Cannot Assign to Part of self
Related to mutation restrictions:
use std::cell::RefCell;
struct Example {
data: RefCell<Vec<i32>>,
}
impl Example {
fn modify(&self, val: i32) {
self.data.borrow_mut().push(val);
}
}
These errors often appear together when working with complex ownership patterns involving interior mutability.