rc_borrow_mut

This crate can be used to mutably borrow the contents of an Rc, if there are no other strong references, despite any Weak references.

⚠ Warning ⚠

This crate is unsound, as it can result in use-after-free in safe code.

let mut rc = Rc::new("asdf".to_string());
std::mem::forget(Rc::borrow_mut(&mut rc));
drop(rc.clone());
println!("use after free: {rc:?}");

Also, this crate requires specific Rust nightly versions. Currently, it targets nightly-2026-03-01 and may not compile on other versions.

Example

use std::rc::Rc;
use rc_borrow_mut::RcBorrowMut;

let mut rc = Rc::new(0);
let weak = Rc::downgrade(&Rc::clone(&rc));

// Wont panic, since there are no other strong references.
let mut mutable = Rc::borrow_mut(&mut rc);

*mutable += 1;

// Weak references don't work during mutation.
assert!(weak.upgrade().is_none());

*mutable += 1;
drop(mutable);

// Weak references start working again after we are done mutating.
assert_eq!(*weak.upgrade().unwrap(), 2);

Other Warnings

• Requires nightly Rust
• Uses unsafe
• Depends on unspecified implementation details of Rc
• Not rigorously verified

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.