unchecked-refcell

UncheckedRefCell is a drop-in alternative to core::cell::RefCell for performance-critical code where it is certain no borrowing rules are violated.

• In debug builds, it behaves exactly like RefCell, enforcing Rust’s borrow rules.
• In release builds, all borrow checks are skipped for maximum performance. Use this only when you are certain borrow violations cannot occur as otherwise this may lead to undefined behavior if multiple mutable references exist simultaneously.

Enabling the checked feature flag (disabled by default) forces borrow checking in release builds too. This is only intended for use with debugging.

It provides all the same APIs as RefCell, including: borrow, borrow_mut, try_borrow, try_borrow_mut, replace, replace_with, swap, get_mut, take, etc.

Features

• checked — enable runtime borrow checking, identical to RefCell.
• debug_refcell — enables tracking of borrow origins for debugging purposes.

Benchmarks

Benchmarks comparing UncheckedRefCell with std::cell::RefCell (release build):

std_refcell_borrow:   [2.5263 ms 2.5345 ms 2.5427 ms]

std_refcell_borrow_mut:   [2.9351 ms 2.9377 ms 2.9406 ms]

unchecked_refcell_borrow:  [677.66 µs 678.54 µs 679.50 µs]

unchecked_refcell_borrow_mut:   [1.0260 ms 1.0268 ms 1.0276 ms]

In release builds without checked, UncheckedRefCell is roughly 2–4x faster than RefCell.