Reborrow

Reborrowing is an action performed on exclusive references which creates a copy of the source reference and marks it disabled for reads and writes. This retains the exclusivity of the reference despite creating a copy, as only one copy can be used at a time.

Today, true reborrowing is only available to Rust’s exclusive &mut T references. Going beyond references means enabling true reborrowing for user-defined exclusive reference types by defining a Reborrow trait.

We want to make it possible for both of the following functions to compile: an exclusive &mut T reference and a user-defined custom exclusive reference CustomMut<'_, u32> should have equivalent semantics.

Example:

#![allow(unused)]
fn main() {
fn f(a: &mut u32) {
    f_x(a);
    f_y(a);
}

fn g(a: CustomMut<'_, u32>) {
    g_x(a);
    g_y(a);
}
}

Use cases

• User-defined exclusive references
• Wrapper types
• Marker types
• Exclusive reference collections

Approaches

The current approach to reborrowing in user-land is based on an explicit method. The current work in the Reborrow traits lang experiment is based on a marker trait.

• Marker trait approach
• Method-based approach

CoerceShared

Exclusive references call for a shared counterpart, into which an exclusive reference can be coerced into. For Rust’s references, this is &T. For user-defined exclusive reference types, a shared counterpart is a second user-defined type that is freely shareable (read: is Copy). Coercing a user-defined exclusive reference into a shared reference type requires defining a CoerceShared trait.

Resources

Tracking Issue for Reborrow trait lang experiment · Issue #145612 · rust-lang/rust

Reborrow traits - Rust Project Goals, Jul 2025

rfcs/text/0000-autoreborrow-traits.md at autoreborrow-traits · aapoalas/rfcs, May 2025

Abusing reborrowing for fun, profit, and a safepoint garbage collector (conference talk with examples), Feb 2025