Evan Ovadia describes Valen’s new flexible borrow checker built on group borrowing (originally proposed by Nick Smith for Mojo). The design relaxes Rust’s strict shared-xor-mutable rule toward “no use-after-free”: the compiler remembers where references point and uses that to verify memory safety while allowing more interaction with reference counting, generational references and Rust interop.
Background
Group borrowing spread to other language efforts after a 2025 article: Google’s Carbon pivoted to it; Ante, Plecra, Zeta, and Fang incorporated related ideas. Valen combines group borrowing with Vale’s approach and supports calling Rust code.
The post argues three challenges matter for a “memory safety holy grail”: a more relaxed borrow checker, reading more complex data structures, and making those systems work together.
Capabilities highlighted
- Group borrowing tracks reference targets so borrows can coexist with some forms of mutation elsewhere, when lifetimes remain safe.
- Rust interop: Valen’s borrow checker is shaped so Valen code can call Rust APIs safely.
- Single borrow reference kind vs Rust’s shared/mutable split in some designs.
The article is a deep technical walkthrough (October 11, 2026) with planned syntactic sugar (auto-deref, struct indexing, type inference) noted as coming in November.
Ovadia positions Valen as pursuing power comparable to Rust’s borrow checking with flexibility closer to GC or reference counting—without abandoning compile-time guarantees.