Definition
The property that safe Rust (unsafe-free) cannot exhibit undefined behavior; soundness bugs are compiler acceptance of programs the type system intends to reject—tracked as I-unsound on rust-lang/rust.
Key Points
- 2026-10-13: Issue #164250 (
P-high,A-borrow-checker,A-variance): covariant field assignment can bypass struct lifetime requirements (T: 'static), letting non-'staticreferences survive intoDrop—use-after-scope in safe code; reproducers on stable ≥1.85.0; variants traced to early Rust releases (2026-10-13-rust-borrow-checker-covariant-lifetime-unsoundness) - Types Team: exploitation likely needs malicious or unusual
Drop; fixing all soundness holes remains high priority; PR #164277 proposed (open at report time)
Not a typical application bug—risk profile is niche unless untrusted code supplies
Dropor proc-macro-generated types.