11 year old unsoundness bugs found in Rust borrow checker

A critical soundness bug has been discovered in Rust's borrow checker that has existed for 11 years, allowing a struct's lifetime requirements to be bypassed through covariant field assignment. The vulnerability enables unsafe code to compile and create use-after-free conditions, causing memory safety violations that can lead to crashes when address sanitizer is enabled. The issue affects Rust versions at least back to 1.85.0 and requires urgent attention from the types team.