Sean McArthur reported a security vulnerability affecting the standard library that caused the Error::downcast family of methods to perform unsound casts when a manual implementation of the Error::type_id method returned the wrong TypeId, leading to security issues such as out of bounds reads/writes/etc.

The Error::type_id method was recently stabilized as part of Rust 1.34.0. This point release destabilizes it, preventing any code on the stable and beta channels to implement or use it, awaiting future plans that will be discussed in issue #60784.

https://blog.rust-lang.org/2019/05/14/Rust-1.34.2.html
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-12083

This is fixed in version 1.34.2.