There are a few places in Rust and its stdlib that people seek to validate with formal methods, but usually these efforts are first focused on proving fundamental pieces (such as std::pin
https://doc.rust-lang.org/std/pin/index.html , which I believe has been integrated into Ralf Jung's formal model), and only later on outlying APIs such as this (the rationale being that fundamental APIs would be enormously more disruptive to fix if unsoundness should be discovered).
As to your conclusion though, obligatory shout-out to Huon Wilson, who discovered the one other instance of unsoundness in a newly-stabilized API (this one due to a typo, therefore much simpler to resolve), mere hours after the stable release of Rust 1.15: https://www.reddit.com/r/rust/comments/5roiq7/announcing_rus... :)