Rust releases spend six weeks in beta, and each individual feature can spend an unlimited amount of time baking in nightly, which is usually enough time to catch potential soundness issues before they find their way into stable (this is the second point release that I can recall having such a purpose). The particular feature under which this API falls has been in nightly for quite a while, so it may be that someone had a half-formed idea of how to trigger memory unsafety and was only reminded of it when they read the release notes where its stabilization was mentioned.