It's either https://github.com/rust-lang/rust/issues/112905 https://github.com/rust-lang/rust/issues/112905, https://github.com/rust-lang/rust/issues/84366 http

It's either https://github.com/rust-lang/rust/issues/112905 https://github.com/rust-lang/rust/issues/112905, https://github.com/rust-lang/rust/issues/84366 https://github.com/rust-lang/rust/issues/84366 or (most likely) https://github.com/rust-lang/rust/issues/25860 https://github.com/rust-lang/rust/issues/25860. It can be argued that those are bugs of the compiler, not the language (but in practice that doesn't matter, it's unsound in the only way that matters now!) and that it's very unlikely to be hit by accident (but someone can leverage it intentionally to cause unsoundness in their library, without the presence of unsafe).