Oh that's just the suggestion I mentioned here https://news.ycombinator.com/item?id=31009572 (I saw it on some internals.rust-lang.org thread), but with no new syntax. It also mirrors the usage of existing beyond-UB safety markers like UnwindSafe https://doc.rust-lang.org/std/panic/trait.UnwindSafe.html but with inverted polarity (instead of asserting something is safe, it asserts that the programmer acknowledges the new kind of unsafety)

I think an interesting part of unsafe Rust is the interplay between positive and negative polarities (unsafe/safe fn vs unsafe blocks basically) and I think that adding new syntax is the way to leverage this kind of idiom in the new kind of unsafe