> forget is not marked as unsafe, because Rust's safety guarantees do not include a guarantee that destructors will always run. For example, a program can create a reference cycle using Rc, or call process::exit to exit without running destructors. Thus, allowing mem::forget from safe code does not fundamentally change Rust's safety guarantees. ... Because forgetting a value is allowed, any unsafe code you write must allow for this possibility. You cannot return a value and expect that the caller will necessarily run the value's destructor.

https://doc.rust-lang.org/std/mem/fn.forget.html