In Rust, you cannot put something on the heap and "forget" to ever free it. Rust doesn't expose any mechanism in safe code to heap-allocate something that won't automatically free it for you when that thing goes out of scope (the primary way to do this is with the Box type).

As evidenced by this whole kerfuffle, you can in fact leak heap-allocated values by constructing an Rc cycle (or taking advantage of various implementation bugs in other things), or by using mem::forget() (which I believe is still marked as unsafe but seems likely to change). But none of these approaches could be characterized as "forget[ting] to ever free it", because they're not a consequence of "oops I forgot to do that".