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std::mem::forget(value) consumes value and skips its destructor; it does not itself free heap memory. If that destructor would have released a heap allocation or another resource, skipping it can leave the resource unreleased. That is allowed in safe Rust, though it can still be a memory or resource-management problem.
Does std::mem::forget free heap memory?
No. It prevents the value’s destructor from running. It is not an allocator operation that frees or reallocates memory. Whether heap memory remains allocated depends on what the value owns and what its destructor would normally do.
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For example, a Vec owns a heap allocation for its elements. When the vector is dropped, its destructor releases that backing allocation. Forgetting the vector skips that cleanup:
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let data = vec![1, 2, 3];
std::mem::forget(data);After the call, the local binding no longer owns the vector, but the vector’s destructor did not run to release its allocation. This example illustrates the mechanism; not every value passed to forget owns heap memory.
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What happens to the value and its resources?
The function takes ownership of its argument. Because that ownership has been consumed, the value will not later be dropped as the local binding goes out of scope. Any cleanup normally performed by its destructor is skipped.
- Heap-backed value: A forgotten
Veccan leave its backing allocation unreleased because the vector’s destructor does not run. - Other managed resource: A forgotten
Fileis not closed by its destructor. The standard-library documentation uses this as an example of a resource leak. - Value with no relevant destructor cleanup: Forgetting it does not necessarily create a heap leak; the effect depends on the value’s ownership and destructor behavior.
Rust’s core documentation describes forget as circumventing a value’s destructor. The Rust core source documentation explains the function and its ownership-transfer example. The standard-library Vec documentation describes the vector’s heap allocation.
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Is using mem::forget unsafe or undefined behavior?
Calling mem::forget is safe; it is not, by itself, undefined behavior. The Rust core documentation explains: “forget is not marked as unsafe, because Rust’s safety guarantees do not include a guarantee that destructors will always run.”
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Safe does not mean harmless. A skipped destructor can retain memory, leave a file descriptor open, or otherwise prevent expected cleanup. The Rustonomicon’s discussion of leaks treats memory leaks as safe in Rust’s model while noting that they can still make a program incorrect.
When is mem::forget used?
Its specialized use is suppressing cleanup when ownership of an underlying resource has been transferred elsewhere. The Rust core documentation describes a File whose raw descriptor has been handed to code outside Rust: forgetting the File prevents its destructor from closing a descriptor now owned by that external code.
This is not a general memory-management technique. If the goal is simply to let Rust release a vector, string, box, or file normally, do not forget it; ordinary ownership and scope cleanup provide that behavior.
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For carefully controlled destruction or ownership-transfer code, ManuallyDrop<T> is generally the more appropriate tool. It inhibits automatic destruction while keeping the wrapped value available for a deliberate operation. The Rust core documentation says it is typically preferred for specialized memory-ownership transfer.
| Question | mem::forget(value) |
ManuallyDrop<T> |
|---|---|---|
| What it does | Consumes the value and skips its destructor. | Wraps a value to prevent automatic destructor execution. |
| Typical role | Suppresses cleanup, including after a resource has been transferred outside Rust. | Allows destruction to be managed deliberately while retaining access to the wrapped value. |
| Main risk | Cleanup is skipped; using it in an ownership transfer can be error-prone. | Manual destruction requires care: exposing or dropping a value after it has already been dropped can violate safety requirements. |
The sequencing can be important in unsafe transfer code. With ManuallyDrop, automatic destruction is disabled before extracting raw parts. With forget, the value is consumed only after extraction, leaving a window in which a panic could trigger an unwanted drop or double-free. The documented ManuallyDrop pattern favors a leak over a double-drop in that scenario.
ManuallyDrop<T> is not an uninitialized-memory wrapper such as MaybeUninit<T>; it has the same layout and bit validity as T. Its wrapper behavior and safety considerations are documented in the standard-library ManuallyDrop documentation. Most Rust code does not need either API for ordinary ownership.
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