Function forget

pub const fn forget<T>(t: T)

Takes ownership and “forgets” about the value without running its destructor.

Any resources the value manages, such as heap memory or a file handle, will linger forever in an unreachable state. However, it does not guarantee that pointers to this memory will remain valid.

• If you want to leak memory, see Box::leak.
• If you want to obtain a raw pointer to the memory, see Box::into_raw.
• If you want to dispose of a value properly, running its destructor, see mem::drop.

Safety

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.

That said, leaking resources such as memory or I/O objects is usually undesirable. The need comes up in some specialized use cases for FFI or unsafe code, but even then, ManuallyDrop is typically preferred.

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.

Examples

The canonical safe use of mem::forget is to circumvent a value’s destructor implemented by the Drop trait. For example, this will leak a File, i.e. reclaim the space taken by the variable but never close the underlying system resource:

use std::mem;
use std::fs::File;

let file = File::open("foo.txt").unwrap();
mem::forget(file);

This is useful when the ownership of the underlying resource was previously transferred to code outside of Rust, for example by transmitting the raw file descriptor to C code.

Relationship with ManuallyDrop

While mem::forget can also be used to transfer memory ownership, doing so is error-prone. ManuallyDrop should be used instead. Consider, for example, this code:

use std::mem;

let mut v = vec![65, 122];
// Build a `String` using the contents of `v`
let s = unsafe { String::from_raw_parts(v.as_mut_ptr(), v.len(), v.capacity()) };
// leak `v` because its memory is now managed by `s`
mem::forget(v);  // ERROR - v is invalid and must not be passed to a function
assert_eq!(s, "Az");
// `s` is implicitly dropped and its memory deallocated.

There are two issues with the above example:

• If more code were added between the construction of String and the invocation of mem::forget(), a panic within it would cause a double free because the same memory is handled by both v and s.
• After calling v.as_mut_ptr() and transmitting the ownership of the data to s, the v value is invalid. Even when a value is just moved to mem::forget (which won’t inspect it), some types have strict requirements on their values that make them invalid when dangling or no longer owned. Using invalid values in any way, including passing them to or returning them from functions, constitutes undefined behavior and may break the assumptions made by the compiler.

Switching to ManuallyDrop avoids both issues:

use std::mem::ManuallyDrop;

let v = vec![65, 122];
// Before we disassemble `v` into its raw parts, make sure it
// does not get dropped!
let mut v = ManuallyDrop::new(v);
// Now disassemble `v`. These operations cannot panic, so there cannot be a leak.
let (ptr, len, cap) = (v.as_mut_ptr(), v.len(), v.capacity());
// Finally, build a `String`.
let s = unsafe { String::from_raw_parts(ptr, len, cap) };
assert_eq!(s, "Az");
// `s` is implicitly dropped and its memory deallocated.

ManuallyDrop robustly prevents double-free because we disable v’s destructor before doing anything else. mem::forget() doesn’t allow this because it consumes its argument, forcing us to call it only after extracting anything we need from v. Even if a panic were introduced between construction of ManuallyDrop and building the string (which cannot happen in the code as shown), it would result in a leak and not a double free. In other words, ManuallyDrop errs on the side of leaking instead of erring on the side of (double-)dropping.

Also, ManuallyDrop prevents us from having to “touch” v after transferring the ownership to s — the final step of interacting with v to dispose of it without running its destructor is entirely avoided.