Managing dynamic memory is hard. Some languages assume users will do it themselves (C, C++), and some languages go to extreme lengths to protect users from themselves (Java, Python). In Rust, how the language uses dynamic memory (also referred to as the heap) is a system called ownership. And as the docs mention, ownership is Rust’s most unique feature.

The heap is used in two situations; when the compiler is unable to predict either the total size of memory needed, or how long the memory is needed for, it allocates space in the heap. This happens pretty frequently; if you want to download the Google home page, you won’t know how large it is until your program runs. And when you’re finished with Google, we deallocate the memory so it can be used to store other webpages. If you’re interested in a slightly longer explanation of the heap, check out The Stack and the Heap in Rust’s documentation.

We won’t go into detail on how the heap is managed; the ownership documentation does a phenomenal job explaining both the “why” and “how” of memory management. Instead, we’re going to focus on understanding “when” heap allocations occur in Rust.

To start off, take a guess for how many allocations happen in the program below:

fn main() {}

It’s obviously a trick question; while no heap allocations occur as a result of that code, the setup needed to call main does allocate on the heap. Here’s a way to show it:

#![feature(integer_atomics)]
use std::alloc::{GlobalAlloc, Layout, System};
use std::sync::atomic::{AtomicU64, Ordering};

static ALLOCATION_COUNT: AtomicU64 = AtomicU64::new(0);

struct CountingAllocator;

unsafe impl GlobalAlloc for CountingAllocator {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        ALLOCATION_COUNT.fetch_add(1, Ordering::SeqCst);
        System.alloc(layout)
    }

    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        System.dealloc(ptr, layout);
    }
}

#[global_allocator]
static A: CountingAllocator = CountingAllocator;

fn main() {
    let x = ALLOCATION_COUNT.fetch_add(0, Ordering::SeqCst);
    println!("There were {} allocations before calling main!", x);
}

– Rust Playground

As of the time of writing, there are five allocations that happen before main is ever called.

But when we want to understand more practically where heap allocation happens, we’ll follow this guide:

• Smart pointers hold their contents in the heap
• Collections are smart pointers for many objects at a time, and reallocate when they need to grow

Finally, there are two “addendum” issues that are important to address when discussing Rust and the heap:

• Non-heap alternatives to many standard library types are available.
• Special allocators to track memory behavior should be used to benchmark code.

Smart pointers

The first thing to note are the “smart pointer” types. When you have data that must outlive the scope in which it is declared, or your data is of unknown or dynamic size, you’ll make use of these types.

The term smart pointer comes from C++, and while it’s closely linked to a general design pattern of “Resource Acquisition Is Initialization”, we’ll use it here specifically to describe objects that are responsible for managing ownership of data allocated on the heap. The smart pointers available in the alloc crate should look mostly familiar:

• Box
• Rc
• Arc
• Cow

The standard library also defines some smart pointers to manage heap objects, though more than can be covered here. Some examples are:

• RwLock
• Mutex

Finally, there is one “gotcha”: cell types (like RefCell) look and behave similarly, but don’t involve heap allocation. The core::cell docs have more information.

When a smart pointer is created, the data it is given is placed in heap memory and the location of that data is recorded in the smart pointer. Once the smart pointer has determined it’s safe to deallocate that memory (when a Box has gone out of scope or a reference count goes to zero), the heap space is reclaimed. We can prove these types use heap memory by looking at code:

use std::rc::Rc;
use std::sync::Arc;
use std::borrow::Cow;

pub fn my_box() {
    // Drop at assembly line 1640
    Box::new(0);
}

pub fn my_rc() {
    // Drop at assembly line 1650
    Rc::new(0);
}

pub fn my_arc() {
    // Drop at assembly line 1660
    Arc::new(0);
}

pub fn my_cow() {
    // Drop at assembly line 1672
    Cow::from("drop");
}

– Compiler Explorer

Collections

Collection types use heap memory because their contents have dynamic size; they will request more memory when needed, and can release memory when it’s no longer necessary. This dynamic property forces Rust to heap allocate everything they contain. In a way, collections are smart pointers for many objects at a time. Common types that fall under this umbrella are Vec, HashMap, and String (not str).

While collections store the objects they own in heap memory, creating new collections will not allocate on the heap. This is a bit weird; if we call Vec::new(), the assembly shows a corresponding call to real_drop_in_place:

pub fn my_vec() {
    // Drop in place at line 481
    Vec::<u8>::new();
}

– Compiler Explorer

But because the vector has no elements to manage, no calls to the allocator will ever be dispatched:

use std::alloc::{GlobalAlloc, Layout, System};
use std::sync::atomic::{AtomicBool, Ordering};

fn main() {
    // Turn on panicking if we allocate on the heap
    DO_PANIC.store(true, Ordering::SeqCst);

    // Interesting bit happens here
    let x: Vec<u8> = Vec::new();
    drop(x);

    // Turn panicking back off, some deallocations occur
    // after main as well.
    DO_PANIC.store(false, Ordering::SeqCst);
}

#[global_allocator]
static A: PanicAllocator = PanicAllocator;
static DO_PANIC: AtomicBool = AtomicBool::new(false);
struct PanicAllocator;

unsafe impl GlobalAlloc for PanicAllocator {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        if DO_PANIC.load(Ordering::SeqCst) {
            panic!("Unexpected allocation.");
        }
        System.alloc(layout)
    }

    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        if DO_PANIC.load(Ordering::SeqCst) {
            panic!("Unexpected deallocation.");
        }
        System.dealloc(ptr, layout);
    }
}

– Rust Playground

Other standard library types follow the same behavior; make sure to check out HashMap::new(), and String::new().

Heap Alternatives

While it is a bit strange to speak of the stack after spending time with the heap, it’s worth pointing out that some heap-allocated objects in Rust have stack-based counterparts provided by other crates. If you have need of the functionality, but want to avoid allocating, there are typically alternatives available.

When it comes to some standard library smart pointers (RwLock and Mutex), stack-based alternatives are provided in crates like parking_lot and spin. You can check out lock_api::RwLock, lock_api::Mutex, and spin::Once if you’re in need of synchronization primitives.

thread_id may be necessary if you’re implementing an allocator because thread::current().id() uses a thread_local! structure that needs heap allocation.

Tracing Allocators

When writing performance-sensitive code, there’s no alternative to measuring your code. If you didn’t write a benchmark, you don’t care about it’s performance You should never rely on your instincts when a microsecond is an eternity.

Similarly, there’s great work going on in Rust with allocators that keep track of what they’re doing (like alloc_counter). When it comes to tracking heap behavior, it’s easy to make mistakes; please write tests and make sure you have tools to guard against future issues.