Let me start with a confession that might ruffle some feathers in the Rust community: Rust doesn’t prevent memory leaks. There, I said it. And before the pitchforks come out, let me clarify—this isn’t a bug, it’s a feature. Or rather, it’s a deliberate design decision that reveals something fascinating about what “memory safety” actually means. You see, when we evangelists talk about Rust being “memory safe,” we’re painting with a rather broad brush. We love to contrast it with C and C++, where a dangling pointer can summon demons through your nose (undefined behavior, for the uninitiated). But here’s the uncomfortable truth: Rust’s memory safety guarantees are more nuanced than the marketing materials suggest. Memory leaks are not just possible in Rust—they’re explicitly considered memory safe. If that sounds paradoxical, buckle up. We’re about to dive deep into one of Rust’s most misunderstood aspects, and I promise it’ll change how you think about safety guarantees in systems programming.

What Rust Actually Promises (And What It Doesn’t)

Let’s get our definitions straight before we go any further. When Rust claims to be “memory safe,” it’s making specific promises about what kinds of bugs it prevents at compile time: Rust prevents:

  • Use-after-free errors
  • Double frees
  • Dangling pointers
  • Data races in concurrent code
  • Buffer overflows (in safe code)
  • Null pointer dereferences Rust does NOT prevent:
  • Memory leaks
  • Logic errors
  • Deadlocks
  • Running out of memory Notice something interesting? Memory leaks aren’t on the “prevented” list. This isn’t an oversight—it’s philosophy. The Rust team decided early on that preventing memory leaks entirely would be incompatible with the language’s zero-cost abstraction goals. And honestly? They might be right, even if it makes our marketing pitches a bit more complicated. The key insight here is that memory leaks don’t violate memory safety in the technical sense. A leak means memory sits unused but allocated—it doesn’t mean you’re reading garbage values or writing to freed memory. Your program is wrong, sure, but it’s wrong in a predictable, contained way.

The Two Faces of Memory Leakage

Rust provides two main pathways to leak memory, each revealing different aspects of the language’s design philosophy. Let’s explore both with working code examples you can run yourself.

The Circular Reference Conundrum

The first and most discussed way to leak memory in Rust involves reference cycles using Rc<T> and RefCell<T>. Here’s where things get interesting from a theoretical standpoint. Rust’s ownership system is brilliant for acyclic data structures. But the moment you need a graph or a doubly-linked list, you hit a wall. The compiler can’t figure out who owns whom when A points to B and B points to A. Enter Rc<T> (reference counting) and RefCell<T> (interior mutability)—tools that let you work around ownership rules at runtime. Here’s a minimal example that leaks memory:

use std::cell::RefCell;
use std::rc::Rc;
#[derive(Debug)]
struct Node {
    value: i32,
    next: Option<Rc<RefCell<Node>>>,
}
fn create_cycle() {
    // Create two nodes
    let node_a = Rc::new(RefCell::new(Node {
        value: 1,
        next: None,
    }));
    let node_b = Rc::new(RefCell::new(Node {
        value: 2,
        next: None,
    }));
    // Create the cycle: A -> B -> A
    node_a.borrow_mut().next = Some(Rc::clone(&node_b));
    node_b.borrow_mut().next = Some(Rc::clone(&node_a));
    println!("Reference count of node_a: {}", Rc::strong_count(&node_a));
    println!("Reference count of node_b: {}", Rc::strong_count(&node_b));
    // When this function ends, both nodes have ref count of 2
    // They reference each other, so neither can be dropped
    // Memory leak achieved!
}
fn main() {
    create_cycle();
    println!("Function returned, but memory is still leaked!");
}

Run this code, and you’ll see both nodes have a reference count of 2 when the function ends. When node_a and node_b go out of scope, their reference counts drop to 1—but never to 0, which means their destructors never run, and the memory is never freed. Here’s what’s happening under the hood:

graph LR A[node_a: Rc RefCount=2] -->|next points to| B[node_b: Rc RefCount=2] B -->|next points to| A A -.->|stack reference dropped| C[Ref count now 1] B -.->|stack reference dropped| D[Ref count now 1] C -->|but still pointing to| D D -->|but still pointing to| C

The fix? Use Weak<T> references to break the cycle:

use std::cell::RefCell;
use std::rc::{Rc, Weak};
#[derive(Debug)]
struct Node {
    value: i32,
    next: Option<Rc<RefCell<Node>>>,
    prev: Option<Weak<RefCell<Node>>>, // Weak reference!
}
fn create_no_cycle() {
    let node_a = Rc::new(RefCell::new(Node {
        value: 1,
        next: None,
        prev: None,
    }));
    let node_b = Rc::new(RefCell::new(Node {
        value: 2,
        next: None,
        prev: None,
    }));
    // A -> B (strong), B -> A (weak)
    node_a.borrow_mut().next = Some(Rc::clone(&node_b));
    node_b.borrow_mut().prev = Some(Rc::downgrade(&node_a));
    println!("Strong count of node_a: {}", Rc::strong_count(&node_a));
    println!("Weak count of node_a: {}", Rc::weak_count(&node_a));
    // No leak! Weak references don't prevent deallocation
}

The Infinite Appetite of Growable Collections

The second way to leak memory is so obvious it’s almost embarrassing: just keep adding elements to a Vec and never stop. Here’s the canonical example:

fn leak_with_vec() {
    let mut data: Vec<i32> = Vec::new();
    loop {
        data.push(42);
        if data.len() % 1_000_000 == 0 {
            println!("Leaked {} MB", data.len() * 4 / 1_000_000);
        }
    }
}

This is a memory leak in the truest sense—we’re continuously allocating memory that we never free. But here’s the kicker: detecting this automatically is impossible. No, really. It’s the Halting Problem in disguise. To detect this leak, a compiler would need to determine whether data.push(42) ever stops being called. But determining if an arbitrary program halts is undecidable—one of computer science’s most famous impossibility results. So unless your loop is trivially obvious (like the one above), no static analyzer can catch it. Here’s a more realistic example that’s harder to detect:

use std::collections::HashMap;
struct LeakyCache {
    cache: HashMap<String, Vec<u8>>,
}
impl LeakyCache {
    fn new() -> Self {
        LeakyCache {
            cache: HashMap::new(),
        }
    }
    fn cache_data(&mut self, key: String, data: Vec<u8>) {
        // Oops! We never remove old entries
        // This will grow unbounded if keys are unique
        self.cache.insert(key, data);
    }
    fn get_data(&self, key: &str) -> Option<&Vec<u8>> {
        self.cache.get(key)
    }
}
fn simulate_leak() {
    let mut cache = LeakyCache::new();
    // Simulate a long-running service
    for i in 0..1_000_000 {
        // Each iteration creates a unique key
        let key = format!("user_session_{}", i);
        let data = vec![0u8; 1024]; // 1KB per entry
        cache.cache_data(key, data);
        if i % 10_000 == 0 {
            println!("Cache size: {} entries", i);
        }
    }
    println!("Total memory leaked: ~1 GB");
}

This is the kind of leak that actually happens in production. You build a cache without an eviction policy, and slowly but surely, your service starts OOMing. The compiler can’t help you here—this is perfectly valid Rust code.

Intentional Leaks: When You Want to Break the Rules

Now for something that’ll really bake your noodle: Rust provides official ways to leak memory on purpose. The standard library includes functions like std::mem::forget and Box::leak that are designed to leak.

The Forgotten Value

use std::mem::forget;
fn intentional_leak() {
    let s = String::from("This string will never be freed");
    let v = vec![1, 2, 3, 4, 5];
    // Take ownership and never run the destructor
    forget(s);
    forget(v);
    // Memory is leaked, destructors never ran
    println!("Values forgotten, memory leaked!");
}

The forget function takes ownership of a value and then… does nothing. It doesn’t run the destructor. The memory stays allocated forever (or until your process ends). Why would you want this? Sometimes you’re interfacing with C code and need to transfer ownership across the FFI boundary without running Rust destructors.

The Leaked Box

fn static_leak() {
    let x = Box::new(vec![1, 2, 3, 4, 5]);
    // Convert Box to a static reference
    let static_ref: &'static mut Vec<i32> = Box::leak(x);
    // Now we have a mutable reference that lives forever
    static_ref.push(6);
    static_ref.push(7);
    println!("Leaked vec: {:?}", static_ref);
    // Memory is never freed (unless you call Box::from_raw)
}

Box::leak is even more explicit—it converts a Box<T> into a &'static mut T. This is useful when you need something to live for the entire program lifetime but don’t want to use actual static variables.

Why This Is “Safe” (And Why That Matters)

Here’s where we need to zoom out and think philosophically about what “safety” means in programming languages. When Rust says memory leaks are “safe,” it’s making a distinction between correctness and safety. A memory leak is incorrect—your program isn’t behaving as intended. But it’s not unsafe in the sense that it violates memory safety guarantees. Let me illustrate with a comparison: Unsafe behavior (prevented by Rust):

fn use_after_free() {
    let mut v = vec![1, 2, 3];
    let ptr = v.as_ptr();
    drop(v); // Free the memory
    // This won't compile!
    // unsafe { println!("{}", *ptr); }
}

This code won’t compile because the pointer ptr would be dangling after drop(v). If we could execute it, we’d be reading freed memory—undefined behavior territory. Safe behavior (allowed by Rust):

fn memory_leak() {
    let mut v = vec![1, 2, 3];
    std::mem::forget(v); // Leak the memory
    // This compiles fine!
    println!("Memory leaked, but safely!");
}

This code leaks memory, but it never accesses invalid memory. The memory is allocated and simply stays allocated. No undefined behavior, no reading garbage values, no exploitable vulnerabilities. It’s wrong, but it’s predictably wrong. Here’s the key insight: memory safety is about preventing undefined behavior, not preventing all possible bugs. Memory leaks are bugs, but they’re not security vulnerabilities in the way that use-after-free or buffer overflows are.

The Unsafe Escape Hatch

Of course, we’d be remiss not to mention unsafe Rust, which is where you can actually create traditional memory safety violations. When you use raw pointers and manual memory management, you’re back in C territory:

fn unsafe_leak_and_use_after_free() {
    unsafe {
        use std::alloc::{alloc, dealloc, Layout};
        // Manually allocate memory
        let layout = Layout::new::<i32>();
        let ptr = alloc(layout) as *mut i32;
        // Write to it
        *ptr = 42;
        println!("Value: {}", *ptr);
        // Free it
        dealloc(ptr as *mut u8, layout);
        // This is a use-after-free bug!
        // Undefined behavior territory
        // println!("Value after free: {}", *ptr);
        // If we never called dealloc, it would be a leak
    }
}

In unsafe blocks, Rust’s guarantees go out the window. You can leak memory, use freed memory, dereference null pointers—the whole nine yards of memory corruption. But that’s the point: unsafe is an explicit opt-out of safety checking. The compiler is warning you: “I can’t help you here.”

Real-World Implications and Best Practices

So what does all this mean for writing production Rust code? Here are some practical takeaways from years of debugging memory leaks in supposedly “safe” Rust programs:

Detecting Reference Cycles

Reference cycles are the sneakiest form of leak because they look innocent. Here’s my go-to strategy for detecting them: Step 1: Use Weak<T> for back-references

use std::rc::{Rc, Weak};
struct Parent {
    children: Vec<Rc<Child>>,
}
struct Child {
    parent: Weak<Parent>, // Always use Weak for back-references
}

Step 2: Add debug logging to destructors

impl Drop for Node {
    fn drop(&mut self) {
        println!("Dropping node with value: {}", self.value);
    }
}

If you don’t see these messages when you expect to, you’ve got a leak. Step 3: Use memory profilers in production Tools like Valgrind (on Linux) or instruments (on macOS) can detect leaks even in Rust:

# On Linux with Valgrind
valgrind --leak-check=full ./your_program
# On macOS
instruments -t Leaks ./your_program

Taming Growable Collections

For the growable collection leaks, the solution is good old-fashioned engineering discipline:

use std::collections::HashMap;
struct BoundedCache<K, V> {
    cache: HashMap<K, V>,
    max_size: usize,
}
impl<K: Eq + std::hash::Hash, V> BoundedCache<K, V> {
    fn new(max_size: usize) -> Self {
        BoundedCache {
            cache: HashMap::new(),
            max_size,
        }
    }
    fn insert(&mut self, key: K, value: V) {
        if self.cache.len() >= self.max_size {
            // Remove oldest entry (in real code, use an LRU)
            if let Some(first_key) = self.cache.keys().next().cloned() {
                self.cache.remove(&first_key);
            }
        }
        self.cache.insert(key, value);
    }
}

Always bound your collections. Always. I don’t care if you think it’ll never grow that large. Murphy’s Law says it will, and it’ll happen at 3 AM on a holiday weekend.

Monitoring Memory Usage

In long-running services, implement memory monitoring:

use std::alloc::{GlobalAlloc, Layout, System};
use std::sync::atomic::{AtomicUsize, Ordering};
struct TrackingAllocator;
static ALLOCATED: AtomicUsize = AtomicUsize::new(0);
unsafe impl GlobalAlloc for TrackingAllocator {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        let ret = System.alloc(layout);
        if !ret.is_null() {
            ALLOCATED.fetch_add(layout.size(), Ordering::SeqCst);
        }
        ret
    }
    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        System.dealloc(ptr, layout);
        ALLOCATED.fetch_sub(layout.size(), Ordering::SeqCst);
    }
}
#[global_allocator]
static GLOBAL: TrackingAllocator = TrackingAllocator;
fn get_allocated_bytes() -> usize {
    ALLOCATED.load(Ordering::SeqCst)
}

This lets you track memory usage trends over time. If it’s monotonically increasing, you’ve got a leak.

The Philosophical Question: Is This a Problem?

Here’s where I’m going to get really opinionated. Is Rust’s acceptance of memory leaks a flaw in the language design? I’d argue: no, and here’s why. The alternative would be to prevent all memory leaks at compile time. But that would require one of two approaches:

  1. Garbage collection: This violates Rust’s zero-cost abstraction principle. You lose deterministic destruction, which is critical for RAII patterns and systems programming.
  2. Overly restrictive ownership rules: You’d have to ban reference counting and probably many other patterns. The language would become unusable for complex data structures. The Rust team made a pragmatic choice: prevent the memory safety violations that lead to security vulnerabilities and undefined behavior, but allow memory leaks as the lesser evil. In practice, this works out well because:
  • Memory leaks are detectable with runtime tools
  • They don’t cause silent corruption or exploits
  • They’re usually logic bugs that testing catches
  • The patterns that cause them (cycles, unbounded growth) are well-understood Compare this to C++, where you have all the same leak potential PLUS use-after-free, PLUS data races, PLUS uninitialized memory. Rust’s trade-off looks pretty good in that light.

The Uncomfortable Truth

Let’s wrap this up with some real talk. The Rust community sometimes oversells the language as a silver bullet for all memory issues. We need to be more honest about what Rust actually provides: Rust gives you:

  • Freedom from undefined behavior in safe code
  • Compile-time prevention of memory corruption bugs
  • Thread safety without data races
  • A fantastic type system for modeling invariants Rust doesn’t give you:
  • Protection from logic errors
  • Automatic memory leak detection
  • Performance for free (you still need to think)
  • A guarantee that your program is correct Memory leaks in Rust are a feature, not a bug. They’re a deliberate design decision that reflects real-world engineering trade-offs. Understanding this makes you a better Rust programmer because you stop relying on the compiler to catch everything and start thinking about the architecture of your systems. The next time someone tells you Rust prevents all memory issues, you can smile and say: “Well, actually…” And then you can show them this article and watch the cognitive dissonance set in.

What This Means for You

If you’re writing Rust in production, here are my concrete recommendations:

  1. Profile your long-running services: Memory leaks are a thing. Monitor them.
  2. Use Weak<T> liberally: Any time you have a potential cycle, use weak references.
  3. Bound all collections: Every Vec, HashMap, or custom data structure that grows needs a maximum size or eviction policy.
  4. Test destructors: Add logging to Drop implementations and verify they’re called when expected.
  5. Don’t fear unsafe: Sometimes you need manual memory management. Just understand the responsibility that comes with it.
  6. Think about ownership: The borrow checker catches most issues, but it’s not omniscient. You still need to design your data structures thoughtfully. The goal isn’t to avoid memory leaks entirely—that’s impossible in any language without GC. The goal is to understand the trade-offs, use the right tools, and build systems that are robust even when they’re not perfect. And remember: a memory leak that you can detect and fix is infinitely better than a use-after-free that silently corrupts your data and opens security holes. That’s the real Rust guarantee, and it’s a damn good one. Now go forth and leak memory safely! Just… try not to leak too much, okay?