Skip to content

Satisfy allocator API growth contract and update documentation #7

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
qaijuang merged 2 commits into from
Jul 7, 2026
Merged

Satisfy allocator API growth contract and update documentation #7

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
8ea052c
satisfy stdlib allocator_api impl growth contract as documented
qaijuang Jul 7, 2026
204449c
update a few outdated areas in docs
qaijuang Jul 7, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
14 changes: 10 additions & 4 deletions README.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,8 @@
Small, fixed-capacity arena allocator for single-threaded Rust programs.

You pick the capacity up front. The arena capacity never grows.
Addresses stay stable. When it is full, fallible allocation returns `None`.
Raw bump allocations stay stable. When the arena is full, fallible allocation
returns `None`.

## Choose an arena

Expand All @@ -23,7 +24,10 @@ Addresses stay stable. When it is full, fallible allocation returns `None`.
- `nightly` requires a nightly Rust toolchain and enables the unstable
standard-library `allocator_api` implementation for `BumpArena`. Typed
arenas backed by `BumpArena` also use that allocator for their internal
tracking storage.
tracking storage. The allocator implementation remains fixed-capacity, but
`grow`, `grow_zeroed`, and `shrink` can relocate blocks into remaining arena
capacity when in-place resizing is not possible, so allocator-managed blocks
must use the pointer returned from successful resize operations.

## Allocation APIs

Expand Down Expand Up @@ -134,5 +138,7 @@ any values stored in the arena must already have been dropped.
With `nightly`, `BumpArena` implements
`core::alloc::Allocator`. Per-block `deallocate` is a no-op -- memory is reclaimed
only by `reset` or by dropping the arena. `grow`, `grow_zeroed`, and `shrink`
resize only the most recent allocation in place. Drop all collections and values
that use an arena allocator before calling `reset`.
reuse the existing block when possible and otherwise may relocate it into
remaining arena capacity. Relocated old blocks are logically deallocated but
their bytes are not physically reclaimed until `reset` or drop. Drop all
collections and values that use an arena allocator before calling `reset`.
197 changes: 168 additions & 29 deletions src/bump_arena.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -23,8 +23,9 @@ use crate::{UninitAllocator, sys};
/// The arena uses incremental commitment: the initial physical footprint is
/// tiny, and pages are committed in chunks as allocations request more memory.
/// Committed memory is never decommitted until the entire arena is dropped.
/// This gives stable addresses, predictable performance, and minimal upfront
/// resource usage.
/// Raw bump allocations keep stable addresses, while allocator API resize
/// operations may return a relocated block. This gives predictable performance
/// and minimal upfront resource usage.
///
/// # Thread safety
///
Expand Down Expand Up @@ -303,7 +304,8 @@ unsafe impl UninitAllocator for BumpArena {
// Safety:
//
// Same contract as for `&BumpArena`, with the addition that `grow` may
// trigger a virtual‑memory commit if the new size requires it.
// trigger a virtual-memory commit or relocate the block into remaining arena
// capacity if in-place growth is not possible.
#[cfg(feature = "nightly")]
unsafe impl core::alloc::Allocator for BumpArena {
fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, core::alloc::AllocError> {
Expand Down Expand Up @@ -332,26 +334,45 @@ unsafe impl core::alloc::Allocator for BumpArena {
let old_end = old_offset.checked_add(old_size).ok_or(core::alloc::AllocError)?;
let is_last = old_end == offset;

if !is_last || new_size <= old_size || !old_ptr.is_multiple_of(new_layout.align()) {
if new_size < old_size {
return Err(core::alloc::AllocError);
}

let required_offset = old_offset.checked_add(new_size).ok_or(core::alloc::AllocError)?;
if required_offset > self.capacity {
return Err(core::alloc::AllocError);
}
if old_ptr.is_multiple_of(new_layout.align()) {
if new_size == old_size {
return Ok(NonNull::slice_from_raw_parts(ptr, new_size));
}

if required_offset > self.commit.get() {
let slice = self
.alloc_uninit_bump(old_offset, required_offset, new_size)
.ok_or(core::alloc::AllocError)?;
let ptr = unsafe { NonNull::new_unchecked(slice.as_mut_ptr().cast()) };
return Ok(NonNull::slice_from_raw_parts(ptr, new_size));
if is_last {
let required_offset =
old_offset.checked_add(new_size).ok_or(core::alloc::AllocError)?;
if required_offset > self.capacity {
return Err(core::alloc::AllocError);
}

if required_offset > self.commit.get() {
let slice = self
.alloc_uninit_bump(old_offset, required_offset, new_size)
.ok_or(core::alloc::AllocError)?;
let ptr = unsafe { NonNull::new_unchecked(slice.as_mut_ptr().cast()) };
return Ok(NonNull::slice_from_raw_parts(ptr, new_size));
}

self.offset.set(required_offset);
let new_ptr = unsafe { NonNull::new_unchecked(self.base.as_ptr().add(old_offset)) };
return Ok(NonNull::slice_from_raw_parts(new_ptr, new_size));
}
}

self.offset.set(required_offset);
let new_ptr = unsafe { NonNull::new_unchecked(self.base.as_ptr().add(old_offset)) };
Ok(NonNull::slice_from_raw_parts(new_ptr, new_size))
let new_block = self.allocate(new_layout)?;
unsafe {
core::ptr::copy_nonoverlapping(
ptr.as_ptr(),
new_block.as_ptr().cast::<u8>(),
old_layout.size(),
);
}
Ok(new_block)
}

unsafe fn grow_zeroed(
Expand Down Expand Up @@ -382,14 +403,27 @@ unsafe impl core::alloc::Allocator for BumpArena {
let old_end = old_offset.checked_add(old_size).ok_or(core::alloc::AllocError)?;
let is_last = old_end == offset;

if !is_last || new_size > old_size || !old_ptr.is_multiple_of(new_layout.align()) {
if new_size > old_size {
return Err(core::alloc::AllocError);
}

let new_offset = old_offset.checked_add(new_size).ok_or(core::alloc::AllocError)?;
self.offset.set(new_offset);
let new_ptr = unsafe { NonNull::new_unchecked(self.base.as_ptr().add(old_offset)) };
Ok(NonNull::slice_from_raw_parts(new_ptr, new_size))
if old_ptr.is_multiple_of(new_layout.align()) {
if is_last {
let new_offset = old_offset.checked_add(new_size).ok_or(core::alloc::AllocError)?;
self.offset.set(new_offset);
}
return Ok(NonNull::slice_from_raw_parts(ptr, new_size));
}

let new_block = self.allocate(new_layout)?;
unsafe {
core::ptr::copy_nonoverlapping(
ptr.as_ptr(),
new_block.as_ptr().cast::<u8>(),
new_layout.size(),
);
}
Ok(new_block)
}
}

Expand Down Expand Up @@ -591,6 +625,101 @@ mod tests {
assert_eq!(&values, &[1, 2]);
}

#[cfg(feature = "nightly")]
#[test]
fn allocator_grow_relocates_non_last_allocation() {
let alloc = &BumpArena::new(128);
let old = Layout::from_size_align(8, 1).unwrap();
let blocker = Layout::from_size_align(8, 1).unwrap();
let grown = Layout::from_size_align(16, 1).unwrap();

let block = alloc.allocate(old).unwrap();
let ptr = block_ptr(block);
unsafe { ptr.as_ptr().write_bytes(0xAB, old.size()) };
let blocker = alloc.allocate(blocker).unwrap();
let blocker_ptr = block_ptr(blocker);
unsafe { blocker_ptr.as_ptr().write_bytes(0xCD, 8) };

let grown_block = unsafe { alloc.grow(ptr, old, grown).unwrap() };
let grown_ptr = block_ptr(grown_block);

assert_ne!(grown_ptr, ptr);
for index in 0..old.size() {
assert_eq!(unsafe { grown_ptr.as_ptr().add(index).read() }, 0xAB);
}
for index in 0..8 {
assert_eq!(unsafe { blocker_ptr.as_ptr().add(index).read() }, 0xCD);
}
}

#[cfg(feature = "nightly")]
#[test]
fn allocator_grow_zeroed_relocation_zeroes_new_tail() {
let alloc = &BumpArena::new(128);
let old = Layout::from_size_align(4, 1).unwrap();
let blocker = Layout::from_size_align(8, 1).unwrap();
let grown = Layout::from_size_align(12, 1).unwrap();

let block = alloc.allocate(old).unwrap();
let ptr = block_ptr(block);
unsafe { ptr.as_ptr().write_bytes(0xAB, old.size()) };
alloc.allocate(blocker).unwrap();

let grown_block = unsafe { alloc.grow_zeroed(ptr, old, grown).unwrap() };
let grown_ptr = block_ptr(grown_block);

assert_ne!(grown_ptr, ptr);
for index in 0..old.size() {
assert_eq!(unsafe { grown_ptr.as_ptr().add(index).read() }, 0xAB);
}
for index in old.size()..grown.size() {
assert_eq!(unsafe { grown_ptr.as_ptr().add(index).read() }, 0);
}
}

#[cfg(feature = "nightly")]
#[test]
fn allocator_grow_error_keeps_old_allocation_untouched() {
let bump = BumpArena::new(24);
let alloc = &bump;
let old = Layout::from_size_align(8, 1).unwrap();
let blocker = Layout::from_size_align(16, 1).unwrap();
let grown = Layout::from_size_align(16, 1).unwrap();

let block = alloc.allocate(old).unwrap();
let ptr = block_ptr(block);
unsafe { ptr.as_ptr().write_bytes(0xAB, old.size()) };
alloc.allocate(blocker).unwrap();
let used = bump.used();

assert!(unsafe { alloc.grow(ptr, old, grown) }.is_err());
assert_eq!(bump.used(), used);
for index in 0..old.size() {
assert_eq!(unsafe { ptr.as_ptr().add(index).read() }, 0xAB);
}
}

#[cfg(feature = "nightly")]
#[test]
fn vec_try_reserve_relocates_when_buffer_is_not_last_allocation() {
let bump = BumpArena::new(128);
let mut values = Vec::with_capacity_in(1, &bump);
values.push(1u64);
let original = values.as_ptr();
let blocker = bump.allocate(Layout::new::<u64>()).unwrap();
let blocker_ptr = block_ptr(blocker);
unsafe { blocker_ptr.as_ptr().write_bytes(0xCD, core::mem::size_of::<u64>()) };

assert!(values.try_reserve(1).is_ok());
values.push(2);

assert_ne!(values.as_ptr(), original);
assert_eq!(&values, &[1, 2]);
for index in 0..core::mem::size_of::<u64>() {
assert_eq!(unsafe { blocker_ptr.as_ptr().add(index).read() }, 0xCD);
}
}

#[test]
fn try_new_reports_os_error_for_unreservable_capacity() {
let err = BumpArena::try_new(usize::MAX).unwrap_err();
Expand All @@ -600,21 +729,31 @@ mod tests {

#[cfg(feature = "nightly")]
#[test]
fn allocator_rejects_resize_when_pointer_does_not_fit_new_alignment() {
fn allocator_relocates_when_pointer_does_not_fit_new_alignment() {
let bump = BumpArena::new(256);
let (ptr, old) = allocate_last_block_misaligned_to(&bump, 8);
let used = bump.used();
unsafe { ptr.as_ptr().write_bytes(0xAB, old.size()) };
let grown = Layout::from_size_align(16, 8).unwrap();

assert!(unsafe { bump.grow(ptr, old, grown) }.is_err());
assert_eq!(bump.used(), used);
let grown_block = unsafe { bump.grow(ptr, old, grown).unwrap() };
let grown_ptr = block_ptr(grown_block);
assert_ne!(grown_ptr, ptr);
assert_eq!((grown_ptr.as_ptr() as usize) % grown.align(), 0);
for index in 0..old.size() {
assert_eq!(unsafe { grown_ptr.as_ptr().add(index).read() }, 0xAB);
}

let bump = BumpArena::new(256);
let (ptr, old) = allocate_last_block_misaligned_to(&bump, 8);
let used = bump.used();
unsafe { ptr.as_ptr().write_bytes(0xAB, old.size()) };
let shrunk = Layout::from_size_align(4, 8).unwrap();

assert!(unsafe { bump.shrink(ptr, old, shrunk) }.is_err());
assert_eq!(bump.used(), used);
let shrunk_block = unsafe { bump.shrink(ptr, old, shrunk).unwrap() };
let shrunk_ptr = block_ptr(shrunk_block);
assert_ne!(shrunk_ptr, ptr);
assert_eq!((shrunk_ptr.as_ptr() as usize) % shrunk.align(), 0);
for index in 0..shrunk.size() {
assert_eq!(unsafe { shrunk_ptr.as_ptr().add(index).read() }, 0xAB);
}
}
}
Loading