Struct Arena 

pub struct Arena<M: MapStorage> {
    state: RefCell<ArenaState<M>>,
    _nosend: PhantomData<Rc<()>>,
}

A reference-counted arena for GC protection, generic over map type.

This provides an alternative to R’s PROTECT stack that:

• Uses reference counting for each SEXP
• Allows releasing protections in any order
• Has no stack size limit (uses heap allocation)

Type Aliases

• RefCountedArena = Arena<BTreeMap<...>> (ordered, good for ref counting)
• HashMapArena = Arena<HashMap<...>> (faster for large collections)

Fields

state: RefCell<ArenaState<M>>

_nosend: PhantomData<Rc<()>>

Implementations

impl<M: MapStorage> Arena<M>

pub unsafe fn new() -> Self

Create a new arena with default capacity (16 slots).

For workloads protecting many distinct SEXPs (e.g., ppsize-scale loops), prefer with_capacity to avoid backing VECSXP growth and map rehashing during operation.

Safety

Must be called from the R main thread.

pub unsafe fn with_capacity(capacity: usize) -> Self

Create a new arena with specific initial capacity.

Pre-sizing the arena avoids growth of the backing VECSXP and rehashing of the internal map. Use this when the expected number of distinct protected values is known or can be estimated.

Safety

Must be called from the R main thread.

pub unsafe fn protect(&self, x: SEXP) -> SEXP

Protect a SEXP, incrementing its reference count.

Safety

Must be called from the R main thread.

pub unsafe fn unprotect(&self, x: SEXP)

Unprotect a SEXP, decrementing its reference count.

Safety

Must be called from the R main thread.

Panics

Panics if x was not protected by this arena.

pub unsafe fn try_unprotect(&self, x: SEXP) -> bool

Try to unprotect a SEXP, returning true if it was protected.

Safety

Must be called from the R main thread.

pub fn is_protected(&self, x: SEXP) -> bool

Check if a SEXP is currently protected by this arena.

pub fn ref_count(&self, x: SEXP) -> usize

Get the reference count for a SEXP (0 if not protected).

pub fn len(&self) -> usize

Get the number of distinct SEXPs currently protected.

pub fn is_empty(&self) -> bool

Check if the arena is empty.

pub fn capacity(&self) -> usize

Get the current capacity.

pub unsafe fn clear(&self)

Clear all protections.

Safety

Must be called from the R main thread.

pub unsafe fn guard(&self, x: SEXP) -> ArenaGuard<'_, M>

Protect a SEXP and return an RAII guard.

Safety

Must be called from the R main thread.

Trait Implementations

impl<M: MapStorage> Default for Arena<M>

fn default() -> Self

Returns the “default value” for a type.

impl<M: MapStorage> Drop for Arena<M>

fn drop(&mut self)

Executes the destructor for this type.

fn pin_drop(self: Pin<&mut Self>)

🔬This is a nightly-only experimental API. (pin_ergonomics)

Execute the destructor for this type, but different to Drop::drop, it requires self to be pinned.

Layout

Note: Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.