In the previous post Interior mutability patterns we looked at four different conventions that can be used to safely use interor mutability. We didn’t look further than the basic API, that was already plenty of material to cover.

Yet there are some creative methods that push the limit of those abstractions. Let’s explore them!

On container types:

method Cell Atomic ¹ AtomicCell ¹ RefCell Mutex RwLock OnceCell QCell TCell LCell into_inner yes yes yes yes yes yes yes yes* yes* yes* get_mut yes — yes* yes yes yes yes yes* yes* yes* as_ptr yes yes yes yes yes yes yes* yes yes yes from_mut yes — yes* ( BoCell ) — — — — yes* yes* replace yes yes yes yes — — — yes* yes* yes* swap yes yes yes yes — — — — — — update yes — — yes — — — —* —* —* as_slice_of_cells yes — — — — — — — yes* yes* with — — — — maybe maybe — — — —

On smart pointers:

method Ref RefMut MutexGuard RwLockReadGuard RwLockWriteGuard ReentrantMutexGuard bump — — yes* yes* yes* yes unlocked — — yes* yes* yes* yes Condvar::wait — — yes yes* yes* yes* map yes yes yes* yes* yes* yes map_split yes yes yes* yes* yes* yes* try_map yes* yes* yes* yes* yes* yes downgrade — — — — yes* — upgrade yes* — — maybe — —

¹: In this post I use Atomic<T> to describe a wrapper based on atomic operations, and AtomicCell<T> to describe a lock-based solution for larger types.

*: possible, but not implemented (or not implemented in the standard library or main crate)

We could place these methods in 9 categories:

Bypass conventions if you have exclusive access

For many uses of types with interior mutability there is a point in the program where the value is not yet shared, and you either own the value, or have exclusive access. Or there is a point where you are done sharing, for example in the Drop implementation of a type.

If you have exclusive, mutable access, you don’t need interior mutability. In that case you can just use the wrapped value, without having to uphold the conventions the wrapper normally asks for in order to use the value with multiple references.

Basic API:

impl < T : ? Sized > Cell < T > { fn into_inner ( self ) -> T fn get_mut ( & mut self ) -> & mut T const fn as_ptr ( & self ) -> * mut T }

into_inner just returns the wrapped value by consuming the wrapper type.

get_mut

If you can provide a mutable reference to the wrapper type, get_mut gives a mutable reference to the wrapped value. This is just as possible with synchronization primitives like Mutex and RwLock , which don’t need to do any locking in that case.

A notable exception for get_mut is a wrapper that treats small types as atomic integers, here named Atomic<T> (although there is no crate that provides such an abstraction without fallback for larger types).

Atomic<T> is currently unsound when used on types with padding bytes. This could be solved in all but one case if there is some kind of “atomic freeze operation”, which “freezes” the value of the padding bytes before any operation on them. The ‘but one’ case is get_mut : it would allow user code to change the value without freezing the padding bytes.

For now this is a bit of a theoretical concern, as there is no such thing as a freeze operation. The discussion around a freeze operation has died down after a generic ptr::freeze method for uninitialized memory turned out to be impossible, although that doesn’t make it impossible for just padding bytes of small structs.

as_ptr

as_ptr is a method that is always safe to call, which will return a raw pointer to the wrapped value. Dereferencing the pointer however requires an unsafe block. Any use of the pointer has to uphold any conventions the wrapper type upholds, which is nigh impossible because you don’t have access to the internal state of the wrapper. That makes as_ptr basically only usable on Cell , or in situations where you already have exclusive access, like into_inner and get_mut .

An as_ptr method was not available for normal integer atomics. Such a function may be useful for FFI, where the receiving library also promises to do only atomic operations on the pointed value. I made a PR, and it is now available on nightly as as_mut_ptr .

Temporary wrap a value to provide interior mutability

Basic API:

impl < T : ? Sized > Cell < T > { fn from_mut ( t : & mut T ) -> & Cell < T > }

If an interior mutability wrapper does not need any extra fields, and does not place any extra restrictions such as a certain alignment, it can be made #[repr(C)] or #[repr(transparent)] .

Such as wrapper can be put around any value where you have mutable access to, allowing you to temporary create multiple references in combination with interior mutability. This is a useful pattern if you have hard to solve problems with the borrow checker that require multiple mutable references.

RFC 1789 established this pattern with Cell::from_mut . It can also be supported by TCell and LCell of the qcell crate. A wrapper like AtomicCell could support it if it always only used a locking method (i.e. no optimization with atomic operations for small types), and uses a set op global locks (instead of putting a lock together with the value).

For Atomic<T> (for small integer-sized types) from_mut is not possible, because the alignment may not be correct. Besides requiring state it wouldn’t make any sense for OnceCell to have from_mut : the only time when it provides mutability is when the value is not yet initialized, and you can’t have mutable references to uninitialized memory.

RefCell , Mutex etc. don’t support wrapping an arbitrary type because they need to hold some extra state. The recent BoCell is a simple solution for RefCell . Instead of storing the state with the wrapped value, you can just store the state somewhere else with a pointer to the value. The signature of BoCell::new matches that of from_mut .

One small missed opportunity for Cell::from_mut was to have it return &mut Cell<T> , which would have allowed using the returned value again with Cell::get_mut .

mem::replace , but for interior mutability

Basic API:

impl < T : ? Sized > Cell < T > { fn replace ( & self , val : T ) -> T fn swap ( & self , other : & Cell < T > ) }

For Cell we have already seen the replace method, which allows reading the old value and setting an new value in one operation. The same is possible for Atomic<T> and AtomicCell<T> .

RefCell offers a replace method too, with the restriction that it panics when there are any active borrows. In theory Mutex and RwLock could also support it, but until now the maintainers wisely took the decision to not add any methods that would secretly take a lock.

What if you want to swap the value of two Cells? You could do so by using replace twice in combination with a temporary value. Cell and RefCell both offer a swap method for convenience, and for large types, performance. I suppose it could also be offered by the types of the qcell crate, but that may get a too complex because you would not only pass two cells but also have to handle two owners.

Updating a value (single-threaded wrappers only)

Basic API:

impl < T : Copy > Cell < T > { fn update < F > ( & self , f : F ) -> T where F : FnOnce ( T ) -> T } impl < T : ? Sized > RefCell < T > { fn replace_with < F > ( & self , f : F ) -> T where F : FnOnce ( & mut T ) -> T }

One trick that is available for single-threaded types is Cell::update (unstable) or RefCell::replace_with . These methods run a closure with the current value in the cell as input, and write the result to the value when done.

A problem is: what happens when you try to access the cell while inside the closure? Cell::update currently has a Copy bound. It will pass a copy of the value to the update closure. Reads from the cell during the closure will return the old value, and writes will be overwritten after the closure is done.

An alternative is requiring a Default bound, so that Cell::update takes out the value, reads from the cell during the closure will see a default value, and a new value will be put back in after the closure is done. Because both options are equally valid and both just for convenience, I wonder how long it will take to reach a decision and stabilize the function.

For RefCell::replace_with it is easy: borrowing it again inside the closure causes a panic, just like other invalid borrows. I wonder if there is much use for replace_with , but it exists.

For synchronization primitives something like Cell::update is tricky, other threads could have modified the value while the closure is running. You really want to use compare_and_swap -like methods instead, or something like the experimental fetch_update on atomic integers.

For synchronization primitives something like RefCell::replace_with is also not recommended, because that would mean secretly taking a lock.

For OnceCell it doesn’t make sense to update the value, because the only time its value can be mutated is while it is uninitialized. An alternative could be OnceFlipCell , which comes with an initial state that can be flipped to a usable state. But I really don’t see much use for such a type.

References in Cell if the structure is ‘flat’

The basic convention that makes Cell safe is to never hand out references to its wrapped value. It turns out here is a case where taking a reference to inside the cell doesn’t violate its soundness. That would be really useful for larger types, as it allows you to read or write only part of the value.

Let’s call the structure of a value ‘flat’ (see this comment) if the references don’t go under indirections ( Box or other wrapper types) or branches ( enum s). Then a write to Cell can’t change the structure, there is only one possible structure. A write to the entire cell will mutate the contents, but it can never switch a part to which we hold a reference to another enum variant, deallocate the referenced memory, or otherwise make the inner reference invalid.

We do have to make sure the inner references also follow the same conventions for interior mutability, meaning we have also have to wrap them in Cell s. Just like temporary wrapping a value to provide interior mutability.

That brings us to a few safe conversions:

tuples: &Cell<(A, B)> → &(Cell<A>, Cell<B>)

→ arrays: &Cell<[A; N]> → &[Cell<A>; N]

→ slices: &Cell<[A]> → &[Cell<A>]

→ structs: &Cell<struct> → &Cell<field> (if the field is not private)

Only the conversion of slices is currently implemented, with Cell::as_slice_of_cells :

impl < T > Cell < [ T ] > { fn as_slice_of_cells ( & self ) -> & [ Cell < T > ] }

Are there any other internal mutability types that can follow this trick? The type has to support the same operation as from_mut , and it should not hand out a regular mutable reference from the parent cell and from the inner cell at the same time ( Cell never hands out any, so that is easy).

AtomicCell also doesn’t hand out references, but uses the address of the wrapped value to synchronize accesses with other threads. Even if we take a reference to something inside it and wrap it in an AtomicCell again, the address will not be the same. So I don’t think it is something that can be supported.

TCell and LCell could implement from_mut , and can also ensure there a mutable reference is unique. The owner ensures either all references are shared, or that there is only one mutable reference. Multiple mutable references can be created with LCellOwner::{rw2, rw3} , but those methods check at runtime the adresses are different, and can be extended to check the addresses don’t fall within the size of one of the values.

Temporary lending out a mutable reference to another thread

Simplified API:

impl < 'a , T : ? Sized > MutexGuard < 'a , T > { fn bump ( s : & mut Self ) fn unlocked < F , U > ( s : & mut Self , f : F ) -> U where F : FnOnce () -> U } impl Condvar { fn wait < T : ? Sized > ( & self , mutex_guard : & mut MutexGuard < T > ) }

bump and unlocked

With a Mutex you may want to temporary release your lock on the primitive to let another thread do some work.

parking_lot offers a MutexGuard::bump method that lets you temporary give up your lock on the mutex, and block until the lock can be acquired again. The similar MutexGuard::unlocked method lets you run a closure in the meantime (and block afterwards). This is functionally equivalent to lacking and unlocking the mutex, but it can be much more efficient in the case where there are no waiting threads.

bump and unlocked take exclusive access to the smart pointer. Another thread which acquires the mutex has mutable access, so in a way you can view those two methods as lending out a mutable reference to another thread.

Is bump also sound for RwLock and ReentrantMutex ? RwLockWriteGuard has exclusive mutable access, so it can lend out that access to another thread with bump . But one thread can hold multiple instances of RwLockReadGuard or ReentrantMutexGuard , in which case bump can’t guarantee exclusive access.

Still they are also safe, because now it is not the type system that guarantees exclusive access, but the synchronization primitive. If the current thread holds more than one lock, only one will be released. Then another thread won’t be able to get a lock for write access (or in the case of ReentrantMutex , won’t be able to get a lock at all). bump will basically have no effect if there are multiple instances of RwLockReadGuard or ReentrantMutexGuard .

Condition variables

Waiting on a condition variable with a mutex with Condvar::wait also gives up the lock, but the thread doesn’t wake up again until notified.

You wait on the Condvar , giving it a mutable reference to the MutexGuard , which temporary releases the lock. Another thread acquires the lock, does the work you are waiting on, calls Condvar::notify_* and releases the lock to wake you up. You now again hold the lock.

Temporary lending out mutable access to another thread by waiting on a Condvar with MutexGuard is safe, for the same reasons as bump and unlocked . The standard library and parking_lot only support this combination, using a Condvar in with a MutexGuard .

The shared-mutex crate has an alternative RW lock implementation that is designed to be useable with condition variables. Again waiting with a SharedMutexWriteGuard is sound, as we can lend a unique reference. It is also correct if the thread holds only one SharedMutexReadGuard . And if the thread holds multiple SharedMutexReadGuard , only one will be unlocked and the thread will wait until notified. As another thread can’t acquire a write lock now, we will probably have a deadlock but no unsoundness.

There is currently no crate (that I know of) that supports waiting on a Condvar in combination with a ReentrantMutex . It would probably quickly lead to deadlocks, because if it was possible to avoid the chance of holding multiple locks, you should not be using ReentrantMutex .

In the common ReentrantMutex<RefCell> case there is a decoupling between holding a number of locks, and holding references. If those two were more closely integrated it would be possible to temporary give up all locks a thread holds if it only has one reference active, when waiting on a Condvar . And to panic otherwise.

Refining smart pointers

Basic API (see documentation of Ref , RefMut ):

impl < T : ? Sized > RefCell < T > { fn borrow ( & self ) -> Ref < T > fn borrow_mut ( & self ) -> RefMut < T > } impl < 'b , T : ? Sized > Ref < 'b , T > { fn map < U , F > ( orig : Ref < 'b , T > , f : F ) -> Ref < 'b , U > where U : ? Sized , F : FnOnce ( & T ) -> & U fn map_split < U , V , F > ( orig : Ref < 'b , T > , f : F ) -> ( Ref < 'b , U > , Ref < 'b , V > ) where U : ? Sized , V : ? Sized , F : FnOnce ( & T ) -> ( & U , & V ) fn try_map < U , F > ( orig : Ref < 'b , T > , f : F ) -> Result < Ref < 'b , U > , Self > where U : ? Sized , F : FnOnce ( & T ) -> Option <& U > fn clone ( orig : & Ref < 'b , T > ) -> Ref < 'b , T > } impl < 'b , T : ? Sized > RefMut < 'b , T > { fn map < U , F > ( orig : RefMut < 'b , T > , f : F ) -> RefMut < 'b , U > where U : ? Sized , F : FnOnce ( & mut T ) -> & mut U fn map_split < U , V , F > ( orig : RefMut < 'b , T > , f : F ) -> ( RefMut < 'b , U > , RefMut < 'b , V > ) where U : ? Sized , V : ? Sized , F : FnOnce ( & mut T ) -> ( & mut U , & mut V ) fn try_map < U , F > ( orig : RefMut < 'b , T > , f : F ) -> Result < RefMut < 'b , U > , Self > where U : ? Sized , F : FnOnce ( & mut T ) -> Option <& mut U > }

Note that all these methods on smart pointers don’t take self as argument, but are associated methods instead. A normal method would interfere with methods of the same name on the contents of a RefCell used through Deref .

Ref::map

If you have a smart pointer type to inside a RefCell , you can take normal references to parts of the wrapped type. But Ref::map allows you to refine the smart pointer to point to a part of the wrapped type.

map was also once implemented for MutexGuard , RwLockReadGuard and RwLockWriteGuard , but removed. The problem is the interaction with Condvar . Notice that if we would refine the smart pointer with MutexGuard::map to only part of the wrapped value, through Condvar we would give the other thread also mutable access to only part of the value. But the other thread gets access to the entire wrapped value. That can easily lead to things like reading from deallocated memory, as discovered in this comment.

The solution is to make map return a different type so it can’t be used as argument for Condvar::wait . parking_lot implements this solution, its MutexGuard::map returns a MappedMutexGuard . Similarly the map methods on RwLockWriteGuard and RwLockReadGuard should return another type, because they can also temporary lend out a mutable reference to the entire value to another thread with bump .

RefMut::map_split

While Ref::map is nice, RefMut::map_split is where it gets really interesting in my opinion. It allows you to refine a smart pointer to two parts of the wrapped value. This is the only way to get more than one mutable reference to inside the RefCell .

Just like map , map_split is not implemented for Mutex and RwLock in the standard library. parking_lot also doesn’t support map_split yet. And because the concept of multiple mutable references goes pretty deep, that may take a long time, if ever, to be supported.

Ref::try_map

Sometimes you want to refine the smart pointer to part of the value, but don’t know yet if it exists. Examples are an enum, vector or hashmap. If it is undesirable to do the lookup twice, once to confirm it exists, and once in map , something like try_map comes in handy.

It was at some time available inside the standard library as the unstable method Ref::filter_map . The ref_filter_map crate provides the functionality for RefCell after it got removed from the standard library by smuggling around a raw pointer.

parking_lot has a try_map method for all its guards, just as shared_mutex has with result_map . In my opinion that API is superior to the original filter_map , because it returns a Result . That way the method can return either a refined reference, or the original one.

Recover

Both parking_lot and shared_mutex return Mapped* types from their map methods. shared_mutex has a potentially useful method: it can recover a reference to the entire value from a mapped reference, without releasing the lock, if you can also provide a reference to the mutex / RW lock.

It is worth noting that a library can’t provide both split_mut and recover on a mutable (mapped) smart pointer. split_mut would allow multiple mutable references to exist, pointing to different parts of the value. Recovering from one a mutable reference to the entire value would alias the other ones.

map and reentrant mutexes

parking_lot provides a map and try_map method on its ReentrantMutexGuard . A reentrant mutex can’t hand out mutable references, so must be used in combination with single-treaded interior mutability types. But with a shared reference in ReentrantMutexGuard::map you can’t descend into such a type, so map is rarely useable in practice.

The upside is that ReentrantMutexGuard::map does not need to return a MappedReentrantMutexGuard type (although parking_lot currently does). Because ReentrantMutex doesn’t hand out mutable references, it doesn’t have the problem that Mutex and RwLock have where a mapped smart pointer can be invalidated when temporary lending out a mutable reference to another thread.

Downgrading or upgrading a smart pointer

Basic API:

impl < 'b , T : ? Sized > RefMut < 'b , T > { fn downgrade ( orig : RefMut < 'b , T > ) -> Ref < 'b , T > } impl < 'b , T : ? Sized > Ref < 'b , T > { fn upgrade ( orig : Ref < 'b , T > ) -> RefMut < 'b , T > }

Downgrade

RwLockWriteGuard in parking_lot has a downgrade method which turns it into a RwLockReadGuard . It is instructive to explore why RefCell s RefMut can’t provide a similar method to turn it into a Ref .

The signature of the closure used in RefMut::map is FnOnce(&mut T) -> &mut U . This gives it an interesting property: it allows you to bypass conventions of wrapped interior mutability types because you have exclusive access.

Because RefMut::map made the promise to wrapped types that its reference is unique, the returned RefMut has to remain unique. It can’t be turned into a Ref , of which multiple can exist. Example of how it can go wrong:

let refcell = RefCell :: new ( Cell :: new ( 10u32 )); let ref_mut = refcell .borrow_mut (); RefMut :: map ( ref_mut , | x | x .get_mut ()); let ref1 = RefMut :: downgrade ( ref_mut ); let cell_ref = &* ref1 ; let ref2 = refcell .borrow (); // We can now mutate the `Cell` through `ref2` while there also exists a // reference to its interior.

For RwLockWriteGuard however downgrade is sound, because it’s map method returns another type. But that returned type, MappedRwLockWriteGuard , now has to remain unique, so MappedRwLockWriteGuard::downgrade is unsound.

Upgrade

For RefCell a Ref::upgrade method can be simple. Ref::map can’t traverse into wrapped interior mutability types because it doesn’t guarantee unique access, so we don’t have to problem RefMut::downgrade has. And if there are multiple shared references to the interior of the RefCell when upgrading, the convention is to panic. But there doesn’t really seem to be an advantage to warrant an upgrade method.

For RwLock an RwLockReadGuard::upgrade method can be useful, as it allows starting an operation without blocking other threads from reading, and only upgrading to a writer lock when necessary. If other threads hold read locks, upgrade will have to block until they are done (just as when acquiring a write lock). And if the current thread holds more than one reader lock, it will deadlock.

The problem is: what happens if two reader locks attempt to upgrade at the same time? As both hold on to their current lock and wait until there is only one active lock, they deadlock. And contrary to other deadlocks wich can be considered programmer errors, this one may be unavoidable.

Two possible solutions:

upgrade must be fallible, where one reader releases its read lock when upgrade fails, so the other can succeed.

must be fallible, where one reader releases its read lock when fails, so the other can succeed. Establish before the upgrade moment that a reader is upgradable, and allow only one upgradable reader.

parking_lot has an RwLockUpgradableReadGuard , the second option. It can probably become the best solution, but it currently misses a number of nice conversions such as deriving it from a normal read lock, or working with mapped smart pointers.

Combined with waiting on a condition variable

Basic API:

impl < 'mutex , T : ? Sized > SharedMutexReadGuard < 'mutex , T > { fn wait_for_write ( self , cond : & Condvar ) -> LockResult < SharedMutexWriteGuard < 'mutex , T >> fn wait_for_read ( self , cond : & Condvar ) -> LockResult < Self > } impl < 'mutex , T : ? Sized > SharedMutexWriteGuard < 'mutex , T > { fn wait_for_write ( self , cond : & Condvar ) -> LockResult < Self > fn wait_for_read ( self , cond : & Condvar ) -> LockResult < SharedMutexReadGuard < 'mutex , T >> }

In the part on condition variables we have seen waiting on those is an operation that ensures the smart pointer is unique. And when waiting the threads temporary give up their lock on the RW lock. That makes this a safe moment to convert a read lock to a write lock.

The SharedMutex RW lock implementation provides such combinations. SharedMutexReadGuard::wait_for_write will upgrade the lock after waiting, and SharedMutexWriteGuard::wait_for_read will downgrade it.

Scopes instead of smart pointers

Smart pointers are not the only way to keep track of the number of references/locks handed out. So can scopes: methods that provide a closure with a reference, and do the bookkeeping before and after running the closure.

For RefCell it doesn’t make any sense to use it with a closure to provide scoped access to the interior: its whole point is to be used in cases where the regular scope-based ‘inherited mutability’ doesn’t work out. For OnceCell , QCell , TCell and LCell it also doesn’t offer a real advantage, they can already hand out plain references.

Just like it is not sound to hand out smart pointers to a Cell , it is not sound to hand out references to its interior when inside a closure. A scope can keep track of how long the reference is alive, but nothing else. It doesn’t prevent another reference to the Cell to be used inside the closure, resulting in a shared reference being able to observe a mutation.

But for synchronization primitives you typically want to keep the structure regarding mutability within a thread clear, dealing with concurrency is already hard enough. So here scope-based methods can be a good alternative, and scopes make it easier to control the lifetime of a lock than smart pointers.

Basic API, from a PR attempting to add such methods to Mutex and RwLock in the standard library:

impl < T : ? Sized > Mutex < T > { fn with < U , F > ( & self , f : F ) -> U where F : FnOnce ( & mut T ) -> U } impl < T : ? Sized > RwLock < T > { fn with_read < U , F > ( & self , func : F ) -> U where F : FnOnce ( & T ) -> U fn with_write < U , F > ( & self , func : F ) -> U where F : FnOnce ( & mut T ) -> U }

One issue that conplicated things is that Mutex and RwLock have a feature: if one thread holding a lock panics, it will poison the mutex / RW lock, signalling the contents are probably in an inconsistent state. To force you to check for this scenario, every time you acquire a lock it doesn’t just return a MutexGuard<T> , but LockResult<MutexGuard<T>> (which is commonly just unwrapped).

Because it is somewhat up for discussion how much value the ability to handle poisoned locks really has, compared to just panicking, and how to translate that into an API for working with scopes, the PR was closed.

For Mutex and RwLock in parking_lot , which don’t support poisoning, I think a method like with can be a nice addition.

Any other creativity?

Appearently rustaceans really like to push the boundaries, to explore the limits of what keeps these interior mutability conventions sound.

I can’t think of any other directions, but I am sure this list will grow outdated at some point ;-).

Do you know of any other creative methods on interior mutability types?

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