Note: The Share trait described in this RFC was later renamed to Sync.

Summary

The high-level idea is to add language features that simultaneously achieve three goals:

move Send and Share out of the language entirely and into the standard library, providing mechanisms for end users to easily implement and use similar “marker” traits of their own devising;
make “normal” Rust types sendable and sharable by default, without the need for explicit opt-in; and,
continue to require “unsafe” Rust types (those that manipulate unsafe pointers or implement special abstractions) to “opt-in” to sendability and sharability with an unsafe declaration.

These goals are achieved by two changes:

Unsafe traits: An unsafe trait is a trait that is unsafe to implement, because it represents some kind of trusted assertion. Note that unsafe traits are perfectly safe to use. Send and Share are examples of unsafe traits: implementing these traits is effectively an assertion that your type is safe for threading.
Default and negative impls: A default impl is one that applies to all types, except for those types that explicitly opt out. For example, there would be a default impl for Send, indicating that all types are Send “by default”.

To counteract a default impl, one uses a negative impl that explicitly opts out for a given type T and any type that contains T. For example, this RFC proposes that unsafe pointers *T will opt out of Send and Share. This implies that unsafe pointers cannot be sent or shared between threads by default. It also implies that any structs which contain an unsafe pointer cannot be sent. In all examples encountered thus far, the set of negative impls is fixed and can easily be declared along with the trait itself.

Safe wrappers like Arc, Atomic, or Mutex can opt to implement Send and Share explicitly. This will then make them be considered sendable (or sharable) even though they contain unsafe pointers etc.

Based on these two mechanisms, we can remove the notion of Send and Share as builtin concepts. Instead, these would become unsafe traits with default impls (defined purely in the library). The library would explicitly opt out of Send/Share for certain types, like unsafe pointers (*T) or interior mutability (Unsafe<T>). Any type, therefore, which contains an unsafe pointer would be confined (by default) to a single thread. Safe wrappers around those types, like Arc, Atomic, or Mutex, can then opt back in by explicitly implementing Send (these impls would have to be designed as unsafe).

Motivation

Since proposing opt-in builtin traits, I have become increasingly concerned about the notion of having Send and Share be strictly opt-in. There are two main reasons for my concern:

Rust is very close to being a language where computations can be parallelized by default. Making Send, and especially Share, opt-in makes that harder to achieve.
The model followed by Send/Share cannot easily be extended to other traits in the future nor can it be extended by end-users with their own similar traits. It is worrisome that I have come across several use cases already which might require such extension (described below).

To elaborate on those two points: With respect to parallelization: for the most part, Rust types are threadsafe “by default”. To make something non-threadsafe, you must employ unsynchronized interior mutability (e.g., Cell, RefCell) or unsynchronized shared ownership (Rc). In both cases, there are also synchronized variants available (Mutex, Arc, etc). This implies that we can make APIs to enable intra-task parallelism and they will work ubiquitously, so long as people avoid Cell and Rc when not needed. Explicit opt-in threatens that future, however, because fewer types will implement Share, even if they are in fact threadsafe.

With respect to extensibility, it is particularly worrisome that if a library forgets to implement Send or Share, downstream clients are stuck. They cannot, for example, use a newtype wrapper, because it would be illegal to implement Send on the newtype. This implies that all libraries must be vigilant about implementing Send and Share (even more so than with other pervasive traits like Eq or Ord). The current plan is to address this via lints and perhaps some convenient deriving syntax, which may be adequate for Send and Share. But if we wish to add new “classification” traits in the future, these new traits won’t have been around from the start, and hence won’t be implemented by all existing code.

Another concern of mine is that end users cannot define classification traits of their own. For example, one might like to define a trait for “tainted” data, and then test to ensure that tainted data doesn’t pass through some generic routine. There is no particular way to do this today.

More examples of classification traits that have come up recently in various discussions:

Snapshot (nee Freeze), which defines logical immutability rather than physical immutability. Rc<int>, for example, would be considered Snapshot. Snapshot could be useful because Snapshot+Clone indicates a type whose value can be safely “preserved” by cloning it.
NoManaged, a type which does not contain managed data. This might be useful for integrating garbage collection with custom allocators which do not wish to serve as potential roots.
NoDrop, a type which does not contain an explicit destructor. This can be used to avoid nasty GC quandries.

All three of these (Snapshot, NoManaged, NoDrop) can be easily defined using traits with default impls.

A final, somewhat weaker, motivator is aesthetics. Ownership has allowed us to move threading almost entirely into libraries. The one exception is that the Send and Share types remain built-in. Opt-in traits makes them less built-in, but still requires custom logic in the “impl matching” code as well as special safety checks when Safe or Share are implemented.

After the changes I propose, the only traits which would be specifically understood by the compiler are Copy and Sized. I consider this acceptable, since those two traits are intimately tied to the core Rust type system, unlike Send and Share.

Detailed design

Unsafe traits

Certain traits like Send and Share are critical to memory safety. Nonetheless, it is not feasible to check the thread-safety of all types that implement Send and Share. Therefore, we introduce a notion of an unsafe trait – this is a trait that is unsafe to implement, because implementing it carries semantic guarantees that, if compromised, threaten memory safety in a deep way.

An unsafe trait is declared like so:

unsafe trait Foo { ... }

To implement an unsafe trait, one must mark the impl as unsafe:

unsafe impl Foo for Bar { ... }

Designating an impl as unsafe does not automatically mean that the body of the methods is an unsafe block. Each method in the trait must also be declared as unsafe if it to be considered unsafe.

Unsafe traits are only unsafe to implement. It is always safe to reference an unsafe trait. For example, the following function is safe:

fn foo<T:Send>(x: T) { ... }

It is also safe to opt out of an unsafe trait (as discussed in the next section).

Default and negative impls

We add a notion of a default impl, written:

impl Trait for .. { }

Default impls are subject to various limitations:

The default impl must appear in the same module as Trait (or a submodule).
Trait must not define any methods.

We further add the notion of a negative impl, written:

impl !Trait for Foo { }

Negative impls are only permitted if Trait has a default impl. Negative impls are subject to the usual orphan rules, but they are permitting to be overlapping. This makes sense because negative impls are not providing an implementation and hence we are not forced to select between them. For similar reasons, negative impls never need to be marked unsafe, even if they reference an unsafe trait.

Intuitively, to check whether a trait Foo that contains a default impl is implemented for some type T, we first check for explicit (positive) impls that apply to T. If any are found, then T implements Foo. Otherwise, we check for negative impls. If any are found, then T does not implement Foo. If neither positive nor negative impls were found, we proceed to check the component types of T (i.e., the types of a struct’s fields) to determine whether all of them implement Foo. If so, then Foo is considered implemented by T.

Oe non-obvious part of the procedure is that, as we recursively examine the component types of T, we add to our list of assumptions that T implements Foo. This allows recursive types like

struct List<T> { data: T, next: Option<List<T>> }

to be checked successfully. Otherwise, we would recursive infinitely. (This procedure is directly analogous to what the existing TypeContents code does.)

Note that there exist types that expand to an infinite tree of types. Such types cannot be successfully checked with a recursive impl; they will simply overflow the builtin depth checking. However, such types also break code generation under monomorphization (we cannot create a finite set of LLVM types that correspond to them) and are in general not supported. Here is an example of such a type:

struct Foo<A> {
    data: Option<Foo<Vec<A>>>
}

The difference between Foo and List above is that Foo<A> references Foo<Vec<A>>, which will then in turn reference Foo<Vec<Vec<A>>> and so on.

The Send and Share traits will be modeled entirely in the library as follows. First, we declare the two traits as follows:

unsafe trait Send { }
unsafe impl Send for .. { }

unsafe trait Share { }
unsafe impl Share for .. { }

Both traits are declared as unsafe because declaring that a type if Send and Share has ramifications for memory safety (and data-race freedom) that the compiler cannot, itself, check.

Next, we will add opt out impls of Send and Share for the various unsafe types:

impl<T> !Send for *T { }
impl<T> !Share for *T { }

impl<T> !Send for *mut T { }
impl<T> !Share for *mut T { }

impl<T> !Share for Unsafe<T> { }

Note that it is not necessary to write unsafe to opt out of an unsafe trait, as that is the default state.

Finally, we will add opt in impls of Send and Share for the various safe wrapper types as needed. Here I give one example, which is Mutex. Mutex is interesting because it has the property that it converts a type T from being Sendable to something Sharable:

unsafe impl<T:Send> Send for Mutex<T> { }
unsafe impl<T:Send> Share for Mutex<T> { }

The `Copy` and `Sized` traits

The final two builtin traits are Copy and Share. This RFC does not propose any changes to those two traits but rather relies on the specification from the original opt-in RFC.

Controlling copy vs move with the `Copy` trait

The Copy trait is “opt-in” for user-declared structs and enums. A struct or enum type is considered to implement the Copy trait only if it implements the Copy trait. This means that structs and enums would move by default unless their type is explicitly declared to be Copy. So, for example, the following code would be in error:

struct Point { x: int, y: int }
...
let p = Point { x: 1, y: 2 };
let q = p;  // moves p
print(p.x); // ERROR

To allow that example, one would have to impl Copy for Point:

struct Point { x: int, y: int }
impl Copy for Point { }
...
let p = Point { x: 1, y: 2 };
let q = p;  // copies p, because Point is Pod
print(p.x); // OK

Effectively, there is a three step ladder for types:

If you do nothing, your type is linear, meaning that it moves from place to place and can never be copied in any way. (We need a better name for that.)
If you implement Clone, your type is cloneable, meaning that it moves from place to place, but it can be explicitly cloned. This is suitable for cases where copying is expensive.
If you implement Copy, your type is copyable, meaning that it is just copied by default without the need for an explicit clone. This is suitable for small bits of data like ints or points.

What is nice about this change is that when a type is defined, the user makes an explicit choice between these three options.

Determining whether a type is `Sized`

Per the DST specification, the array types [T] and object types like Trait are unsized, as are any structs that embed one of those types. The Sized trait can never be explicitly implemented and membership in the trait is always automatically determined.

Matching and coherence for the builtin types `Copy` and `Sized`

In general, determining whether a type implements a builtin trait can follow the existing trait matching algorithm, but it will have to be somewhat specialized. The problem is that we are somewhat limited in the kinds of impls that we can write, so some of the implementations we would want must be “hard-coded”.

Specifically we are limited around tuples, fixed-length array types, proc types, closure types, and trait types:

Fixed-length arrays: A fixed-length array [T, ..n] is Copy if T is Copy. It is always Sized as T is required to be Sized.
Tuples: A tuple (T_0, ..., T_n) is Copy/Sized depending if, for all i, T_i is Copy/Sized.
Trait objects (including procs and closures): A trait object type Trait:K (assuming DST here ;) is never Copy nor Sized.

We cannot currently express the above conditions using impls. We may at some point in the future grow the ability to express some of them. For now, though, these “impls” will be hardcoded into the algorithm as if they were written in libstd.

Per the usual coherence rules, since we will have the above impls in libstd, and we will have impls for types like tuples and fixed-length arrays baked in, the only impls that end users are permitted to write are impls for struct and enum types that they define themselves. Although this rule is in the general spirit of the coherence checks, it will have to be written specially.

Design discussion

Why unsafe traits

Without unsafe traits, it would be possible to create data races without using the unsafe keyword:

struct MyStruct { foo: Cell<int> }
impl Share for MyStruct { }

Balancing abstraction, safety, and convenience.

In general, the existence of default traits is anti-abstraction, in the sense that it exposes implementation details a library might prefer to hide. Specifically, adding new private fields can cause your types to become non-sendable or non-sharable, which may break downstream clients without your knowing. This is a known challenge with parallelism: knowing whether it is safe to parallelize relies on implementation details we have traditionally tried to keep secret from clients (often it is said that parallelism is “anti-modular” or “anti-compositional” for this reason).

I think this risk must be weighed against the limitations of requiring total opt in. Requiring total opt in not only means that some types will accidentally fail to implement send or share when they could, but it also means that libraries which wish to employ marker traits cannot be composed with other libraries that are not aware of those marker traits. In effect, opt-in is anti-modular in its own way.

To be more specific, imagine that library A wishes to define a Untainted trait, and it specifically opts out of Untainted for some base set of types. It then wishes to have routines that only operate on Untainted data. Now imagine that there is some other library B that defines a nifty replacement for Vector, NiftyVector. Finally, some library C wishes to use a NiftyVector<uint>, which should not be considered tainted, because it doesn’t reference any tainted strings. However, NiftyVector<uint> does not implement Untainted (nor can it, without either library A or library B knowing about one another). Similar problems arise for any trait, of course, due to our coherence rules, but often they can be overcome with new types. Not so with Send and Share.

Other use cases

Part of the design involves making space for other use cases. I’d like to sketch out how some of those use cases can be implemented briefly. This is not included in the Detailed design section of the RFC because these traits generally concern other features and would be added under RFCs of their own.

Isolating snapshot types. It is useful to be able to identify types which, when cloned, result in a logical snapshot. That is, a value which can never be mutated. Note that there may in fact be mutation under the covers, but this mutation is not visible to the user. An example of such a type is Rc<T> – although the ref count on the Rc may change, the user has no direct access and so Rc<T> is still logically snapshotable. However, not all Rc instances are snapshottable – in particular, something like Rc<Cell<int>> is not.

trait Snapshot { }
impl Snapshot for .. { }

// In general, anything that can reach interior mutability is not
// snapshotable.
impl<T> !Snapshot for Unsafe<T> { }

// But it's ok for Rc<T>.
impl<T:Snapshot> Snapshot for Rc<T> { }

Note that these definitions could all occur in a library. That is, the Rc type itself doesn’t need to know about the Snapshot trait.

Preventing access to managed data. As part of the GC design, we expect it will be useful to write specialized allocators or smart pointers that explicitly do not support tracing, so as to avoid any kind of GC overhead. The general idea is that there should be a bound, let’s call it NoManaged, that indicates that a type cannot reach managed data and hence does not need to be part of the GC’s root set. This trait could be implemented as follows:

unsafe trait NoManaged { }
unsafe impl NoManaged for .. { }
impl<T> !NoManaged for Gc<T> { }

Preventing access to destructors. It is generally recognized that allowing destructors to escape into managed data – frequently referred to as finalizers – is a bad idea. Therefore, we would generally like to ensure that anything is placed into a managed box does not implement the drop trait. Instead, we would prefer to regular the use of drop through a guardian-like API, which basically means that destructors are not asynchronously executed by the GC, as they would be in Java, but rather enqueued for the mutator thread to run synchronously at its leisure. In order to handle this, though, we presumably need some sort of guardian wrapper types that can take a value which has a destructor and allow it to be embedded within managed data. We can summarize this in a trait GcSafe as follows:

unsafe trait GcSafe { }
unsafe impl GcSafe for .. { }

// By default, anything which has drop trait is not GcSafe.
impl<T:Drop> !GcSafe for T { }

// But guardians are, even if `T` has drop.
impl<T> GcSafe for Guardian<T> { }

Why are `Copy` and `Sized` different?

The Copy and Sized traits remain builtin to the compiler. This makes sense because they are intimately tied to analyses the compiler performs. For example, the running of destructors and tracking of moves requires knowing which types are Copy. Similarly, the allocation of stack frames need to know whether types are fully Sized. In contrast, sendability and sharability has been fully exported to libraries at this point.

In addition, opting in to Copy makes sense for several reasons:

Experience has shown that “data-like structs”, for which Copy is most appropriate, are a very small percentage of the total.
Changing a public API from being copyable to being only movable has a outsized impact on users of the API. It is common however that as APIs evolve they will come to require owned data (like a Vec), even if they do not initially, and hence will change from being copyable to only movable. Opting in to Copy is a way of saying that you never foresee this coming to pass.
Often it is useful to create linear “tokens” that do not themselves have data but represent permissions. This can be done today using markers but it is awkward. It becomes much more natural under this proposal.

Drawbacks

API stability. The main drawback of this approach over the existing opt-in approach seems to be that a type may be “accidentally” sendable or sharable. I discuss this above under the heading of “balancing abstraction, safety, and convenience”. One point I would like to add here, as it specifically pertains to API stability, is that a library may, if they choose, opt out of Send and Share pre-emptively, in order to “reserve the right” to add non-sendable things in the future.

Alternatives

The existing opt-in design is of course an alternative.
We could also simply add the notion of unsafe traits and not default impls and then allow types to unsafely implement Send or Share, bypassing the normal safety guidelines. This gives an escape valve for a downstream client to assert that something is sendable which was not declared as sendable. However, such a solution is deeply unsatisfactory, because it rests on the downstream client making an assertion about the implementation of the library it uses. If that library should be updated, the client’s assumptions could be invalidated, but no compilation errors will result (the impl was already declared as unsafe, after all).

Phasing

Many of the mechanisms described in this RFC are not needed immediately. Therefore, we would like to implement a minimal “forwards compatible” set of changes now and then leave the remaining work for after the 1.0 release. The builtin rules that the compiler currently implements for send and share are quite close to what is proposed in this RFC. The major change is that unsafe pointers and the UnsafeCell type are currently considered sendable.

Therefore, to be forwards compatible in the short term, we can use the same hybrid of builtin and explicit impls for Send and Share that we use for Copy, with the rule that unsafe pointers and UnsafeCell are not considered sendable. We must also implement the unsafe trait and unsafe impl concept.

What this means in practice is that using *const T, *mut T, and UnsafeCell will make a type T non-sendable and non-sharable, and T must then explicitly implement Send or Share.

Unresolved questions

The terminology of “unsafe trait” seems somewhat misleading, since it seems to suggest that “using” the trait is unsafe, rather than implementing it. One suggestion for an alternate keyword was trusted trait, which might dovetail with the use of trusted to specify a trusted block of code. If we did use trusted trait, it seems that all impls would also have to be trusted impl.
Perhaps we should declare a trait as a “default trait” directly, rather than using the impl Drop for .. syntax. I don’t know precisely what syntax to use, though.
Currently, there are special rules relating to object types and the builtin traits. If the “builtin” traits are no longer builtin, we will have to generalize object types to be simply a set of trait references. This is already planned but merits a second RFC. Note that no changes here are required for the 1.0, since the phasing plan dictates that builtin traits remain special until after 1.0.

The Rust RFC Book