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// NOTE: Both these features have accepted RFCs #![feature( const_panic, // RFC 2345 - Const asserts )] #![deny(missing_docs)] #![cfg_attr(not(feature = "std"), no_std)] //! Zero overhead tracing garbage collection for rust, //! by abusing the borrow checker. //! //! ## Features //! 1. Easy to use, since `Gc<T>` is `Copy` and coerces to a reference. //! 2. Absolutely zero overhead when modifying pointers, since `Gc<T>` is `Copy`. //! 3. Implementation agnostic API //! 4. Unsafe code has complete freedom to manipulate garbage collected pointers, and it doesn't need to understand the distinction //! 5. Uses rust's lifetime system to ensure all roots are known at explicit safepoints, without any runtime overhead. //! 6. Collection can only happen with an explicit `safepoint` call and has no overhead between these calls, //! 7. API supports moving objects (allowing copying/generational GCs) #![cfg(any(feature = "alloc", feature = "std"))] extern crate alloc; /* * I want this library to use 'mostly' stable features, * unless there's good justification to use an unstable feature. */ use core::mem; use core::ops::{Deref, DerefMut}; use core::ptr::NonNull; use core::marker::PhantomData; use core::hash::{Hash, Hasher}; use core::fmt::{self, Debug, Formatter}; #[macro_use] mod manually_traced; pub mod cell; pub mod prelude; pub mod dummy_impl; /// Invoke the closure with a temporary [GcContext], /// then perform a safepoint afterwards. /// /// Normally returns a tuple `($updated_root, $closure_result)`. /// /// If a value is provided it is considered as a root of garbage collection /// both for the safepoint and the duration of the entire context. /// /// # Safety /// This macro is completely safe, although it expands to unsafe code internally. // TODO: Document all forms of this macro #[macro_export(local_inner_macros)] macro_rules! safepoint_recurse { ($context:ident, |$sub_context:ident| $closure:expr) => {{ let ((), result) = safepoint_recurse!($context, (), |$sub_context, new_root| { let () = new_root; $closure }); result }}; ($context:ident, $root:expr, |$sub_context:ident, $new_root:ident| $closure:expr) => {{ let mut root = $root; let result = unsafe { __recurse_context!($context, &mut root, |$sub_context, $new_root| { $closure }) }; /* * NOTE: We're assuming result is unmanaged here * The borrow checker will verify this is true (by marking this as a mutation). * If you need a manged result, use the @managed_result variant */ let updated_root = safepoint!($context, $root); (updated_root, result) }}; ($context:ident, $root:expr, @managed_result, |$sub_context:ident, $new_root:ident| $closure:expr) => {{ use $crate::{GcContext}; let mut root = $root; let erased_result = unsafe { __recurse_context!( $context, &mut root, |$sub_context, $new_root| { let result = $closure; $sub_context.rebrand_static(result) } ) }; /* * Rebrand back to the current collector lifetime * It could have possibly been allocated from the sub-context inside the closure, * but as long as it was valid at the end of the closure it's valid now. * We trust that GcContext::recurse_context * did not perform a collection after calling the closure. */ let result = unsafe { $context.rebrand_self(**erased_result) }; safepoint!($context, (root, result)) }}; } /// Create a new sub-context for the duration of the closure /// /// The specified `root` object will be appended to the shadow-stack /// and is guarenteed to live for the entire lifetime of the closure /// (and the created sub-context). /// /// Unlike [safepoint_recurse!] this doesn't imply a safepoint anywhere. /// /// # Safety /// This doesn't actually mutate the original collector. /// It is possible user code could trigger a collection in the closure /// without the borrow checker noticing invalid pointers elsewhere. /// (See docs for [GcContext::recurse_context]) /// /// It is not publicly exposed for this reason #[macro_export] #[doc(hidden)] macro_rules! __recurse_context { ($context:ident, $root:expr, |$sub_context:ident, $new_root:ident| $closure:expr) => {{ use $crate::{GcContext}; // TODO: Panic safety $context.recurse_context(&mut $root, |mut $sub_context, erased_root| { /* * NOTE: Guarenteed to live for the lifetime of the entire closure. * However, it could be relocated if 'sub_collector' is collected */ let $new_root = $sub_context.rebrand_self(*erased_root); $closure }) }}; } /// Indicate it's safe to begin a garbage collection, /// while keeping the specified root alive. /// /// All other garbage collected pointers that aren't reachable /// from the root are invalidated. /// They have a lifetime that references the [GcContext] /// and the borrow checker considers the safepoint a 'mutation'. /// /// The root is exempted from the "mutation" and rebound to the new lifetime. /// /// ## Example /// ``` /// # use ::zerogc::safepoint; /// # let mut context = zerogc::dummy_impl::DummySystem::new().new_context(); /// # // TODO: Can we please get support for non-Sized types like `String`?!?!?! /// let root = zerogc::dummy_impl::leaked(String::from("potato")); /// let root = safepoint!(context, root); /// assert_eq!(**root, "potato"); /// ``` /// /// ## Safety /// This macro is completely safe, although it expands to unsafe code internally. #[macro_export] macro_rules! safepoint { ($context:ident, $value:expr) => {unsafe { use $crate::{GcContext}; // TODO: What happens if we panic during a collection /* * Some collectors support multiple running instances * with different ids, handing out different GC pointers. * TODO: Should we be checking somehow that the ids match? */ let mut erased = $context.rebrand_static($value); $context.basic_safepoint(&mut &mut erased); $context.rebrand_self(erased) }}; } /// Indicate its safe to begin a garbage collection (like [safepoint!]) /// and then "freeze" the specified context. /// /// Until it's unfrozen, the context can't be used for allocation. /// Its roots are marked invalid, since the collector could be relocating them. /// However, the roots of any parent contexts are still considered valid. /// /// This allows other threads to perform collections *without blocking this thread*. #[macro_export] macro_rules! freeze_context { ($context:ident) => {unsafe { use $crate::{GcContext, FrozenContext}; let mut context = $context; context.freeze(); FrozenContext::new(context) }}; } /// Unfreeze the context, allowing it to be used again /// /// Returns a [GcContext] struct. #[macro_export] macro_rules! unfreeze_context { ($frozen:ident) => {unsafe { use $crate::{FrozenContext, GcContext}; let mut context = FrozenContext::into_context($frozen); context.unfreeze(); context }}; } /// A garbage collector implementation. /// /// These implementations should be completely safe and zero-overhead. pub unsafe trait GcSystem { /// The type of collector IDs given by this system type Id: CollectorId; /// The type of contexts used in this sytem type Context: GcContext<Id=Self::Id>; } /// A system which supports creating handles to [Gc] references. /// /// This type-system hackery is needed because /// we need to place bounds on `T as GcBrand` // TODO: Remove when we get more powerful types pub unsafe trait GcHandleSystem<'gc, 'a, T: GcSafe + ?Sized + 'gc>: GcSystem where T: GcErase<'a, Self::Id>, <T as GcErase<'a, Self::Id>>::Erased: GcSafe { /// The type of handles to this object. type Handle: GcHandle<<T as GcErase<'a, Self::Id>>::Erased, System=Self>; /// Create a handle to the specified GC pointer, /// which can be used without a context /// /// NOTE: Users should only use from [Gc::create_handle]. /// /// The system is implicit in the [Gc] #[doc(hidden)] fn create_handle(gc: Gc<'gc, T, Self::Id>) -> Self::Handle; } /// The context of garbage collection, /// which can be frozen at a safepoint. /// /// This is essentially used to maintain a shadow-stack to a set of roots, /// which are guarenteed not to be collected until a safepoint. /// /// This context doesn't necessarily support allocation (see [GcSimpleAlloc] for that). pub unsafe trait GcContext: Sized { /// The system used with this context type System: GcSystem<Context=Self, Id=Self::Id>; /// The type of ids used in the system type Id: CollectorId; /// Inform the garbage collection system we are at a safepoint /// and are ready for a potential garbage collection. /// /// ## Safety /// This method is unsafe and should never be invoked by user code. /// /// See the [safepoint!] macro for a safe wrapper. unsafe fn basic_safepoint<T: Trace>(&mut self, value: &mut &mut T); /// Inform the garbage collection system we are at a safepoint /// and are ready for a potential garbage collection. /// /// Unlike a `basic_safepoint`, the collector continues to /// stay at the safepoint instead of returning immediately. /// The context can't be used for anything (including allocations), /// until it is unfrozen. /// /// This allows other threds to perform collections while this /// thread does other work (without using the GC). /// /// The current contexts roots are considered invalid /// for the duration of the collection, /// since the collector could potentially relocate them. /// /// Any parent contexts are fine and their roots will be /// preserved by collections. /// /// ## Safety /// Assumes this context is valid and not already frozen. /// /// Don't invoke this directly unsafe fn freeze(&mut self); /// Unfreeze this context, allowing it to be used again. /// /// ## Safety /// Must be a valid context! /// Must be currently frozen! /// /// Don't invoke this directly unsafe fn unfreeze(&mut self); #[inline(always)] #[doc(hidden)] unsafe fn rebrand_static<'a, T>(&self, value: T) -> T::Erased where T: GcErase<'a, Self::Id> { let branded = mem::transmute_copy(&value); mem::forget(value); branded } #[inline(always)] #[doc(hidden)] unsafe fn rebrand_self<'gc, T>(&'gc self, value: T) -> T::Branded where T: GcRebrand<'gc, Self::Id> { let branded = mem::transmute_copy(&value); mem::forget(value); branded } /// Invoke the closure with a temporary [GcContext]. /// /// The specified value is /// guarenteed to live throughout the created context for the closure. /// However, because it could possibly be relocated by a collection, /// it's bound to the lifetime of the sub-collector. /// /// ## Safety /// This macro doesn't imply garbage collection, /// so it doesn't mutate the collector directly. /// However the specified closure could trigger a collection in the sub-context. /// This would in undefined behavior if the collection /// invalidates a pointer tied to this context. /// /// For this reason, this function should never be invoked by user code. /// /// See the [safepoint_recurse!] macro for a safe wrapper unsafe fn recurse_context<T, F, R>(&self, value: &mut &mut T, func: F) -> R where T: Trace, F: for <'gc> FnOnce(&'gc mut Self, &'gc mut T) -> R; } /// A simple interface to allocating from a [GcContext]. /// /// Some garbage collectors implement more complex interfaces, /// so implementing this is optional pub unsafe trait GcSimpleAlloc<'gc, T: GcSafe + 'gc>: GcContext + 'gc { /// Allocate the specified object in this garbage collector, /// binding it to the lifetime of this collector. /// /// The object will never be collected until the next safepoint, /// which is considered a mutation by the borrow checker and will be statically checked. /// Therefore, we can statically guarantee the pointers will survive until the next safepoint. /// /// See `safepoint!` docs on how to properly invoke a safepoint /// and transfer values across it. /// /// This gives a immutable reference to the resulting object. /// Once allocated, the object can only be correctly modified with a `GcCell` fn alloc(&'gc self, value: T) -> Gc<'gc, T, Self::Id>; } /// The internal representation of a frozen context /// /// ## Safety /// Don't use this directly!!! #[doc(hidden)] #[must_use] pub struct FrozenContext<C: GcContext> { /// The frozen context context: C, } impl<C: GcContext> FrozenContext<C> { #[doc(hidden)] #[inline] pub unsafe fn new(context: C) -> Self { FrozenContext { context } } #[doc(hidden)] #[inline] pub unsafe fn into_context(self) -> C { self.context } } /// Uniquely identifies the collector in case there are /// multiple collectors. /// /// ## Safety /// To simply the typing, this contains no references to the /// lifetime of the associated [GcSystem]. /// /// It's implicitly held and is unsafe to access. /// As long as the collector is valid, /// this id should be too. /// /// It should be safe to assume that a collector exists /// if any of its pointers still do! pub unsafe trait CollectorId: Copy + Eq + Debug + NullTrace + 'static { /// The type of the garbage collector system type System: GcSystem<Id=Self>; /// Perform a write barrier before writing to a garbage collected field /// /// ## Safety /// Smilar to the [GcDirectBarrier] trait, it can be assumed that /// the field offset is correct and the types match. unsafe fn gc_write_barrier<'gc, T: GcSafe + ?Sized + 'gc, V: GcSafe + ?Sized + 'gc>( owner: &Gc<'gc, T, Self>, value: &Gc<'gc, V, Self>, field_offset: usize ); /// Assume the ID is valid and use it to access the [GcSystem] /// /// NOTE: The system is bound to the lifetime of *THIS* id. /// A CollectorId may have an internal pointer to the system /// and the pointer may not have a stable address. In other words, /// it may be difficult to reliably take a pointer to a pointer. /// /// ## Safety /// Undefined behavior if the associated collector no longer exists. unsafe fn assume_valid_system(&self) -> &Self::System; } /// A garbage collected pointer to a value. /// /// This is the equivalent of a garbage collected smart-pointer. /// It's so smart, you can even coerce it to a reference bound to the lifetime of the `GarbageCollectorRef`. /// However, all those references are invalidated by the borrow checker as soon as /// your reference to the collector reaches a safepoint. /// The objects can only survive garbage collection if they live in this smart-pointer. /// /// The smart pointer is simply a guarantee to the garbage collector /// that this points to a garbage collected object with the correct header, /// and not some arbitrary bits that you've decided to heap allocate. #[repr(C)] pub struct Gc<'gc, T: GcSafe + ?Sized + 'gc, Id: CollectorId> { value: NonNull<T>, /// Used to uniquely identify the collector, /// to ensure we aren't modifying another collector's pointers collector_id: Id, /* * TODO: I think this lifetime variance is safe * Better add some tests and an explination. */ marker: PhantomData<&'gc T>, } impl<'gc, T: GcSafe + ?Sized + 'gc, Id: CollectorId> Gc<'gc, T, Id> { /// Create a GC pointer from a raw ID/pointer pair /// /// ## Safety /// Undefined behavior if the underlying pointer is not valid /// and associated with the collector corresponding to the id. #[inline(always)] pub unsafe fn from_raw(id: Id, value: NonNull<T>) -> Self { Gc { collector_id: id, value, marker: PhantomData } } /// The value of the underlying pointer #[inline(always)] pub fn value(&self) -> &'gc T { unsafe { *(&self.value as *const NonNull<T> as *const &'gc T) } } /// Cast this reference to a raw pointer /// /// ## Safety /// It's undefined behavior to mutate the /// value. /// The pointer is only valid as long as /// the reference is. #[inline] pub unsafe fn as_raw_ptr(&self) -> *mut T { self.value.as_ptr() as *const T as *mut T } /// Create a handle to this object, which can be used without a context #[inline] pub fn create_handle<'a>(&self) -> <Id::System as GcHandleSystem<'gc, 'a, T>>::Handle where Id::System: GcHandleSystem<'gc, 'a, T>, T: GcErase<'a, Id> + 'a, <T as GcErase<'a, Id>>::Erased: GcSafe + 'a { <Id::System as GcHandleSystem<'gc, 'a, T>>::create_handle(*self) } /// Get a reference to the system /// /// ## Safety /// This is based on the assumption that a [GcSystem] must outlive /// all of the pointers it owns. /// Although it could be restricted to the lifetime of the [CollectorId] /// (in theory that may have an internal pointer) it will still live for '&self'. #[inline] pub fn system(&self) -> &'_ Id::System { // This assumption is safe - see the docs unsafe { self.collector_id.assume_valid_system() } } /// Get a copy of the collector's id /// /// The underlying collector it points to is not necessarily always valid #[inline] pub fn collector_id(&self) -> Id { self.collector_id } } /// Double-indirection is completely safe unsafe impl<'gc, T: GcSafe + 'gc, Id: CollectorId> GcSafe for Gc<'gc, T, Id> { const NEEDS_DROP: bool = true; // We are Copy } /// Rebrand unsafe impl<'gc, 'new_gc, T, Id> GcRebrand<'new_gc, Id> for Gc<'gc, T, Id> where T: GcSafe + GcRebrand<'new_gc, Id>, <T as GcRebrand<'new_gc, Id>>::Branded: GcSafe, Id: CollectorId, { type Branded = Gc<'new_gc, <T as GcRebrand<'new_gc, Id>>::Branded, Id>; } unsafe impl<'gc, 'a, T, Id> GcErase<'a, Id> for Gc<'gc, T, Id> where T: GcSafe + GcErase<'a, Id>, <T as GcErase<'a, Id>>::Erased: GcSafe, Id: CollectorId, { type Erased = Gc<'a, <T as GcErase<'a, Id>>::Erased, Id>; } unsafe impl<'gc, T: GcSafe + 'gc, Id: CollectorId> Trace for Gc<'gc, T, Id> { // We always need tracing.... const NEEDS_TRACE: bool = true; #[inline] fn visit<V: GcVisitor>(&mut self, visitor: &mut V) -> Result<(), V::Err> { unsafe { // We're doing this correctly! V::visit_gc(visitor, self) } } } impl<'gc, T: GcSafe + 'gc, Id: CollectorId> Deref for Gc<'gc, T, Id> { type Target = &'gc T; #[inline(always)] fn deref(&self) -> &Self::Target { unsafe { &*(&self.value as *const NonNull<T> as *const &'gc T) } } } unsafe impl<'gc, O, V, Id> GcDirectBarrier<'gc, Gc<'gc, O, Id>> for Gc<'gc, V,Id> where O: GcSafe + 'gc, V: GcSafe + 'gc, Id: CollectorId { #[inline(always)] unsafe fn write_barrier(&self, owner: &Gc<'gc, O, Id>, field_offset: usize) { Id::gc_write_barrier(owner, self, field_offset) } } // We can be copied freely :) impl<'gc, T: GcSafe + ?Sized + 'gc, Id: CollectorId> Copy for Gc<'gc, T, Id> {} impl<'gc, T: GcSafe + ?Sized + 'gc, Id: CollectorId> Clone for Gc<'gc, T, Id> { #[inline(always)] fn clone(&self) -> Self { *self } } // Delegating impls impl<'gc, T: GcSafe + Hash + 'gc, Id: CollectorId> Hash for Gc<'gc, T, Id> { #[inline] fn hash<H: Hasher>(&self, state: &mut H) { self.value().hash(state) } } impl<'gc, T: GcSafe + PartialEq + 'gc, Id: CollectorId> PartialEq for Gc<'gc, T, Id> { #[inline] fn eq(&self, other: &Self) -> bool { // NOTE: We compare by value, not identity self.value() == other.value() } } impl<'gc, T: GcSafe + Eq + 'gc, Id: CollectorId> Eq for Gc<'gc, T, Id> {} impl<'gc, T: GcSafe + PartialEq + 'gc, Id: CollectorId> PartialEq<T> for Gc<'gc, T, Id> { #[inline] fn eq(&self, other: &T) -> bool { self.value() == other } } impl<'gc, T: GcSafe + Debug + 'gc, Id: CollectorId> Debug for Gc<'gc, T, Id> { fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result { if !f.alternate() { // Pretend we're a newtype by default f.debug_tuple("Gc").field(self.value()).finish() } else { // Alternate spec reveals `collector_id` f.debug_struct("Gc") .field("collector_id", &self.collector_id) .field("value", self.value()) .finish() } } } /// In order to send *references* between threads, /// the underlying type must be sync. /// /// This is the same reason that `Arc<T>: Send` requires `T: Sync` unsafe impl<'gc, T, Id> Send for Gc<'gc, T, Id> where T: GcSafe + ?Sized + Sync, Id: CollectorId + Sync {} /// If the underlying type is `Sync`, it's safe /// to share garbage collected references between threads. /// /// The safety of the collector itself depends on whether [CollectorId] is Sync. /// If it is, the whole garbage collection implenentation should be as well. unsafe impl<'gc, T, Id> Sync for Gc<'gc, T, Id> where T: GcSafe + ?Sized + Sync, Id: CollectorId + Sync {} /// A owned handle which points to a garbage collected object. /// /// This is considered a root by the garbage collector that is independent /// of any specific [GcContext]. Safepoints /// don't need to be informed of this object for collection to start. /// The root is manually managed by user-code, much like a [Box] or /// a reference counted pointer. /// /// This can be cloned and stored independently from a context, /// bridging the gap between native memory and managed memory. /// These are useful to pass to C APIs or any other code /// that doesn't cooperate with zerogc. /// /// ## Tracing /// The object behind this handle is already considered a root of the collection. /// It should always be considered reachable by the garbage collector. /// /// Validity is tracked by this smart-pointer and not by tracing. /// Therefore it is safe to implement [NullTrace] for handles. /* * TODO: Should we drop the Clone requirement? */ pub unsafe trait GcHandle<T: GcSafe + ?Sized>: Clone + NullTrace { /// The type of the system used with this handle type System: GcSystem<Id=Self::Id>; /// The type of [CollectorId] used with this sytem type Id: CollectorId; /// Access this handle inside the closure, /// possibly associating it with the specified /// /// This is accesses the object within "critical section" /// that will **block collections** /// for as long as the closure is in use. /// /// These calls cannot be invoked recursively or they /// may cause a deadlock. /// /// This is similar in purpose to JNI's [GetPrimitiveArrayCritical](https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetPrimitiveArrayCritical_ReleasePrimitiveArrayCritical). /// However it never performs a copy, it is just guarenteed to block any collections. /* * TODO: Should we require this of all collectors? * How much does it limit flexibility? */ fn use_critical<R>(&self, func: impl FnOnce(&T) -> R) -> R; } /// Trait for binding [GcHandle]s to contexts /// using [GcBindHandle::bind_to] /// /// This is separate from the [GcHandle] trait /// because Rust doesn't have Generic Associated Types /// /// TODO: Remove when we get more powerful types pub unsafe trait GcBindHandle<'new_gc, T: GcSafe + ?Sized>: GcHandle<T> where T: GcRebrand<'new_gc, Self::Id>, <T as GcRebrand<'new_gc, Self::Id>>::Branded: GcSafe { /// Associate this handle with the specified context, /// allowing its underlying object to be accessed /// as long as the context is valid. /// /// The underlying object can be accessed just like any /// other object that would be allocated from the context. /// It'll be properly collected and can even be used as a root /// at the next safepoint. fn bind_to(&self, context: &'new_gc <Self::System as GcSystem>::Context) -> Gc< 'new_gc, <T as GcRebrand<'new_gc, Self::Id>>::Branded, Self::Id >; } /// Safely trigger a write barrier before /// writing to a garbage collected value. /// /// The value must be in managed memory, /// a *direct* part of a garbage collected object. /// Write barriers (and writes) must include a reference /// to its owning object. /// /// ## Safety /// It is undefined behavior to forget to trigger a write barrier. /// /// Field offsets are unchecked. They must refer to the correct /// offset (in bytes). /// /// ### Indirection /// This trait only support "direct" writes, /// where the destination field is inline with the source object. /// /// For example it's correct to implement `GcDirectWrite<Value=A> for (A, B)`, /// since since `A` is inline with the owning tuple. /// /// It is **incorrect** to implement `GcDirectWrite<Value=T> for Vec<T>`, /// since it `T` is indirectly referred to by the vector. /// There's no "field offset" we can use to get from `*mut Vec` -> `*mut T`. /// /// The only exception to this rule is [Gc] itself. /// GcRef can freely implement [GcDirectWrite] for any (and all values), /// even though it's just a pointer. /// It's the final destination of all write barriers and is expected /// to internally handle the indirection. pub unsafe trait GcDirectBarrier<'gc, OwningRef>: Trace { /// Trigger a write barrier, /// before writing to one of the owning object's managed fields /// /// It is undefined behavior to mutate a garbage collected field /// without inserting a write barrier before it. /// /// Generational, concurrent and incremental GCs need this to maintain /// the tricolor invariant. /// /// ## Safety /// The specified field offset must point to a valid field /// in the source object. /// /// The type of this value must match the appropriate field unsafe fn write_barrier(&self, owner: &OwningRef, field_offset: usize); } /// Indicates that a type can be safely allocated by a garbage collector. /// /// ## Safety /// Custom destructors must never reference garbage collected pointers. /// The garbage collector may have already freed the other objects /// before calling this type's drop function. /// /// Unlike java finalizers, this allows us to deallocate objects normally /// and avoids a second pass over the objects /// to check for resurrected objects. pub unsafe trait GcSafe: Trace { /// If this type needs a destructor run /// /// This is usually equivalent to `std::mem::needs_drop`. /// However procedurally derived code can sometimes provide /// a no-op drop implementation (for safety), /// which would lead to a false positive with `std::mem::needs_drop()` const NEEDS_DROP: bool; /// Assert this type is GC safe /// /// Only used by procedural derive #[doc(hidden)] fn assert_gc_safe() {} } /// Assert that a type implements Copy /// /// Used by the derive code #[doc(hidden)] pub fn assert_copy<T: Copy>() {} /// A wrapper type that assumes its contents don't need to be traced #[repr(transparent)] #[derive(Copy, Clone, Debug)] pub struct AssumeNotTraced<T>(T); impl<T> AssumeNotTraced<T> { /// Assume the specified value doesn't need to be traced /// /// ## Safety /// Undefined behavior if the value contains anything that need to be traced /// by a garbage collector. #[inline] pub unsafe fn new(value: T) -> Self { AssumeNotTraced(value) } /// Unwrap the inner value of this wrapper #[inline] pub fn into_inner(self) -> T { self.0 } } impl<T> Deref for AssumeNotTraced<T> { type Target = T; #[inline] fn deref(&self) -> &Self::Target { &self.0 } } impl<T> DerefMut for AssumeNotTraced<T> { #[inline] fn deref_mut(&mut self) -> &mut Self::Target { &mut self.0 } } unsafe impl<T> Trace for AssumeNotTraced<T> { const NEEDS_TRACE: bool = false; #[inline(always)] // This method does nothing and is always a win to inline fn visit<V: GcVisitor>(&mut self, _visitor: &mut V) -> Result<(), V::Err> { Ok(()) } } unsafe impl<T> TraceImmutable for AssumeNotTraced<T> { #[inline(always)] fn visit_immutable<V: GcVisitor>(&self, _visitor: &mut V) -> Result<(), V::Err> { Ok(()) } } unsafe impl<T> NullTrace for AssumeNotTraced<T> {} /// No tracing implies GcSafe unsafe impl<T> GcSafe for AssumeNotTraced<T> { const NEEDS_DROP: bool = core::mem::needs_drop::<T>(); } unsafe_gc_brand!(AssumeNotTraced, T); /// Changes all references to garbage collected /// objects to match a specific lifetime. /// /// This indicates that its safe to transmute to the new `Branded` type /// and all that will change is the lifetimes. // TODO: Can we support lifetimes that are smaller than 'new_gc pub unsafe trait GcRebrand<'new_gc, Id: CollectorId>: Trace { /// This type with all garbage collected lifetimes /// changed to `'new_gc` /// /// This must have the same in-memory repr as `Self`, /// so that it's safe to transmute. type Branded: Trace + 'new_gc; /// Assert this type can be rebranded /// /// Only used by procedural derive #[doc(hidden)] fn assert_rebrand() {} } /// Indicates that it's safe to erase all GC lifetimes /// and change them to 'static (logically an 'unsafe) /// /// This trait is the opposite of [GcRebrand] /// /// The lifetime '`a` is the minimum lifetime of all other non-gc references. pub unsafe trait GcErase<'a, Id: CollectorId>: Trace { /// This type with all garbage collected lifetimes /// changed to `'static` (or erased) /// /// This must have the same in-memory repr as `Self`, /// so that it's safe to transmute. type Erased: 'a; /// Assert this type can be erased /// /// Only used by procedural derive #[doc(hidden)] fn assert_erase() {} } /// Indicates that a type can be traced by a garbage collector. /// /// This doesn't necessarily mean that the type is safe to allocate in a garbage collector ([GcSafe]). /// /// ## Safety /// See the documentation of the `trace` method for more info. /// Essentially, this object must faithfully trace anything that /// could contain garbage collected pointers or other `Trace` items. pub unsafe trait Trace { /// Whether this type needs to be traced by the garbage collector. /// /// Some primitive types don't need to be traced at all, /// and can be simply ignored by the garbage collector. /// /// Collections should usually delegate this decision to their element type, /// claiming the need for tracing only if their elements do. /// For example, to decide `Vec<u32>::NEEDS_TRACE` you'd check whether `u32::NEEDS_TRACE` (false), /// and so then `Vec<u32>` doesn't need to be traced. /// By the same logic, `Vec<Gc<u32>>` does need to be traced, /// since it contains a garbage collected pointer. /// /// If there are multiple types involved, you should check if any of them need tracing. /// One perfect example of this is structure/tuple types which check /// `field1::NEEDS_TRACE || field2::NEEDS_TRACE || field3::needs_trace`. /// The fields which don't need tracing will always ignored by `GarbageCollector::trace`, /// while the fields that do will be properly traced. /// /// False negatives will always result in completely undefined behavior. /// False positives could result in un-necessary tracing, but are perfectly safe otherwise. /// Therefore, when in doubt you always assume this is true. /// /// If this is true `NullTrace` should (but doesn't have to) be implemented. const NEEDS_TRACE: bool; /// Visit each field in this type /// /// Users should never invoke this method, and always call the `V::visit` instead. /// Only the collector itself is premitted to call this method, /// and **it is undefined behavior for the user to invoke this**. /// /// Structures should trace each of their fields, /// and collections should trace each of their elements. /// /// ### Safety /// Some types (like `Gc`) need special actions taken when they're traced, /// but those are somewhat rare and are usually already provided by the garbage collector. /// /// Unless I explicitly document actions as legal I may decide to change i. /// I am only bound by the constraints of [semantic versioning](http://semver.org/) in the trace function /// if I explicitly document it as safe behavior in this method's documentation. /// If you try something that isn't explicitly documented here as permitted behavior, /// the collector may choose to override your memory with `0xDEADBEEF`. /// ## Always Permitted /// - Reading your own memory (includes iteration) /// - Interior mutation is undefined behavior, even if you use `GcCell` /// - Calling `GcVisitor::visit` with the specified collector /// - `GarbageCollector::trace` already verifies that it owns the data, so you don't need to do that /// - Panicking /// - This should be reserved for cases where you are seriously screwed up, /// and can't fulfill your contract to trace your interior properly. /// - One example is `Gc<T>` which panics if the garbage collectors are mismatched /// - This rule may change in future versions, depending on how we deal with multi-threading. /// ## Never Permitted Behavior /// - Forgetting a element of a collection, or field of a structure /// - If you forget an element undefined behavior will result /// - This is why we always prefer automatically derived implementations where possible. /// - You will never trigger undefined behavior with an automatic implementation, /// and it'll always be completely sufficient for safe code (aside from destructors). /// - With an automatically derived implementation you will never miss a field /// - It is undefined behavior to mutate any of your own data. /// - The mutable `&mut self` is just so copying collectors can relocate GC pointers /// - Invoking this function directly, without delegating to `GcVisitor` fn visit<V: GcVisitor>(&mut self, visitor: &mut V) -> Result<(), V::Err>; } /// A type that can be safely traced/relocated /// without having to use a mutable reference /// /// Types with interior mutability (like `RefCell` or `Cell<Gc<T>>`) /// can safely implement this, since they allow safely relocating the pointer /// without a mutable reference. /// Likewise primitives (with new garbage collected data) can also /// implement this (since they have nothing to trace). pub unsafe trait TraceImmutable: Trace { /// Visit an immutable reference to this type /// /// The visitor may want to relocate garbage collected pointers, /// which this type must support. fn visit_immutable<V: GcVisitor>(&self, visitor: &mut V) -> Result<(), V::Err>; } /// Marker types for types that don't need to be traced /// /// If this trait is implemented `Trace::NEEDS_TRACE` must be false pub unsafe trait NullTrace: Trace + TraceImmutable {} /// Visits garbage collected objects /// /// This should only be used by a [GcSystem] pub unsafe trait GcVisitor: Sized { /// The type of errors returned by this visitor type Err: Debug; /// Visit a reference to the specified value #[inline(always)] fn visit<T: Trace + ?Sized>(&mut self, value: &mut T) -> Result<(), Self::Err> { value.visit(self) } /// Visit a reference to the specified value #[inline(always)] fn visit_immutable<T: TraceImmutable + ?Sized>(&mut self, value: &T) -> Result<(), Self::Err> { value.visit_immutable(self) } /// Visit a garbage collected pointer /// /// ## Safety /// Undefined behavior if the GC pointer isn't properly visited. unsafe fn visit_gc<'gc, T: GcSafe + 'gc, Id: CollectorId>( &mut self, gc: &mut Gc<'gc, T, Id> ) -> Result<(), Self::Err>; }