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/* LICENSE BEGIN This file is part of the Sixty FPS Project Copyright (c) 2020 Olivier Goffart <olivier.goffart@sixtyfps.io> Copyright (c) 2020 Simon Hausmann <simon.hausmann@sixtyfps.io> SPDX-License-Identifier: GPL-3.0-only LICENSE END */ /*! This crate allows you to create ffi-friendly virtual tables. ## Features - A `#[vtable]` macro to annotate a VTable struct to generate the traits and structure to safely work with it. - `VRef`/`VRefMut`/`VBox` types. They are fat reference/box types which wrap a pointer to the vtable, and a pointer to the object. - Ability to store constants in a vtable. - These constants can even be a field offset. ## Example of use: ``` use vtable::*; // we are going to declare a VTable structure for an Animal trait #[vtable] #[repr(C)] struct AnimalVTable { /// pointer to a function that makes a noise. The `VRef<AnimalVTable>` is the type of /// the self object. /// /// Note: the #[vtable] macro will automatically add `extern "C"` if that is missing. make_noise: fn(VRef<AnimalVTable>, i32) -> i32, /// if there is a 'drop' member, it is considered as the destructor. drop: fn(VRefMut<AnimalVTable>), } struct Dog(i32); // The #[vtable] macro created the Animal Trait impl Animal for Dog { fn make_noise(&self, intensity: i32) -> i32 { println!("Wof!"); return self.0 * intensity; } } // the vtable macro also exposed a macro to create a vtable AnimalVTable_static!(static DOG_VT for Dog); // with that, it is possible to instantiate a VBox let animal_box = VBox::<AnimalVTable>::new(Dog(42)); assert_eq!(animal_box.make_noise(2), 42 * 2); ``` The `#[vtable]` macro created the "Animal" trait. Note that the `#[vtable]` macro is applied to the VTable struct so that `cbindgen` can see the actual vtable. */ #[doc(no_inline)] pub use const_field_offset::*; use core::marker::PhantomData; use core::ops::{Deref, DerefMut, Drop}; use core::{pin::Pin, ptr::NonNull}; #[doc(inline)] pub use vtable_macro::*; /// Internal trait that is implemented by the `#[vtable]` macro. /// /// Safety: The Target object needs to be implemented correctly. /// And there should be a VTable::VTable::new<T> function that returns a /// VTable suitable for the type T. pub unsafe trait VTableMeta { /// That's the trait object that implements the functions /// /// NOTE: the size must be `2*size_of::<usize>` /// and a `repr(C)` with `(vtable, ptr)` so it has the same layout as /// the inner and VBox/VRef/VRefMut type Target; /// That's the VTable itself (so most likely Self) type VTable: 'static; } /// This trait is implemented by the `#[vtable]` macro. /// /// It is implemented if the macro has a "drop" function. pub trait VTableMetaDrop: VTableMeta { /// Safety: the Target needs to be pointing to a valid allocated pointer unsafe fn drop(ptr: *mut Self::Target); fn new_box<X: HasStaticVTable<Self>>(value: X) -> VBox<Self>; } /// Allow to associate a VTable with a type. /// /// Safety: the VTABLE and STATIC_VTABLE need to be a a valid virtual table /// corresponding to pointer to Self instance. pub unsafe trait HasStaticVTable<VT> where VT: ?Sized + VTableMeta, { /// Safety: must be a valid VTable for Self fn static_vtable() -> &'static VT::VTable; } #[derive(Copy, Clone)] /// The inner structure of VRef, VRefMut, and VBox. /// /// Invariant: _vtable and _ptr are valid pointer for the lifetime of the container. /// _ptr is an instance of the object represented by _vtable #[allow(dead_code)] #[repr(C)] struct Inner { vtable: *const u8, ptr: *const u8, } impl Inner { /// Transmute a reference to self into a reference to T::Target. fn deref<T: ?Sized + VTableMeta>(&self) -> *const T::Target { debug_assert_eq!(core::mem::size_of::<T::Target>(), core::mem::size_of::<Inner>()); self as *const Inner as *const T::Target } } /// An equivalent of a Box that holds a pointer to a VTable and a pointer to an instance. /// A VBox frees the instance when dropped. /// /// The type parameter is supposed to be the VTable type. /// /// The VBox implements Deref so one can access all the members of the vtable. /// /// This is only valid if the VTable has a `drop` type (so that the `#[vtable]` macro /// implements the `VTableMetaDrop` trait for it) #[repr(transparent)] pub struct VBox<T: ?Sized + VTableMetaDrop> { inner: Inner, phantom: PhantomData<T::Target>, } impl<T: ?Sized + VTableMetaDrop> Deref for VBox<T> { type Target = T::Target; fn deref(&self) -> &Self::Target { unsafe { &*self.inner.deref::<T>() } } } impl<T: ?Sized + VTableMetaDrop> DerefMut for VBox<T> { fn deref_mut(&mut self) -> &mut Self::Target { unsafe { &mut *(self.inner.deref::<T>() as *mut _) } } } impl<T: ?Sized + VTableMetaDrop> Drop for VBox<T> { fn drop(&mut self) { unsafe { T::drop(self.inner.deref::<T>() as *mut _); } } } impl<T: ?Sized + VTableMetaDrop> VBox<T> { /// Create a new VBox from an instance of a type that can be associated with a VTable. /// /// Will move the instance on the heap. /// /// (the `HasStaticVTable` is implemented by the `“MyTrait”VTable_static!` macro generated by /// the #[vtable] macro) pub fn new<X: HasStaticVTable<T>>(value: X) -> Self { T::new_box(value) } /// Safety: the `ptr` needs to be a valid for the `vtable`, and properly allocated so it can be dropped pub unsafe fn from_raw(vtable: NonNull<T::VTable>, ptr: NonNull<u8>) -> Self { Self { inner: Inner { vtable: vtable.cast().as_ptr(), ptr: ptr.cast().as_ptr() }, phantom: PhantomData, } } /// Gets a VRef pointing to this box pub fn borrow<'b>(&'b self) -> VRef<'b, T> { unsafe { VRef::from_inner(self.inner) } } /// Gets a VRefMut pointing to this box pub fn borrow_mut<'b>(&'b mut self) -> VRefMut<'b, T> { unsafe { VRefMut::from_inner(self.inner) } } /// Leaks the content of the box. pub fn leak(self) -> VRefMut<'static, T> { let inner = self.inner; core::mem::forget(self); unsafe { VRefMut::from_inner(inner) } } } /// `VRef<'a MyTraitVTable>` can be thought as a `&'a dyn MyTrait` /// /// It will dereference to a structure that has the same members as MyTrait. #[repr(transparent)] pub struct VRef<'a, T: ?Sized + VTableMeta> { inner: Inner, phantom: PhantomData<&'a T::Target>, } // Need to implement manually otheriwse it is not implemented if T do not implement Copy / Clone impl<'a, T: ?Sized + VTableMeta> Copy for VRef<'a, T> {} impl<'a, T: ?Sized + VTableMeta> Clone for VRef<'a, T> { fn clone(&self) -> Self { Self { inner: self.inner, phantom: PhantomData } } } impl<'a, T: ?Sized + VTableMeta> Deref for VRef<'a, T> { type Target = T::Target; fn deref(&self) -> &Self::Target { unsafe { &*self.inner.deref::<T>() } } } impl<'a, T: ?Sized + VTableMeta> VRef<'a, T> { /// Create a new VRef from an reference of a type that can be associated with a VTable. /// /// (the `HasStaticVTable` is implemented by the `“MyTrait”VTable_static!` macro generated by /// the #[vtable] macro) pub fn new<X: HasStaticVTable<T>>(value: &'a X) -> Self { Self { inner: Inner { vtable: X::static_vtable() as *const T::VTable as *const u8, ptr: value as *const X as *const u8, }, phantom: PhantomData, } } /// Create a new Pin<VRef<_>> from a pinned reference. This is similar to `VRef::new`. pub fn new_pin<X: HasStaticVTable<T>>(value: core::pin::Pin<&'a X>) -> Pin<Self> { // Since Value is pinned, this means it is safe to construct a Pin unsafe { Pin::new_unchecked(Self { inner: Inner { vtable: X::static_vtable() as *const T::VTable as *const u8, ptr: value.get_ref() as *const X as *const u8, }, phantom: PhantomData, }) } } unsafe fn from_inner(inner: Inner) -> Self { Self { inner, phantom: PhantomData } } /// Safety: the `ptr` needs to be a valid for the `vtable`, and properly allocated so it can be dropped pub unsafe fn from_raw(vtable: NonNull<T::VTable>, ptr: NonNull<u8>) -> Self { Self { inner: Inner { vtable: vtable.cast().as_ptr(), ptr: ptr.cast().as_ptr() }, phantom: PhantomData, } } /// Return a reference of the given type if the type is matching. pub fn downcast<X: HasStaticVTable<T>>(&self) -> Option<&X> { if self.inner.vtable == X::static_vtable() as *const _ as *const u8 { // Safety: We just checked that the vtable fits unsafe { Some(&*(self.inner.ptr as *const X)) } } else { None } } /// Return a reference of the given type if the type is matching pub fn downcast_pin<X: HasStaticVTable<T>>(this: Pin<Self>) -> Option<Pin<&'a X>> { let inner = unsafe { Pin::into_inner_unchecked(this).inner }; if inner.vtable == X::static_vtable() as *const _ as *const u8 { // Safety: We just checked that the vtable fits unsafe { Some(Pin::new_unchecked(&*(inner.ptr as *const X))) } } else { None } } } /// `VRefMut<'a MyTraitVTable>` can be thought as a `&'a mut dyn MyTrait` /// /// It will dereference to a structure that has the same members as MyTrait. #[repr(transparent)] pub struct VRefMut<'a, T: ?Sized + VTableMeta> { inner: Inner, phantom: PhantomData<&'a mut T::Target>, } impl<'a, T: ?Sized + VTableMeta> Deref for VRefMut<'a, T> { type Target = T::Target; fn deref(&self) -> &Self::Target { unsafe { &*self.inner.deref::<T>() } } } impl<'a, T: ?Sized + VTableMeta> DerefMut for VRefMut<'a, T> { fn deref_mut(&mut self) -> &mut Self::Target { unsafe { &mut *(self.inner.deref::<T>() as *mut _) } } } impl<'a, T: ?Sized + VTableMeta> VRefMut<'a, T> { /// Create a new VRef from a mutable reference of a type that can be associated with a VTable. /// /// (the `HasStaticVTable` is implemented by the `“MyTrait”VTable_static!` macro generated by /// the #[vtable] macro) pub fn new<X: HasStaticVTable<T>>(value: &'a mut X) -> Self { Self { inner: Inner { vtable: X::static_vtable() as *const T::VTable as *const u8, ptr: value as *mut X as *const u8, }, phantom: PhantomData, } } unsafe fn from_inner(inner: Inner) -> Self { Self { inner, phantom: PhantomData } } /// Safety: the `ptr` needs to be a valid for the `vtable`, and properly allocated so it can be dropped pub unsafe fn from_raw(vtable: NonNull<T::VTable>, ptr: NonNull<u8>) -> Self { Self { inner: Inner { vtable: vtable.cast().as_ptr(), ptr: ptr.cast().as_ptr() }, phantom: PhantomData, } } /// Borrow this to obtain a VRef. pub fn borrow<'b>(&'b self) -> VRef<'b, T> { unsafe { VRef::from_inner(self.inner) } } /// Borrow this to obtain a new VRefMut. pub fn borrow_mut<'b>(&'b mut self) -> VRefMut<'b, T> { unsafe { VRefMut::from_inner(self.inner) } } /// Create a VRef with the same lifetime as the original lifetime. pub fn into_ref(self) -> VRef<'a, T> { unsafe { VRef::from_inner(self.inner) } } /// Return a reference of the given type if the type is matching. pub fn downcast<X: HasStaticVTable<T>>(&mut self) -> Option<&mut X> { if self.inner.vtable == X::static_vtable() as *const _ as *const u8 { // Safety: We just checked that the vtable fits unsafe { Some(&mut *(self.inner.ptr as *mut X)) } } else { None } } } /** Creates a `VRef` or a `VRefMut` suitable for an instance that implements the trait When possible, `VRef::new` or `VRefMut::new` should be preferred, as they use a static vtable. But when using the generated `XxxVTable_static!` macro that is not possible and this macro can be used instead. Note that the `downcast` will not work with references created with this macro. ``` use vtable::*; #[vtable] struct MyVTable { /* ... */ } struct Something { /* ... */}; impl My for Something {}; let mut s = Something { /* ... */}; // declare a my_vref variable for the said VTable new_vref!(let my_vref : VRef<MyVTable> for My = &s); // same but mutable new_vref!(let mut my_vref_m : VRefMut<MyVTable> for My = &mut s); ``` */ #[macro_export] macro_rules! new_vref { (let $ident:ident : VRef<$vtable:ty> for $trait_:path = $e:expr) => { // ensure that the type of the expression is correct let vtable = { use $crate::VTableMeta; fn get_vt<X: $trait_>(_: &X) -> <$vtable as VTableMeta>::VTable { <$vtable as VTableMeta>::VTable::new::<X>() } get_vt($e) }; let $ident = { use $crate::VTableMeta; fn create<'a, X: $trait_>( vtable: &'a <$vtable as VTableMeta>::VTable, val: &'a X, ) -> $crate::VRef<'a, <$vtable as VTableMeta>::VTable> { use ::core::ptr::NonNull; // Safety: we constructed the vtable such that it fits for the value unsafe { $crate::VRef::from_raw(NonNull::from(vtable), NonNull::from(val).cast()) } } create(&vtable, $e) }; }; (let mut $ident:ident : VRefMut<$vtable:ty> for $trait_:path = $e:expr) => { // ensure that the type of the expression is correct let vtable = { use $crate::VTableMeta; fn get_vt<X: $trait_>(_: &X) -> <$vtable as VTableMeta>::VTable { <$vtable as VTableMeta>::VTable::new::<X>() } get_vt($e) }; let mut $ident = { use $crate::VTableMeta; fn create<'a, X: $trait_>( vtable: &'a <$vtable as VTableMeta>::VTable, val: &'a mut X, ) -> $crate::VRefMut<'a, <$vtable as VTableMeta>::VTable> { use ::core::ptr::NonNull; // Safety: we constructed the vtable such that it fits for the value unsafe { $crate::VRefMut::from_raw(NonNull::from(vtable), NonNull::from(val).cast()) } } create(&vtable, $e) }; }; } /// Represents an offset to a field of type matching the vtable, within the Base container structure. #[repr(C)] pub struct VOffset<Base, T: ?Sized + VTableMeta, PinFlag = NotPinned> { vtable: &'static T::VTable, /// Safety invariant: the vtable is valid, and the field at the given offset within Base is /// matching with the vtable offset: usize, phantom: PhantomData<FieldOffset<Base, (), PinFlag>>, } impl<Base, T: ?Sized + VTableMeta, PinFlag> core::fmt::Debug for VOffset<Base, T, PinFlag> { fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result { write!(f, "VOffset({})", self.offset) } } impl<Base, T: ?Sized + VTableMeta, Flag> VOffset<Base, T, Flag> { #[inline] pub fn apply<'a>(self, x: &'a Base) -> VRef<'a, T> { let ptr = x as *const Base as *const u8; unsafe { VRef::from_raw( NonNull::from(self.vtable), NonNull::new_unchecked(ptr.add(self.offset) as *mut _), ) } } #[inline] pub fn apply_mut<'a>(self, x: &'a mut Base) -> VRefMut<'a, T> { let ptr = x as *mut Base as *mut u8; unsafe { VRefMut::from_raw( NonNull::from(self.vtable), NonNull::new_unchecked(ptr.add(self.offset)), ) } } #[inline] pub fn new<X: HasStaticVTable<T>>(o: FieldOffset<Base, X, Flag>) -> Self { Self { vtable: X::static_vtable(), offset: o.get_byte_offset(), phantom: PhantomData } } /// Create a new VOffset from raw data /// /// Safety: there must be a field that matches the vtable at offset T in base. #[inline] pub unsafe fn from_raw(vtable: &'static T::VTable, offset: usize) -> Self { Self { vtable, offset, phantom: PhantomData } } } impl<Base, T: ?Sized + VTableMeta> VOffset<Base, T, AllowPin> { #[inline] pub fn apply_pin<'a>(self, x: Pin<&'a Base>) -> Pin<VRef<'a, T>> { let ptr = x.get_ref() as *const Base as *mut u8; unsafe { Pin::new_unchecked(VRef::from_raw( NonNull::from(self.vtable), NonNull::new_unchecked(ptr.add(self.offset)), )) } } } // Need to implement manually otheriwse it is not implemented if T do not implement Copy / Clone impl<Base, T: ?Sized + VTableMeta, Flag> Copy for VOffset<Base, T, Flag> {} impl<Base, T: ?Sized + VTableMeta, Flag> Clone for VOffset<Base, T, Flag> { fn clone(&self) -> Self { Self { vtable: self.vtable, offset: self.offset, phantom: PhantomData } } } #[cfg(doctest)] mod compile_fail_tests;