vtable/

lib.rs

// Copyright © SixtyFPS GmbH <info@slint.dev>
// SPDX-License-Identifier: MIT OR Apache-2.0

/*!
This crate allows you to create ffi-friendly virtual tables.

## Features

 - A [`#[vtable]`](macro@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.
 - [`VRc`]/[`VWeak`] types: equivalent to `std::rc::{Rc, Weak}` types but works with a vtable pointer.
 - 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@vtable) macro created the "Animal" trait.

Note that the [`#[vtable]`](macro@vtable) macro is applied to the VTable struct so
that `cbindgen` can see the actual vtable.

*/

#![warn(missing_docs)]
#![no_std]
extern crate alloc;

#[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::*;

mod vrc;
pub use vrc::*;

/// Internal trait that is implemented by the [`#[vtable]`](macro@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@vtable) macro.
///
/// It is implemented if the macro has a "drop" function.
///
/// # Safety
/// Only the [`#[vtable]`](macro@vtable) macro should implement this trait.
pub unsafe trait VTableMetaDrop: VTableMeta {
    /// # Safety
    /// `ptr` needs to be pointing to a valid allocated pointer
    unsafe fn drop(ptr: *mut Self::Target);
    /// allocate a new [`VBox`]
    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 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: NonNull<u8>,
    ptr: NonNull<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
    }

    /// Same as [`Self::deref`].
    fn deref_mut<T: ?Sized + VTableMeta>(&mut self) -> *mut T::Target {
        debug_assert_eq!(core::mem::size_of::<T::Target>(), core::mem::size_of::<Inner>());
        self as *mut Inner as *mut 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` function (so that the [`#[vtable]`](macro@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_mut::<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)
    }

    /// Create a new VBox from raw pointers
    /// # Safety
    /// The `ptr` needs to be a valid object fitting the `vtable`.
    /// ptr must be 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(), ptr }, phantom: PhantomData }
    }

    /// Gets a VRef pointing to this box
    pub fn borrow(&self) -> VRef<'_, T> {
        unsafe { VRef::from_inner(self.inner) }
    }

    /// Gets a VRefMut pointing to this box
    pub fn borrow_mut(&mut self) -> VRefMut<'_, 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 otherwise it is not implemented if T does 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
    }
}

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: NonNull::from(X::static_vtable()).cast(),
                ptr: NonNull::from(value).cast(),
            },
            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: NonNull::from(X::static_vtable()).cast(),
                    ptr: NonNull::from(value.get_ref()).cast(),
                },
                phantom: PhantomData,
            })
        }
    }

    unsafe fn from_inner(inner: Inner) -> Self {
        Self { inner, phantom: PhantomData }
    }

    /// Create a new VRef from raw pointers
    /// # Safety
    /// The `ptr` needs to be a valid object fitting the `vtable`.
    /// Both vtable and ptr lifetime must outlive 'a
    pub unsafe fn from_raw(vtable: NonNull<T::VTable>, ptr: NonNull<u8>) -> Self {
        Self { inner: Inner { vtable: vtable.cast(), 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 == NonNull::from(X::static_vtable()).cast() {
            // Safety: We just checked that the vtable fits
            unsafe { Some(self.inner.ptr.cast().as_ref()) }
        } 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 == NonNull::from(X::static_vtable()).cast() {
            // Safety: We just checked that the vtable fits
            unsafe { Some(Pin::new_unchecked(inner.ptr.cast().as_ref())) }
        } else {
            None
        }
    }

    /// Returns a pointer to the VRef's instance. This is primarily useful for comparisons.
    pub fn as_ptr(this: Self) -> NonNull<u8> {
        this.inner.ptr
    }
}

/// `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_mut::<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: NonNull::from(X::static_vtable()).cast(),
                ptr: NonNull::from(value).cast(),
            },
            phantom: PhantomData,
        }
    }

    unsafe fn from_inner(inner: Inner) -> Self {
        Self { inner, phantom: PhantomData }
    }

    /// Create a new VRefMut from raw pointers
    /// # Safety
    /// The `ptr` needs to be a valid object fitting the `vtable`.
    /// Both vtable and ptr lifetime must outlive 'a.
    /// Can create mutable reference to ptr, so no other code can create mutable reference of ptr
    /// during the life time 'a.
    pub unsafe fn from_raw(vtable: NonNull<T::VTable>, ptr: NonNull<u8>) -> Self {
        Self { inner: Inner { vtable: vtable.cast(), ptr }, phantom: PhantomData }
    }

    /// Borrow this to obtain a VRef.
    pub fn borrow(&self) -> VRef<'_, T> {
        unsafe { VRef::from_inner(self.inner) }
    }

    /// Borrow this to obtain a new VRefMut.
    pub fn borrow_mut(&mut self) -> VRefMut<'_, 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 == NonNull::from(X::static_vtable()).cast() {
            // Safety: We just checked that the vtable fits
            unsafe { Some(self.inner.ptr.cast().as_mut()) }
        } 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: $trait_>(_: &mut X) -> <$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> {
    /// Apply this offset to a reference to the base to obtain a [`VRef`] with the same
    /// lifetime as the base lifetime
    #[inline]
    pub fn apply(self, base: &Base) -> VRef<'_, T> {
        let ptr = base as *const Base as *const u8;
        unsafe {
            VRef::from_raw(
                NonNull::from(self.vtable),
                NonNull::new_unchecked(ptr.add(self.offset) as *mut _),
            )
        }
    }

    /// Apply this offset to a reference to the base to obtain a [`VRefMut`] with the same
    /// lifetime as the base lifetime
    #[inline]
    pub fn apply_mut(self, base: &mut Base) -> VRefMut<'_, T> {
        let ptr = base as *mut Base as *mut u8;
        unsafe {
            VRefMut::from_raw(
                NonNull::from(self.vtable),
                NonNull::new_unchecked(ptr.add(self.offset)),
            )
        }
    }

    /// Create an new VOffset from a [`FieldOffset`] where the target type implement the
    /// [`HasStaticVTable`] trait.
    #[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> {
    /// Apply this offset to a reference to the base to obtain a `Pin<VRef<'a, T>>` with the same
    /// lifetime as the base lifetime
    #[inline]
    pub fn apply_pin(self, base: Pin<&Base>) -> Pin<VRef<T>> {
        let ptr = base.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 otherwise it is not implemented if T does 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
    }
}

#[cfg(doctest)]
mod compile_fail_tests;

/// re-export for the macro
#[doc(hidden)]
pub mod internal {
    pub use alloc::alloc::dealloc;
    pub use alloc::boxed::Box;
}