[pointer] Support Box and Arc

src/impls.rs

@@ -648,7 +648,7 @@ unsafe impl<T: TryFromBytes + ?Sized> TryFromBytes for UnsafeCell<T> {
     }
 
     #[inline]
-    fn is_bit_valid<A: invariant::Reference>(candidate: Maybe<'_, Self, A>) -> bool {
+    fn is_bit_valid<A: invariant::Aliasing>(candidate: Maybe<'_, Self, A>) -> bool {
         // The only way to implement this function is using an exclusive-aliased
         // pointer. `UnsafeCell`s cannot be read via shared-aliased pointers
         // (other than by using `unsafe` code, which we can't use since we can't
@@ -1124,15 +1124,15 @@ mod tests {
 
             pub(super) trait TestIsBitValidShared<T: ?Sized> {
                 #[allow(clippy::needless_lifetimes)]
-                fn test_is_bit_valid_shared<'ptr, A: invariant::Reference>(
+                fn test_is_bit_valid_shared<'ptr, A: invariant::Aliasing>(
                     &self,
                     candidate: Maybe<'ptr, T, A>,
                 ) -> Option<bool>;
             }
 
             impl<T: TryFromBytes + Immutable + ?Sized> TestIsBitValidShared<T> for AutorefWrapper<T> {
                 #[allow(clippy::needless_lifetimes)]
-                fn test_is_bit_valid_shared<'ptr, A: invariant::Reference>(
+                fn test_is_bit_valid_shared<'ptr, A: invariant::Aliasing>(
                     &self,
                     candidate: Maybe<'ptr, T, A>,
                 ) -> Option<bool> {
@@ -1222,7 +1222,7 @@ mod tests {
                 #[allow(unused, non_local_definitions)]
                 impl AutorefWrapper<$ty> {
                     #[allow(clippy::needless_lifetimes)]
-                    fn test_is_bit_valid_shared<'ptr, A: invariant::Reference>(
+                    fn test_is_bit_valid_shared<'ptr, A: invariant::Aliasing>(
                         &mut self,
                         candidate: Maybe<'ptr, $ty, A>,
                     ) -> Option<bool> {

src/lib.rs

@@ -3480,10 +3480,22 @@ pub unsafe trait FromBytes: FromZeros {
     where
         Self: KnownLayout + Immutable,
     {
-        static_assert_dst_is_not_zst!(Self);
-        match Ptr::from_ref(source).try_cast_into_no_leftover::<_, BecauseImmutable>(None) {
-            Ok(ptr) => Ok(ptr.bikeshed_recall_valid().as_ref()),
-            Err(err) => Err(err.map_src(|src| src.as_ref())),
+        Self::from_bytes(source)
+    }
+
+    #[must_use = "has no side effects"]
+    #[inline]
+    fn from_bytes<'a, P: pointer::Pointer<'a, Self>, R>(
+        source: P::To<'a, [u8]>,
+    ) -> Result<P, CastError<P::To<'a, [u8]>, Self>>
+    where
+        Self: 'a + KnownLayout + invariant::Read<P::Aliasing, R>,
+    {
+        match Ptr::<'_, _, (P::Aliasing, _, _)>::from_pointer(source)
+            .try_cast_into_no_leftover::<_, R>(None)
+        {
+            Ok(ptr) => Ok(ptr.bikeshed_recall_valid().into_pointer()),
+            Err(err) => Err(err.map_src(|src| src.into_pointer())),
         }
     }
 
@@ -3716,11 +3728,7 @@ pub unsafe trait FromBytes: FromZeros {
     where
         Self: IntoBytes + KnownLayout,
     {
-        static_assert_dst_is_not_zst!(Self);
-        match Ptr::from_mut(source).try_cast_into_no_leftover::<_, BecauseExclusive>(None) {
-            Ok(ptr) => Ok(ptr.bikeshed_recall_valid().as_mut()),
-            Err(err) => Err(err.map_src(|src| src.as_mut())),
-        }
+        Self::from_bytes(source)
     }
 
     /// Interprets the prefix of the given `source` as a `&mut Self` without

src/pointer/inner.rs

@@ -25,7 +25,8 @@ mod _def {
     /// `Ptr<'a, T>` is [covariant] in `'a` and `T`.
     ///
     /// [covariant]: https://doc.rust-lang.org/reference/subtyping.html
-    pub(crate) struct PtrInner<'a, T>
+    #[allow(missing_debug_implementations)]
+    pub struct PtrInner<'a, T>
     where
         T: ?Sized,
     {

src/pointer/invariant.rs

@@ -89,6 +89,15 @@ pub trait Aliasing: Sealed {
     /// Aliasing>::Variance<'a, T>` to inherit this variance.
     #[doc(hidden)]
     type Variance<'a, T: 'a + ?Sized>;
+
+    // #[doc(hidden)]
+    // type Applied<'a, T: 'a + ?Sized>;
+
+    // #[doc(hidden)]
+    // fn into_ptr<'a, T: 'a + ?Sized>(ptr: Self::Applied<'a, T>) -> PtrInner<'a, T>;
+
+    // #[doc(hidden)]
+    // unsafe fn from_ptr<'a, T: 'a + ?Sized>(ptr: PtrInner<'a, T>) -> Self::Applied<'a, T>;
 }
 
 // NOTE: The `AlignmentInner`/`Alignment` distinction is required so that we can
@@ -186,6 +195,34 @@ impl Aliasing for Exclusive {
 }
 impl Reference for Exclusive {}
 
+#[cfg(feature = "alloc")]
+mod _alloc {
+    use alloc::boxed::Box as Bx;
+
+    use super::*;
+
+    pub enum Box {}
+    impl Aliasing for Box {
+        const IS_EXCLUSIVE: bool = false;
+        type Variance<'a, T: 'a + ?Sized> = Bx<T>;
+    }
+}
+
+#[cfg(feature = "std")]
+pub use _std::*;
+#[cfg(feature = "std")]
+mod _std {
+    use std::sync::Arc as Ac;
+
+    use super::*;
+
+    pub enum Arc {}
+    impl Aliasing for Arc {
+        const IS_EXCLUSIVE: bool = false;
+        type Variance<'a, T: 'a + ?Sized> = Ac<T>;
+    }
+}
+
 /// The referent is aligned: for `Ptr<T>`, the referent's address is a multiple
 /// of the `T`'s alignment.
 pub enum Aligned {}
@@ -237,6 +274,18 @@ impl ValidityInner for Valid {
     type MappedTo<M: ValidityMapping> = M::FromValid;
 }
 
+// Shared, Arc, etc
+pub trait SharedFoo: Aliasing {}
+impl SharedFoo for Shared {}
+#[cfg(feature = "std")]
+impl SharedFoo for Arc {}
+
+// Exclusive, Box, etc
+pub trait ExclusiveFoo: Aliasing {}
+impl ExclusiveFoo for Exclusive {}
+#[cfg(feature = "alloc")]
+impl ExclusiveFoo for Box {}
+
 /// [`Ptr`](crate::Ptr) referents that permit unsynchronized read operations.
 ///
 /// `T: Read<A, R>` implies that a pointer to `T` with aliasing `A` permits
@@ -261,8 +310,7 @@ define_because!(
     #[doc(hidden)]
     pub BecauseExclusive
 );
-// SAFETY: The aliasing parameter is `Exclusive`.
-unsafe impl<T: ?Sized> Read<Exclusive, BecauseExclusive> for T {}
+unsafe impl<A: ExclusiveFoo, T: ?Sized> Read<A, BecauseExclusive> for T {}
 
 define_because!(
     /// Unsynchronized reads are permitted because no live [`Ptr`](crate::Ptr)s
@@ -283,6 +331,10 @@ mod sealed {
 
     impl Sealed for Shared {}
     impl Sealed for Exclusive {}
+    #[cfg(feature = "alloc")]
+    impl Sealed for Box {}
+    #[cfg(feature = "std")]
+    impl Sealed for Arc {}
 
     impl Sealed for Aligned {}
 

src/pointer/mod.rs

@@ -86,3 +86,92 @@ where
 {
     ptr.as_bytes::<BecauseImmutable>().as_ref().iter().all(|&byte| byte == 0)
 }
+
+pub use _pointer::*;
+mod _pointer {
+    #[cfg(feature = "alloc")]
+    use alloc::boxed::Box;
+    #[cfg(feature = "std")]
+    use std::sync::Arc;
+
+    use super::{inner::PtrInner, invariant::*};
+
+    pub unsafe trait Pointer<'t, T: ?Sized> {
+        type To<'u, U: 'u + ?Sized>: Pointer<'u, U, Aliasing = Self::Aliasing>;
+
+        #[doc(hidden)]
+        type Aliasing: Aliasing;
+
+        #[doc(hidden)]
+        fn into_ptr(self) -> PtrInner<'t, T>;
+
+        #[doc(hidden)]
+        unsafe fn from_ptr(ptr: PtrInner<'t, T>) -> Self;
+    }
+
+    unsafe impl<'t, T: ?Sized> Pointer<'t, T> for &'t T {
+        type To<'u, U: 'u + ?Sized> = &'u U;
+
+        type Aliasing = Shared;
+
+        #[inline(always)]
+        fn into_ptr(self) -> PtrInner<'t, T> {
+            PtrInner::from_ref(self)
+        }
+
+        #[inline(always)]
+        unsafe fn from_ptr(ptr: PtrInner<'t, T>) -> Self {
+            unsafe { ptr.as_non_null().as_ref() }
+        }
+    }
+
+    unsafe impl<'t, T: ?Sized> Pointer<'t, T> for &'t mut T {
+        type To<'u, U: 'u + ?Sized> = &'u mut U;
+
+        type Aliasing = Exclusive;
+
+        #[inline(always)]
+        fn into_ptr(self) -> PtrInner<'t, T> {
+            PtrInner::from_ref(self)
+        }
+
+        #[inline(always)]
+        unsafe fn from_ptr(ptr: PtrInner<'t, T>) -> Self {
+            unsafe { ptr.as_non_null().as_mut() }
+        }
+    }
+
+    #[cfg(feature = "alloc")]
+    unsafe impl<'t, T: ?Sized> Pointer<'t, T> for Box<T> {
+        type To<'u, U: 'u + ?Sized> = Box<U>;
+
+        type Aliasing = super::invariant::Box;
+
+        #[inline(always)]
+        fn into_ptr(self) -> PtrInner<'t, T> {
+            PtrInner::from_box(self)
+        }
+
+        #[inline(always)]
+        unsafe fn from_ptr(ptr: PtrInner<'t, T>) -> Box<T> {
+            unsafe { Box::from_raw(ptr.as_non_null().as_ptr()) }
+        }
+    }
+
+    #[cfg(feature = "std")]
+    unsafe impl<'t, T: ?Sized> Pointer<'t, T> for Arc<T> {
+        type To<'u, U: 'u + ?Sized> = Arc<U>;
+
+        type Aliasing = super::invariant::Arc;
+
+        #[inline(always)]
+        fn into_ptr(self) -> PtrInner<'t, T> {
+            PtrInner::from_arc(self)
+        }
+
+        #[inline(always)]
+        unsafe fn from_ptr(ptr: PtrInner<'t, T>) -> Arc<T> {
+            unsafe { Arc::from_raw(ptr.as_non_null().as_ptr()) }
+        }
+    }
+}

src/pointer/ptr.rs

@@ -176,6 +176,29 @@ mod _conversions {
         TransmuteFromAlignment, TransmuteFromPtr, TransmuteFromPtrOld,
     };
 
+    // TODO: How to make this a `From` impl without blanket impl conflicts?
+    impl<'a, T, A> Ptr<'a, T, (A, Aligned, Valid)>
+    where
+        T: 'a + ?Sized,
+        A: Aliasing,
+    {
+        /// Constructs a `Ptr` from an existing smart pointer or reference type.
+        #[doc(hidden)]
+        #[inline]
+        pub fn from_pointer<P: Pointer<'a, T, Aliasing = A>>(ptr: P) -> Self {
+            let ptr = P::into_ptr(ptr);
+            unsafe { Self::from_inner(ptr) }
+        }
+
+        /// Constructs `self` to a smart pointer or reference type.
+        #[doc(hidden)]
+        #[inline]
+        #[must_use]
+        pub fn into_pointer<P: Pointer<'a, T, Aliasing = A>>(self) -> P {
+            unsafe { P::from_ptr(self.as_inner()) }
+        }
+    }
+
     /// `&'a T` → `Ptr<'a, T>`
     impl<'a, T> Ptr<'a, T, (Shared, Aligned, Valid)>
     where
@@ -927,6 +950,9 @@ mod _casts {
         /// - If this is a prefix cast, `ptr` has the same address as `self`.
         /// - If this is a suffix cast, `remainder` has the same address as
         ///   `self`.
+        // TODO: This is currently unsound - need to bound with `I::Aliasing:
+        // Reference` in order to prove that splitting is okay (it isn't for
+        // e.g. Box and Arc).
         #[inline(always)]
         pub(crate) fn try_cast_into<U, R>(
             self,