[wip] UnalignUnsized

.github/workflows/ci.yml

@@ -56,6 +56,7 @@ jobs:
           # which a particular feature is supported.
           "zerocopy-core-error-1-81-0",
           "zerocopy-diagnostic-on-unimplemented-1-78-0",
+          "zerocopy-unsized-needs-drop-1-63-0",
           "zerocopy-generic-bounds-in-const-fn-1-61-0",
           "zerocopy-target-has-atomics-1-60-0",
           "zerocopy-aarch64-simd-1-59-0",
@@ -93,6 +94,8 @@ jobs:
             features: "--all-features"
           - toolchain: "zerocopy-diagnostic-on-unimplemented-1-78-0"
             features: "--all-features"
+          - toolchain: "zerocopy-unsized-needs-drop-1-63-0"
+            features: "--all-features"
           - toolchain: "zerocopy-generic-bounds-in-const-fn-1-61-0"
             features: "--all-features"
           - toolchain: "zerocopy-target-has-atomics-1-60-0"
@@ -117,6 +120,8 @@ jobs:
             toolchain: "zerocopy-core-error-1-81-0"
           - crate: "zerocopy-derive"
             toolchain: "zerocopy-diagnostic-on-unimplemented-1-78-0"
+          - crate: "zerocopy-derive"
+            toolchain: "zerocopy-unsized-needs-drop-1-63-0"
           - crate: "zerocopy-derive"
             toolchain: "zerocopy-generic-bounds-in-const-fn-1-61-0"
           - crate: "zerocopy-derive"
@@ -212,6 +217,28 @@ jobs:
             target: "thumbv6m-none-eabi"
           - toolchain: "zerocopy-generic-bounds-in-const-fn-1-61-0"
             target: "wasm32-wasi"
+          # Exclude most targets targets from the
+          # `zerocopy-unsized-needs-drop-1-63-0` toolchain since the
+          # `zerocopy-unsized-needs-drop-1-63-0` feature is unrelated to
+          # compilation target. This only leaves i686 and x86_64 targets.
+          - toolchain: "zerocopy-unsized-needs-drop-1-63-0"
+            target: "arm-unknown-linux-gnueabi"
+          - toolchain: "zerocopy-unsized-needs-drop-1-63-0"
+            target: "aarch64-unknown-linux-gnu"
+          - toolchain: "zerocopy-unsized-needs-drop-1-63-0"
+            target: "powerpc-unknown-linux-gnu"
+          - toolchain: "zerocopy-unsized-needs-drop-1-63-0"
+            target: "powerpc64-unknown-linux-gnu"
+          - toolchain: "zerocopy-unsized-needs-drop-1-63-0"
+            target: "riscv64gc-unknown-linux-gnu"
+          - toolchain: "zerocopy-unsized-needs-drop-1-63-0"
+            target: "s390x-unknown-linux-gnu"
+          - toolchain: "zerocopy-unsized-needs-drop-1-63-0"
+            target: "x86_64-pc-windows-msvc"
+          - toolchain: "zerocopy-unsized-needs-drop-1-63-0"
+            target: "thumbv6m-none-eabi"
+          - toolchain: "zerocopy-unsized-needs-drop-1-63-0"
+            target: "wasm32-wasi"
           # Exclude `thumbv6m-none-eabi` combined with any feature that implies
           # the `std` feature since `thumbv6m-none-eabi` does not include a
           # pre-compiled std.

Cargo.toml

@@ -42,6 +42,9 @@ zerocopy-core-error-1-81-0 = "1.81.0"
 # From 1.78.0, Rust supports the `#[diagnostic::on_unimplemented]` attribute.
 zerocopy-diagnostic-on-unimplemented-1-78-0 = "1.78.0"
 
+# From 1.63.0, Rust supports generic types with trait bounds in `const fn`.
+zerocopy-unsized-needs-drop-1-63-0 = "1.63.0"
+
 # From 1.61.0, Rust supports generic types with trait bounds in `const fn`.
 zerocopy-generic-bounds-in-const-fn-1-61-0 = "1.61.0"
 

src/layout.rs

@@ -94,7 +94,7 @@ pub(crate) enum MetadataCastError {
 
 impl DstLayout {
     /// The minimum possible alignment of a type.
-    const MIN_ALIGN: NonZeroUsize = match NonZeroUsize::new(1) {
+    pub(crate) const MIN_ALIGN: NonZeroUsize = match NonZeroUsize::new(1) {
         Some(min_align) => min_align,
         None => const_unreachable!(),
     };
@@ -598,6 +598,11 @@ impl DstLayout {
 
         Ok((elems, split_at))
     }
+
+    /// Produces `true` if `self.align` equals 1; otherwise `false`.
+    pub(crate) const fn is_trivially_aligned(&self) -> bool {
+        matches!(self.align, DstLayout::MIN_ALIGN)
+    }
 }
 
 // TODO(#67): For some reason, on our MSRV toolchain, this `allow` isn't

src/lib.rs

@@ -797,6 +797,30 @@ pub unsafe trait KnownLayout {
         // resulting size would not fit in a `usize`.
         meta.size_for_metadata(Self::LAYOUT)
     }
+
+    /// Run the destructor of `ptr`'s referent.
+    ///
+    /// # Panics
+    ///
+    /// Implementations of this function never panic.
+    ///
+    /// # Compile-Time Assertions
+    ///
+    /// Implementations of this function must emit a post-monomorphization error
+    /// if `ptr`'s referent has a non-trivial drop that cannot be run.
+    ///
+    /// # Safety
+    ///
+    /// This function may only be called from the destructor (i.e.,
+    /// `Drop::drop`) of transitive owner of `ptr`'s referent. After invoking
+    /// this function, it is forbidden to re-use `ptr` or its referent.
+    #[doc(hidden)]
+    #[inline]
+    unsafe fn destroy(ptr: MaybeAligned<'_, Self, invariant::Exclusive>) {
+        // SAFETY: The preconditions of `destroy_unsized` are identical to that
+        // of `destroy` and are ensured by the caller.
+        unsafe { crate::util::destroy::destroy_unsized(ptr) }
+    }
 }
 
 /// The metadata associated with a [`KnownLayout`] type.

src/pointer/mod.rs

@@ -55,6 +55,28 @@ where
         unsafe { core::ptr::read_unaligned(raw) }
     }
 
+    /// Reads the value from `MaybeAligned`.
+    ///
+    /// # Safety
+    ///
+    /// If `T` has a non-trivial destructor, using the returned `T` (including
+    /// dropping it) and the original referent may cause undefined behavior. The
+    /// caller ensures this does not occur.
+    #[must_use]
+    #[inline]
+    pub(crate) unsafe fn read_unaligned_unchecked<R>(self) -> T
+    where
+        R: AliasingSafeReason,
+        T: AliasingSafe<T, Aliasing, R> + Sized,
+    {
+        let raw = self.as_non_null().as_ptr();
+        // SAFETY: By invariant on `MaybeAligned`, `raw` contains
+        // validly-initialized data for `T`. By `T: AliasingSafe`, we are
+        // permitted to perform a read of `raw`'s referent. The caller ensures
+        // that subsequent uses of `T` do not induce UB.
+        unsafe { core::ptr::read_unaligned(raw) }
+    }
+
     /// Views the value as an aligned reference.
     ///
     /// This is only available if `T` is [`Unaligned`].

src/pointer/ptr.rs

@@ -424,6 +424,36 @@ mod _conversions {
         }
     }
 
+    /// `Box<mut T>` → `Ptr<'static, T>`
+    #[cfg(feature = "alloc")]
+    impl<'a, T> Ptr<'a, T, (Exclusive, Aligned, Valid)>
+    where
+        T: 'a + ?Sized,
+    {
+        /// Constructs a `Ptr` from a `Box`.
+        ///
+        /// This leaks the `Box`.
+        #[inline]
+        pub(crate) fn from_box(ptr: alloc::boxed::Box<T>) -> Self {
+            let ptr = alloc::boxed::Box::into_raw(ptr);
+            let ptr = unsafe { NonNull::new_unchecked(ptr) };
+            unsafe { Self::new(ptr) }
+        }
+
+        /// Constructs a `Box` from a `Ptr`.
+        ///
+        /// # Safety
+        ///
+        /// This must be invoked with a `Ptr<T>` to a referent allocated by a
+        /// `Box<U>`, where the size and alignments of `T` and `U` are equal.
+        #[inline]
+        pub(crate) unsafe fn into_box(ptr: Self) -> alloc::boxed::Box<T> {
+            let ptr = ptr.as_mut() as *mut _;
+            // SAFETY: TODO
+            unsafe { alloc::boxed::Box::from_raw(ptr) }
+        }
+    }
+
     /// `Ptr<'a, T>` → `&'a T`
     impl<'a, T, I> Ptr<'a, T, I>
     where
@@ -815,6 +845,25 @@ mod _transitions {
             unsafe { self.assume_alignment::<Aligned>() }
         }
 
+        /// Attempt to recall that `self`'s referent is trivially aligned.
+        #[inline]
+        // TODO(#859): Reconsider the name of this method before making it
+        // public.
+        pub(crate) fn try_recall_trivially_aligned(
+            self,
+        ) -> Result<Ptr<'a, T, (I::Aliasing, Aligned, I::Validity)>, Self>
+        where
+            T: KnownLayout,
+        {
+            if T::LAYOUT.is_trivially_aligned() {
+                // SAFETY: The above check ensures that `T` has no non-trivial
+                // alignment requirement.
+                Ok(unsafe { self.assume_alignment::<Aligned>() })
+            } else {
+                Err(self)
+            }
+        }
+
         /// Assumes that `self`'s referent conforms to the validity requirement
         /// of `V`.
         ///
@@ -954,6 +1003,26 @@ mod _casts {
         T: 'a + ?Sized,
         I: Invariants,
     {
+        /// Casts to a different (unsized) target type.
+        #[inline]
+        pub fn try_cast<U>(self) -> Result<Ptr<'a, U, (I::Aliasing, Any, Any)>, Self>
+        where
+            T: KnownLayout<PointerMetadata = U::PointerMetadata>,
+            U: 'a + ?Sized + KnownLayout,
+        {
+            let ptr = self.as_non_null();
+            let src_size = T::size_of_val_raw(ptr);
+            let meta = T::pointer_to_metadata(ptr.as_ptr());
+            if meta.size_for_metadata(U::LAYOUT) <= src_size {
+                let bytes = ptr.cast::<u8>();
+                let ptr = U::raw_from_ptr_len(bytes, meta);
+                // SAFETY: TODO
+                Ok(unsafe { Ptr::new(ptr) })
+            } else {
+                Err(self)
+            }
+        }
+
         /// Casts to a different (unsized) target type.
         ///
         /// # Safety
@@ -1641,6 +1710,40 @@ mod _project {
     }
 }
 
+mod _misc {
+    use super::*;
+
+    impl<T, I> Ptr<'_, T, I>
+    where
+        T: ?Sized,
+        I: Invariants<Aliasing = Exclusive, Alignment = Aligned, Validity = Valid>,
+    {
+        /// Executes the referent's destructor.
+        ///
+        /// # Safety
+        ///
+        /// This function may only be invoked from the destructor of an
+        /// transitive owner `ptr`'s referent. After invoking this function, it
+        /// is forbidden to re-use `ptr`'s referent.
+        pub(crate) unsafe fn drop_in_place(self) {
+            let ptr = self.as_non_null().as_ptr();
+            // SAFETY: This invocation satisfies `drop_in_place`'s safety
+            // invariants [1]:
+            // - `ptr` is valid for both reads and writes, because it derived
+            //   from a `Ptr` whose referent is exclusively aliased,
+            //   well-aligned, and valid.
+            // - `ptr` is well-aligned; see above.
+            // - `ptr` is non-null; see above.
+            // - `ptr`'s referent is presumed to be a library-valid
+            // - `ptr` is exclusively aliased and thus is the sole pointer to
+            //   its referent.
+            //
+            // [1] https://doc.rust-lang.org/1.82.0/std/ptr/fn.drop_in_place.html#safety
+            unsafe { core::ptr::drop_in_place(ptr) }
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use core::mem::{self, MaybeUninit};

src/util/macros.rs

@@ -582,6 +582,14 @@ macro_rules! impl_known_layout {
                 #[inline(always)]
                 fn pointer_to_metadata(_ptr: *mut Self) -> () {
                 }
+
+                #[inline]
+                unsafe fn destroy(ptr: crate::MaybeAligned<'_, Self, crate::invariant::Exclusive>) {
+                    // SAFETY: The preconditions of `destroy_sized` are
+                    // identical to that of `destroy` and are ensured by the
+                    // caller.
+                    unsafe { crate::util::destroy::destroy_sized(ptr) }
+                }
             }
         };
     };
@@ -599,7 +607,7 @@ macro_rules! impl_known_layout {
 /// - It must be valid to perform an `as` cast from `*mut $repr` to `*mut $ty`,
 ///   and this operation must preserve referent size (ie, `size_of_val_raw`).
 macro_rules! unsafe_impl_known_layout {
-    ($($tyvar:ident: ?Sized + KnownLayout =>)? #[repr($repr:ty)] $ty:ty) => {
+    ($($tyvar:ident: ?Sized + KnownLayout =>)? #[repr($(packed,)? $repr:ty)] $ty:ty) => {
         const _: () = {
             use core::ptr::NonNull;
 
@@ -620,7 +628,7 @@ macro_rules! unsafe_impl_known_layout {
                 // TODO(#429): Add documentation to `NonNull::new_unchecked`
                 // that it preserves provenance.
                 #[inline(always)]
-                fn raw_from_ptr_len(bytes: NonNull<u8>, meta: <$repr as KnownLayout>::PointerMetadata) -> NonNull<Self> {
+                fn raw_from_ptr_len(bytes: NonNull<u8>, meta: Self::PointerMetadata) -> NonNull<Self> {
                     #[allow(clippy::as_conversions)]
                     let ptr = <$repr>::raw_from_ptr_len(bytes, meta).as_ptr() as *mut Self;
                     // SAFETY: `ptr` was converted from `bytes`, which is non-null.