[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

@@ -762,6 +762,24 @@ pub unsafe trait KnownLayout {
     #[doc(hidden)]
     const LAYOUT: DstLayout;
 
+    /// Does `Self` have a non-trivial destructor?
+    ///
+    /// This defaulted implementation is appropriate for all types except
+    /// `UnalignUnsized<T>` which has an explicit `Drop` implementation and is
+    /// thus unconditionally `mem::needs_drop`, even if `T` is not
+    /// `mem::needs_drop`.
+    ///
+    /// # Safety
+    ///
+    /// Unsafe code may not assume anything about the value of `NEEDS_DROP`.
+    const NEEDS_DROP: bool = {
+        #[cfg(zerocopy_unsized_needs_drop_1_63_0)]
+        let val = core::mem::needs_drop::<Self>();
+        #[cfg(not(zerocopy_unsized_needs_drop_1_63_0))]
+        let val = true;
+        val
+    };
+
     /// SAFETY: The returned pointer has the same address and provenance as
     /// `bytes`. If `Self` is a DST, the returned pointer's referent has `elems`
     /// elements in its trailing slice.
@@ -797,6 +815,34 @@ 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.
+        //
+        // This defaulted implementation works for all types, but for sized
+        // types, delegating to `crate::util::destroy::destroy_sized` — which
+        // does not allocate — is preferable.
+        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`].