Add KnownLayout::size_for_metadata (#2674)

Computes the size of an object of type `Self` with the given
pointer metadata.

Makes progress towards #2541

Author: jswrenn

Cargo.toml

@@ -95,6 +95,8 @@ zerocopy-derive = { version = "=0.8.26", path = "zerocopy-derive" }
 either = "=1.13.0"
 # FIXME(#381) Remove this dependency once we have our own layout gadgets.
 elain = "0.3.0"
+# More recent versions of `glob` have an MSRV higher than ours.
+glob = "=0.3.2"
 itertools = "0.11"
 rand = { version = "0.8.5", default-features = false, features = ["small_rng"] }
 rustversion = "1.0"

src/lib.rs

@@ -821,7 +821,7 @@ pub unsafe trait KnownLayout {
         let meta = Self::pointer_to_metadata(ptr.as_ptr());
         // SAFETY: `size_for_metadata` promises to only return `None` if the
         // resulting size would not fit in a `usize`.
-        meta.size_for_metadata(Self::LAYOUT)
+        Self::size_for_metadata(meta)
     }
 
     #[doc(hidden)]
@@ -831,6 +831,37 @@ pub unsafe trait KnownLayout {
         let meta = Self::PointerMetadata::from_elem_count(0);
         Self::raw_from_ptr_len(NonNull::dangling(), meta)
     }
+
+    /// Computes the size of an object of type `Self` with the given pointer
+    /// metadata.
+    ///
+    /// # Safety
+    ///
+    /// `size_for_metadata` promises to return `None` if and only if the
+    /// resulting size would not fit in a `usize`. Note that the returned size
+    /// could exceed the actual maximum valid size of an allocated object,
+    /// `isize::MAX`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use zerocopy::KnownLayout;
+    ///
+    /// assert_eq!(u8::size_for_metadata(()), Some(1));
+    /// assert_eq!(u16::size_for_metadata(()), Some(2));
+    /// assert_eq!(<[u8]>::size_for_metadata(42), Some(42));
+    /// assert_eq!(<[u16]>::size_for_metadata(42), Some(84));
+    ///
+    /// // This size exceeds the maximum valid object size (`isize::MAX`):
+    /// assert_eq!(<[u8]>::size_for_metadata(usize::MAX), Some(usize::MAX));
+    ///
+    /// // This size, if computed, would exceed `usize::MAX`:
+    /// assert_eq!(<[u16]>::size_for_metadata(usize::MAX), None);
+    /// ```
+    #[inline(always)]
+    fn size_for_metadata(meta: Self::PointerMetadata) -> Option<usize> {
+        meta.size_for_metadata(Self::LAYOUT)
+    }
 }
 
 /// Efficiently produces the [`TrailingSliceLayout`] of `T`.

src/ref.rs

@@ -479,7 +479,7 @@ where
     #[inline]
     pub fn from_bytes_with_elems(source: B, count: usize) -> Result<Ref<B, T>, CastError<B, T>> {
         static_assert_dst_is_not_zst!(T);
-        let expected_len = match count.size_for_metadata(T::LAYOUT) {
+        let expected_len = match T::size_for_metadata(count) {
             Some(len) => len,
             None => return Err(SizeError::new(source).into()),
         };
@@ -533,7 +533,7 @@ where
         count: usize,
     ) -> Result<(Ref<B, T>, B), CastError<B, T>> {
         static_assert_dst_is_not_zst!(T);
-        let expected_len = match count.size_for_metadata(T::LAYOUT) {
+        let expected_len = match T::size_for_metadata(count) {
             Some(len) => len,
             None => return Err(SizeError::new(source).into()),
         };
@@ -579,7 +579,7 @@ where
         count: usize,
     ) -> Result<(B, Ref<B, T>), CastError<B, T>> {
         static_assert_dst_is_not_zst!(T);
-        let expected_len = match count.size_for_metadata(T::LAYOUT) {
+        let expected_len = match T::size_for_metadata(count) {
             Some(len) => len,
             None => return Err(SizeError::new(source).into()),
         };

src/util/mod.rs

@@ -323,7 +323,7 @@ pub(crate) unsafe fn new_box<T>(
 where
     T: ?Sized + crate::KnownLayout,
 {
-    let size = match meta.size_for_metadata(T::LAYOUT) {
+    let size = match T::size_for_metadata(meta) {
         Some(size) => size,
         None => return Err(AllocError),
     };
@@ -515,7 +515,7 @@ mod len_of {
                 // This can return `None` if the metadata describes an object
                 // which can't fit in an `isize`.
                 Some(meta) => {
-                    let size = match meta.size_for_metadata(T::LAYOUT) {
+                    let size = match T::size_for_metadata(meta) {
                         Some(size) => size,
                         None => return Err(MetadataCastError::Size),
                     };