Add size_of, align_of, and layout methods to DynMetadata

Author: SimonSapin

library/core/src/ptr/metadata.rs

@@ -2,7 +2,6 @@
 
 use crate::fmt;
 use crate::hash::{Hash, Hasher};
-use crate::ptr::NonNull;
 
 /// FIXME docs
 #[lang = "pointee_trait"]
@@ -62,17 +61,48 @@ impl<T: ?Sized> Clone for PtrComponents<T> {
 /// The metadata for a `dyn SomeTrait` trait object type.
 #[lang = "dyn_metadata"]
 pub struct DynMetadata<Dyn: ?Sized> {
-    #[allow(unused)]
-    vtable_ptr: NonNull<()>,
+    vtable_ptr: &'static VTable,
     phantom: crate::marker::PhantomData<Dyn>,
 }
 
+/// The common prefix of all vtables. It is followed by function pointers for trait methods.
+///
+/// Private implementation detail of `DynMetadata::size_of` etc.
+#[repr(C)]
+struct VTable {
+    drop_in_place: fn(*mut ()),
+    size_of: usize,
+    align_of: usize,
+}
+
+impl<Dyn: ?Sized> DynMetadata<Dyn> {
+    /// Returns the size of the type associated with this vtable.
+    #[inline]
+    pub fn size_of(self) -> usize {
+        self.vtable_ptr.size_of
+    }
+
+    /// Returns the alignment of the type associated with this vtable.
+    #[inline]
+    pub fn align_of(self) -> usize {
+        self.vtable_ptr.align_of
+    }
+
+    /// Returns the size and alignment together as a `Layout`
+    #[inline]
+    pub fn layout(self) -> crate::alloc::Layout {
+        // SAFETY: the compiler emitted this vtable for a concrete Rust type which
+        // is known to have a valid layout. Same rationale as in `Layout::for_value`.
+        unsafe { crate::alloc::Layout::from_size_align_unchecked(self.size_of(), self.align_of()) }
+    }
+}
+
 unsafe impl<Dyn: ?Sized> Send for DynMetadata<Dyn> {}
 unsafe impl<Dyn: ?Sized> Sync for DynMetadata<Dyn> {}
 
 impl<Dyn: ?Sized> fmt::Debug for DynMetadata<Dyn> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.write_str("DynMetadata { … }")
+        f.debug_tuple("DynMetadata").field(&(self.vtable_ptr as *const VTable)).finish()
     }
 }
 
@@ -94,27 +124,27 @@ impl<Dyn: ?Sized> Eq for DynMetadata<Dyn> {}
 impl<Dyn: ?Sized> PartialEq for DynMetadata<Dyn> {
     #[inline]
     fn eq(&self, other: &Self) -> bool {
-        self.vtable_ptr == other.vtable_ptr
+        crate::ptr::eq::<VTable>(self.vtable_ptr, other.vtable_ptr)
     }
 }
 
 impl<Dyn: ?Sized> Ord for DynMetadata<Dyn> {
     #[inline]
     fn cmp(&self, other: &Self) -> crate::cmp::Ordering {
-        self.vtable_ptr.cmp(&other.vtable_ptr)
+        (self.vtable_ptr as *const VTable).cmp(&(other.vtable_ptr as *const VTable))
     }
 }
 
 impl<Dyn: ?Sized> PartialOrd for DynMetadata<Dyn> {
     #[inline]
     fn partial_cmp(&self, other: &Self) -> Option<crate::cmp::Ordering> {
-        Some(self.vtable_ptr.cmp(&other.vtable_ptr))
+        Some(self.cmp(other))
     }
 }
 
 impl<Dyn: ?Sized> Hash for DynMetadata<Dyn> {
     #[inline]
     fn hash<H: Hasher>(&self, hasher: &mut H) {
-        self.vtable_ptr.hash(hasher)
+        crate::ptr::hash::<VTable, _>(self.vtable_ptr, hasher)
     }
 }

library/core/tests/ptr.rs

@@ -505,3 +505,23 @@ fn ptr_metadata_bounds() {
     {
     }
 }
+
+#[test]
+#[cfg(not(bootstrap))]
+fn dyn_metadata() {
+    #[derive(Debug)]
+    #[repr(align(32))]
+    struct Something([u8; 47]);
+
+    let value = Something([0; 47]);
+    let trait_object: &dyn Debug = &value;
+    let meta = metadata(trait_object);
+
+    assert_eq!(meta.size_of(), 64);
+    assert_eq!(meta.size_of(), std::mem::size_of::<Something>());
+    assert_eq!(meta.align_of(), 32);
+    assert_eq!(meta.align_of(), std::mem::align_of::<Something>());
+    assert_eq!(meta.layout(), std::alloc::Layout::new::<Something>());
+
+    assert!(format!("{:?}", meta).starts_with("DynMetadata(0x"));
+}