add intrinsic to access vtable size and align

library/core/src/intrinsics.rs

@@ -2291,6 +2291,16 @@ extern "rust-intrinsic" {
     /// [`std::hint::black_box`]: crate::hint::black_box
     #[rustc_const_unstable(feature = "const_black_box", issue = "none")]
     pub fn black_box<T>(dummy: T) -> T;
+
+    /// `ptr` must point to a vtable.
+    /// The intrinsic will return the size stored in that vtable.
+    #[cfg(not(bootstrap))]
+    pub fn vtable_size(ptr: *const ()) -> usize;
+
+    /// `ptr` must point to a vtable.
+    /// The intrinsic will return the alignment stored in that vtable.
+    #[cfg(not(bootstrap))]
+    pub fn vtable_align(ptr: *const ()) -> usize;
 }
 
 // Some functions are defined here because they accidentally got made

library/core/src/ptr/metadata.rs

@@ -180,10 +180,20 @@ pub struct DynMetadata<Dyn: ?Sized> {
     phantom: crate::marker::PhantomData<Dyn>,
 }
 
+/// Opaque type for accessing vtables.
+///
+/// Private implementation detail of `DynMetadata::size_of` etc.
+/// Must be zero-sized since there is conceptually not actually any Abstract Machine memory behind this pointer.
+/// However, we can require pointer alignment.
+#[repr(C)]
+#[cfg(not(bootstrap))]
+struct VTable([usize; 0]);
+
 /// 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)]
+#[cfg(bootstrap)]
 struct VTable {
     drop_in_place: fn(*mut ()),
     size_of: usize,
@@ -194,13 +204,25 @@ 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
+        #[cfg(bootstrap)]
+        return self.vtable_ptr.size_of;
+        #[cfg(not(bootstrap))]
+        // SAFETY: DynMetadata always contains a valid vtable pointer
+        return unsafe {
+            crate::intrinsics::vtable_size(self.vtable_ptr as *const VTable as *const ())
+        };
     }
 
     /// Returns the alignment of the type associated with this vtable.
     #[inline]
     pub fn align_of(self) -> usize {
-        self.vtable_ptr.align_of
+        #[cfg(bootstrap)]
+        return self.vtable_ptr.align_of;
+        #[cfg(not(bootstrap))]
+        // SAFETY: DynMetadata always contains a valid vtable pointer
+        return unsafe {
+            crate::intrinsics::vtable_align(self.vtable_ptr as *const VTable as *const ())
+        };
     }
 
     /// Returns the size and alignment together as a `Layout`