use extern type for extra opaqueness

Author: RalfJung

library/core/src/ptr/metadata.rs

@@ -180,14 +180,15 @@ 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]);
+extern "C" {
+    /// 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.
+    type VTable;
+}
 
 /// The common prefix of all vtables. It is followed by function pointers for trait methods.
 ///