the memory_deallocated hook can make good use of knowing the size

src/librustc_mir/interpret/machine.rs

@@ -199,6 +199,7 @@ pub trait Machine<'a, 'mir, 'tcx>: Sized {
     fn memory_deallocated(
         _alloc: &mut Allocation<Self::PointerTag, Self::AllocExtra>,
         _ptr: Pointer<Self::PointerTag>,
+        _size: Size,
     ) -> EvalResult<'tcx> {
         Ok(())
     }

src/librustc_mir/interpret/memory.rs

@@ -232,7 +232,8 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> Memory<'a, 'mir, 'tcx, M> {
         }
 
         // Let the machine take some extra action
-        M::memory_deallocated(&mut alloc, ptr)?;
+        let size = Size::from_bytes(alloc.bytes.len() as u64);
+        M::memory_deallocated(&mut alloc, ptr, size)?;
 
         // Don't forget to remember size and align of this now-dead allocation
         let old = self.dead_alloc_map.insert(