share BinOp::Offset between CTFE and Miri

compiler/rustc_const_eval/src/interpret/operator.rs

@@ -299,6 +299,30 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         Ok((val, false, ty))
     }
 
+    fn binary_ptr_op(
+        &self,
+        bin_op: mir::BinOp,
+        left: &ImmTy<'tcx, M::Provenance>,
+        right: &ImmTy<'tcx, M::Provenance>,
+    ) -> InterpResult<'tcx, (Scalar<M::Provenance>, bool, Ty<'tcx>)> {
+        use rustc_middle::mir::BinOp::*;
+
+        match bin_op {
+            // Pointer ops that are always supported.
+            Offset => {
+                let ptr = left.to_scalar().to_pointer(self)?;
+                let offset_count = right.to_scalar().to_target_isize(self)?;
+                let pointee_ty = left.layout.ty.builtin_deref(true).unwrap().ty;
+
+                let offset_ptr = self.ptr_offset_inbounds(ptr, pointee_ty, offset_count)?;
+                Ok((Scalar::from_maybe_pointer(offset_ptr, self), false, left.layout.ty))
+            }
+
+            // Fall back to machine hook so Miri can support more pointer ops.
+            _ => M::binary_ptr_op(self, bin_op, left, right),
+        }
+    }
+
     /// Returns the result of the specified operation, whether it overflowed, and
     /// the result type.
     pub fn overflowing_binary_op(
@@ -368,7 +392,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     right.layout.ty
                 );
 
-                M::binary_ptr_op(self, bin_op, left, right)
+                self.binary_ptr_op(bin_op, left, right)
             }
             _ => span_bug!(
                 self.cur_span(),