Document which discriminant we write

Co-authored-by: Oli Scherer <[email protected]>

Author: ivarflakstad

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -185,6 +185,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                     let vtable_ptr = self.get_vtable_ptr(tp_ty, preds)?;
                     self.write_pointer(vtable_ptr, dest)?;
                 } else {
+                    // Write `None`
                     self.write_discriminant(FIRST_VARIANT, dest)?;
                 }
             }