fix alignment handling for Repeat expressions

compiler/rustc_const_eval/src/interpret/step.rs

@@ -208,13 +208,13 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     let rest_ptr = first_ptr.offset(elem_size, self)?;
                     // For the alignment of `rest_ptr`, we crucially do *not* use `first.align` as
                     // that place might be more aligned than its type mandates (a `u8` array could
-                    // be 4-aligned if it sits at the right spot in a struct). Instead we use
-                    // `first.layout.align`, i.e., the alignment given by the type.
+                    // be 4-aligned if it sits at the right spot in a struct). We have to also factor
+                    // in element size.
                     self.mem_copy_repeatedly(
                         first_ptr,
-                        first.align,
+                        dest.align,
                         rest_ptr,
-                        first.layout.align.abi,
+                        dest.align.restrict_for_offset(elem_size),
                         elem_size,
                         length - 1,
                         /*nonoverlapping:*/ true,