error[E0594]: cannot assign to `self.0`, which is behind a `&` reference
  --> $DIR/issue-68049-1.rs:7:9
   |
LL |     unsafe fn alloc(&self, _layout: Layout) -> *mut u8 {
   |                     ----- this is an immutable reference
LL |         self.0 += 1;
   |         ^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be written

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0594`.