test test_downcast_mut ... error: Undefined Behavior: trying to retag from <137701> for Unique permission at alloc44937[0x38], but that tag only grants SharedReadOnly permission for this location
       --> src/ptr.rs:170:18
        |
    170 |         unsafe { &mut *self.ptr.as_ptr() }
        |                  ^^^^^^^^^^^^^^^^^^^^^^^ this error occurs as part of retag at alloc44937[0x38..0x50]
        |
        = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
        = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
    help: <137701> was created by a SharedReadOnly retag at offsets [0x38..0x50]
       --> src/ptr.rs:89:18
        |
     89 |             ptr: NonNull::from(ptr),
        |                  ^^^^^^^^^^^^^^^^^^
        = note: this is on thread `test_downcast_mut`
        = note: stack backtrace:
                0: anyhow::ptr::Mut::<'_, HighLevel>::deref_mut
                    at src/ptr.rs:170:18: 170:41
                1: anyhow::error::<impl anyhow::Error>::downcast_mut::<HighLevel>
                    at src/error.rs:560:18: 560:46
                2: test_downcast_mut
                    at tests/test_context.rs:171:5: 171:36
                3: test_downcast_mut::{closure#0}
                    at tests/test_context.rs:168:23: 168:23

Eliminate pointer->reference->pointer during downcast