RUSTSEC-2026-0190 Unsoundness in `Error::downcast_mut()`

Affected versions of this crate violate borrow rules, resulting in undefined behavior, when the user adds context to an error via Error::context and then later calls Error::downcast_mut on the returned Error.

The flaw was corrected in commit 6e8c000 by revising how the mutable reference is constructed, avoiding inclusion of a shared reference in the resulting borrow chain.

Example

use anyhow::Error;
use std::fmt;

#[derive(Debug)]
struct ErrorContext(&'static str);

impl fmt::Display for ErrorContext {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&self.0, f)
    }
}

fn main() {
    let mut error = Error::msg("inner error").context(ErrorContext("old context"));
    let context: &mut ErrorContext = error.downcast_mut().unwrap();
    context.0 = "new context";
    println!("{:?}", error);
}

Miri output

error: Undefined Behavior: trying to retag from <1538> for Unique permission at alloc602[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 alloc602[0x38..0x48]
    |
    = 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: <1538> was created by a SharedReadOnly retag at offsets [0x38..0x48]
   --> src/ptr.rs:89:18
    |
 89 |             ptr: NonNull::from(ptr),
    |                  ^^^^^^^^^^^^^^^^^^
    = note: stack backtrace:
            0: anyhow::ptr::Mut::<'_, ErrorContext>::deref_mut
                at src/ptr.rs:170:18: 170:41
            1: anyhow::error::<impl anyhow::Error>::downcast_mut::<ErrorContext>
                at src/error.rs:560:18: 560:46
            2: main
                at examples/downcast_mut.rs:15:38: 15:58