Description

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 {...
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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...
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Mitigation

Aliases

References