downcast_mut unsoundness advisories for anyhow, eyre, and miette

[dfoxfranke]

This PR contains three advisories for the same bug occurring in three popular error-handling crates. The unsound code originally appeared in anyhow. eyre copied it, and then miette copied it from eyre.

Affected crate(s)

• anyhow (163,113,997)
• eyre (16,375,087)
• miette (12,898,466)

Links to upstream issue(s) or PR(s)

• anyhow: Issue, PR
• eyre: Issue, PR
• miette: Issue

Severity

Informational advisory for soundness issues in safe public APIs. Advisories include example code that crashes in MIRI, but in my limited testing I was unable to find any evidence that rustc currently exploits the UB or compiles the code according to other than its author-intended semantics.

Checklist

• Advisory filename(s) starts with RUSTSEC-0000-0000 as the ID
• date field is set to the public disclosure date
• Contains a concise and descriptive title after advisory metadata
• Asked maintainer(s) if publishing an advisory is appropriate - see timeline

Communication timeline

• 2026-05-31: @Shnatsel reported the issue to miette.
• 2026-06-25: I reported the issue to anyhow. Fixed released on the same day by @dtolnay and okayed a RustSec advisory.
• 2026-06-26: I reported the issue to eyre and opened a PR with a fix.

eyre maintainers have not yet responded, and this PR will remain in draft status until they've had adequate time to do so and release a fix. I'm not sure of miette's intentions, since there has been no maintainer response to the issue thread, but we're past two weeks from public disclosure.