cap-primitives 4.0.3 panics with TryFromIntError on a negative macOS st_rdev

Inizia in src/rustix/fs/metadata_ext.rs alla riga 171 e confronta la conversione di st_rdev con la conversione protetta di st_dev immediatamente sopra. Segui il percorso symlink_metadata di cap_std per capire l’impatto a runtime, quindi verifica che la gestione dei metadati non vada più in panic per valori st_rdev negativi su macOS e che i test pertinenti del progetto passino.

Summary

ImplMetadataExt::from_rustix panics with TryFromIntError(()) on macOS because it converts st_rdev with u64::try_from(...).unwrap(), while the dev field a few lines above guards the same signedness problem explicitly.

Where

cap-primitives 4.0.3 (newest 4.x on crates.io), rustix backend, src/rustix/fs/metadata_ext.rs:171:

dev: if stat.st_dev < 0 {            // ← guarded
    i64::try_from(stat.st_dev).unwrap() as u64
} else {
    u64::try_from(stat.st_dev).unwrap()
},
ino: stat.st_ino.into(),
...
rdev: u64::try_from(stat.st_rdev).unwrap(),   // ← line 171, unguarded

The asymmetry is the whole report. dev handles a negative dev_t and carries a comment acknowledging the platform difference — "platforms where it's unsigned since the first branch here will never be taken" — while rdev, immediately below it, assumes non-negative and unwraps. On macOS dev_t is a signed i32, so st_rdev can be negative and the conversion fails.

Observed

Reached from cap_std's symlink_metadata path, matching the backtrace shape in #328:

called `Result::unwrap()` on an `Err` value: TryFromIntError(())
  at cap-primitives-4.0.3/src/rustix/fs/metadata_ext.rs:171

Environment: x86_64-apple-darwin inside a macOS Ventura Recovery guest under QEMU/KVM, stat'ing ordinary files in a temporary directory. The filesystem there reports a negative st_rdev for regular files, which is what triggers it.

I cannot offer a minimal reproduction. I hit this from CI on a Linux host driving a macOS guest, and I have no macOS machine to narrow it to a specific filesystem or file. I am reporting the code asymmetry, which is verifiable by reading the twelve lines above, rather than claiming to have characterised the trigger. If st_rdev is negative only on some filesystems, the guard below is still the right shape.

Relation to #328

Same file, same error type, same class of assumption — an unguarded try_from on a field that is not always non-negative. That one was about negative timestamps; this is a sibling field that the same reasoning applies to. I did not check whether the dev guard was added by that fix, so I am not claiming rdev was missed by it — only that it has the same problem now.

Suggested fix

Guard st_rdev as st_dev is:

rdev: if stat.st_rdev < 0 {
    i64::try_from(stat.st_rdev).unwrap() as u64
} else {
    u64::try_from(stat.st_rdev).unwrap()
},

or restructure both into a small helper so the next field cannot repeat the mistake. I have not opened a PR because I cannot test the macOS path; the change is yours to make with a reproduction you can actually run.

Impact beyond tests

This is not only a test-fixture problem. It panics inside symlink_metadata, so any cap-std consumer stat'ing a file whose st_rdev is negative fails at runtime on macOS, not merely under a profiler.