Eq + Hash rule is broken when mixing OsStr and Path

[mzabaluev]

This program surprisingly fails:

use std::{
    ffi::OsStr,
    path::Path,
    collections::hash_map::DefaultHasher,
    hash::{Hash, Hasher},
};

fn hash<T: ?Sized + Hash>(v: &T) -> u64 {
    let mut sh = DefaultHasher::new();
    v.hash(&mut sh);
    sh.finish()
}

fn main() {
    let s = OsStr::new("/dev/null");
    let p = Path::new("/dev/null");
    assert_eq!(s, p);
    assert!(
        hash(s) == hash(p),
        "Hashes differ for values that compare as equal"
    );
}

The problem is that the provided impls of PartialEq, while convenient, cross the data domains and break the intuition on matching behavior of Hash and ==. This does not affect HashMap due to lack of a cross-type Borrow impl, but the inconsistency can bite the unwary.

[mzabaluev]

There is another problem with Hash and Eq behavior for Path: #29008

[Enselic]

Triage: I think the rule is written with a single type in mind, but this example of cross-type rule violation seems unnecessarily inconsistent.

[Enselic]

Triage: Maybe all that's needed to close this issue is adding some clarification in the docs that the rule only applies to one type, and not across different (but related) types?