Following cmp_owned hint leads to different behaviour

[Schaeff]

I have code which uses different PartialEq implementations, which is arguably not the best design, but clippy complains about unnecessary allocations and its suggestion ends up using another PartialEq implementation.

Lint name: cmp_owned

I tried this code, before the suggestion it prints Strict! which is the derived PartialEq, after it prints Optimistic! which is the more optimistic PartialEq.

#[derive(PartialEq)]
enum MaybeU32 {
    Unknown,
    Known(u32),
}

impl From<u32> for MaybeU32 {
    fn from(a: u32) -> Self {
        MaybeU32::Known(a)
    }
}

impl PartialEq<u32> for MaybeU32 {
    fn eq(&self, other: &u32) -> bool {
        use MaybeU32::*;
        match self {
            Known(number) => number == other,
            _ => true, // we optimistically return true now because it may end up being the case
        }
    }
}

fn main() {
    if MaybeU32::Unknown == MaybeU32::from(32u32) {
        println!("Optimistic!");
    } else {
        println!("Strict!");
    }
}

Meta

Rust version (rustc -Vv):

rustc 1.46.0-nightly (f455e46ea 2020-06-20)
binary: rustc
commit-hash: f455e46eae1a227d735091091144601b467e1565
commit-date: 2020-06-20
host: x86_64-unknown-linux-gnu
release: 1.46.0-nightly
LLVM version: 10.0

[camsteffen]

Your PartialEq implementation does not satisfy the transitive equality requirement.

[flip1995]

PartialEq has the documented contract:

The equality relation == must satisfy the following conditions (for all a, b, c of type A, B, C):

• Symmetric: if A: PartialEq<B> and B: PartialEq<A>, then a == b implies b == a; and
• Transitive: if A: PartialEq<B> and B: PartialEq<C> and A: PartialEq<C>, then a == b and b == c implies a == c.

Since your implementation violates this, Clippy can't make a sensible suggestion. This is because Clippy assumes that you're implementation meets all preconditions.

This is therefore not a bug in Clippy.