clippy::non_canonical_partial_ord_impl: Checks for non-canonical implementations of PartialOrd when Ord is already implemented

#[derive(Eq, PartialEq)]
struct A(u32);

impl Ord for A {
    fn cmp(&self, other: &Self) -> Ordering {
        // ...
    }
}

impl PartialOrd for A {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        // ...
    }
}

#[derive(Eq, PartialEq)]
struct A(u32);

impl Ord for A {
    fn cmp(&self, other: &Self) -> Ordering {
        // ...
    }
}

impl PartialOrd for A {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))   // or self.cmp(other).into()
    }
}

#[allow(clippy::non_canonical_partial_ord_impl)]
fn my_function() { /* ... */ }

[lints.clippy]
non_canonical_partial_ord_impl = "allow"

cargo clippy -- -A clippy::non-canonical-partial-ord-impl

Clippy's own documentation

What it does

Checks for non-canonical implementations of PartialOrd when Ord is already implemented.

Why is this bad?

If both PartialOrd and Ord are implemented, they must agree. This is commonly done by wrapping the result of cmp in Some for partial_cmp. Not doing this may silently introduce an error upon refactoring.

Known issues

Code that calls the .into() method instead will be flagged, despite .into() wrapping it in Some.

Example

#[derive(Eq, PartialEq)]
struct A(u32);

impl Ord for A {
    fn cmp(&self, other: &Self) -> Ordering {
        // ...
    }
}

impl PartialOrd for A {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        // ...
    }
}

Use instead:

#[derive(Eq, PartialEq)]
struct A(u32);

impl Ord for A {
    fn cmp(&self, other: &Self) -> Ordering {
        // ...
    }
}

impl PartialOrd for A {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))   // or self.cmp(other).into()
    }
}

Past names

• incorrect_partial_ord_impl_on_ord_type

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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All 847 Clippy lints · Rust compiler errors · Cargo.toml checker