PartialOrd and Ord

Partial ordering & Total ordering.

Derivable: ✅

When to implement: Almost always.

# // Copyright 2025 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
// pub trait PartialOrd<Rhs = Self>: PartialEq<Rhs>
// {
//     // Required method
//     fn partial_cmp(&self, other: &Rhs) -> Option<Ordering>;
//
//     /* Provided methods omitted */
// }
// pub trait Ord: Eq + PartialOrd {
//     // Required method
//     fn cmp(&self, other: &Self) -> Ordering;
//
//     /* Provided methods omitted */
// }

#[derive(PartialEq, PartialOrd)]
pub struct Partially(f32);

#[derive(PartialEq, Eq, PartialOrd, Ord)]
pub struct Totally {
    id: u32,
    name: String,
}

Ord gives access to min, max, and clamp methods.

• When derived, compares things in the order they are defined.

  For enums this means each variant is considered "greater than" the last as they are written.

  For structs this means fields are compared as they are written, so id fields are compared before name fields in Totally.

• Prerequisites: PartialEq for PartialOrd, Eq for Ord.

  To implement Ord, a type must also implement PartialEq, Eq, and PartialOrd.

• Like with PartialEq and Eq, a type cannot implement Ord without implementing PartialOrd.

  Like those equality traits, PartialOrd exists to separate types with non-total ordering (particularly floating-point numbers) from types with total ordering.

• Used for sorting/searching algorithms and maintaining the ordering of BTreeMap/BTreeSet style data types.