Appendix C: Derivable Traits

#[derive] is a compiler built-in that generates trait implementations mechanically from a type's fields or variants. You list the traits you want derived in one attribute:

#[derive(PartialEq, Eq, Hash, Display)]
struct GridCell {
    row: i64,
    col: i64,
}

(Note the field types are integers. Eq and Hash require every field to be Eq/Hash, and f64 is neither — NaN != NaN breaks reflexivity — so a struct with f64 fields can derive PartialEq/PartialOrd/Display but not Eq/Ord/Hash. See the per-trait requirements below.)

The compiler resolves derive dependencies automatically regardless of the order you list them. Writing #[derive(Hash)] when PartialEq and Eq are not yet derived causes the compiler to derive them first, in the correct order.

Equality and ordering

PartialEq

Generates == and != by comparing fields pairwise in declaration order. For enums, first checks that the variants match, then compares fields.

No dependencies.

Eq

A marker trait indicating that == is a total equivalence relation (reflexive, symmetric, transitive, and always defined). Adds no method body — it is a promise to the type system.

Requires: PartialEq

PartialOrd

Generates <, <=, >, >= with lexicographic field-order comparison. Returns Option[Ordering] because NaN != NaN for floats; for types without NaN, every comparison returns Some(...).

Requires: PartialEq

Ord

Total ordering: every pair of values is comparable. Generates a complete lexicographic comparison in declaration order.

Requires: PartialOrd + Eq

Hashing

Hash

Generates a hash method that feeds each field into a Hasher. Used by Map and Set as key types.

Requires: Eq (reflects the consistency contract: a == b must imply hash(a) == hash(b))

Display and debugging

Display

Generates a human-readable string representation.

• Structs: emits TypeName { field: value, ... } for pub fields. Use #[derive(Display(all_fields))] to include private fields.
• Enums: emits the variant name. Use #[derive(Display(snake_case))] to emit in snake_case instead of the declared PascalCase. For variants with data, appends the fields in parentheses.

Use Display for values shown to end users. Implement it manually to override the generated representation.

No dependencies.

Debug

Generates a developer-oriented representation. Used by {expr:?} in interpolated strings and by the test runner when printing unexpected values.

• Structs: always includes all fields, regardless of visibility.
• Enums: includes variant name and all fields.

No dependencies.

Default values

Default

Generates a T.default() method that returns a "zero-like" value for the type. The derived implementation calls .default() on each field in declaration order and constructs the struct. For enums, the first declared variant is used, with each of its fields defaulted.

#[derive(Default)]
struct Config {
    timeout_ms: i64,   // defaults to 0
    retries: i64,      // defaults to 0
    verbose: bool,     // defaults to false
}

let cfg = Config.default();

Requires: every field must also implement Default.

Copying

Clone

Generates a .clone() method that produces a deep copy of the value. For reference-semantics (shared) types, cloning produces a new RC handle, not a new heap allocation.

No dependencies.

Copy

Marks a type as trivially copyable (bitwise copy semantics). Assignment and passing to functions copy the value silently instead of moving it. All primitive types are Copy.

Requires: every field of the type must also be Copy.

Auto-derives Clone: #[derive(Copy)] automatically adds Clone if not already present.

Arithmetic on distinct types

Arithmetic

Available on distinct (newtype) types only. Generates +, -, *, /, % by forwarding to the underlying type's operations and wrapping the result back in the newtype. Without this derive, arithmetic between two values of the same distinct type is a type error (intentional: distinct types are supposed to be incompatible units).

#[derive(Arithmetic)]
distinct type Metres = f64;   // now Metres + Metres → Metres

Only valid on distinct types.

Dependency summary