deriving_via

This library is a slightly more convenient version of derive_more for newtype pattern.

=> Deriving via

Basic Usage

#[derive(DerivingVia)] and then write the #[deriving] attribute on struct and list the trait you want to derive in it.

simple

Derives From<i32> for D and Display for D.

#[derive(DerivingVia)]
#[deriving(From, Display)]
pub struct D(i32);

with generics

#[derive(DerivingVia)]
#[deriving(From, Display)]
pub struct D<T: Display>(T);

with newtype pattern

If you have more than one field, specify #[underlying] for one. Note that the other fields require default initialisation by the Default trait.

#[derive(DerivingVia)]
#[deriving(From, Display)]
pub struct Test<T>(#[underlying] i32, std::marker::PhantomData<T>);

Syntax

Derive DerivingVia and list the traits you want to derive in the #[deriving] attribute.

#[derive(DerivingVia)]
#[deriving(<Derive>...)]
struct Target(Base);

The syntax of <Derive> is defined as follows.

Derive := <Trait> | <Trait>(via: <Type>)

Deriving Via

Using the deriving via feature, it is possible to generate derives from the impl of a base of a multi-layered wrapped type.

DerivingVia uses transitive type coercion for type conversion. All newtypes must be dereferenceable to the underlying type. Therefore, DerivingVia automatically generates a Deref trait.

Example

use deriving_via::DerivingVia;

#[derive(DerivingVia)]
pub struct A(i32);

#[derive(DerivingVia)]
pub struct B(A);

#[derive(DerivingVia)]
#[deriving(Display(via: i32))]
pub struct C(B);

fn main() {
  let c = C(B(A(42)));
  println!("{c}"); // 42
}

Deref trait works transitive, but how we re-constructs a Self type? Unfortunately, no convenience mechanism exists in the language, so it is necessary to teach how to revert using the #[transitive] attribute. Some trait require #[transitive] attribute (see Available Derives section).

Example

use std::fmt::Display;

use deriving_via::DerivingVia;

#[derive(DerivingVia)]
#[deriving(From)]
pub struct A(i32);

#[derive(DerivingVia)]
#[deriving(From)]
pub struct B(A);

#[derive(DerivingVia)]
#[deriving(From, Add(via: i32), Display(via: i32))]
#[transitive(i32 -> A -> B -> C)]
pub struct C(B);

fn main() {
  let c = C(B(A(42))) + C(B(A(42)));
  println!("{c}");
}

Available Derives

struct Base(Underlying);

#[derive(DerivingVia)]
#[deriving(<Derive>)]
struct Target(Base);

• fmt
  • Display
    • requires: Base: Display or (via = <Type>) and Type: Display
• ops
  • Eq
    • requires: Base: Eq or (via = <Type>) and Type: Eq
  • Ord
    • requires: Base: Ord or (via = <Type>) and Type: Ord
  • Add-lile (Add, Sub)
    • requires: Base: From<Underlying>
    • limitations: one hop or #[transitive]
  • Mul-like (Mul, Div)
    • requires: Base: From<Underlying>
    • limitations: one hop or #[transitive]
  • Arithmetic (Add, Sub, Mul, Div)
    • requires: Base: From<Underlying>
    • limitations: one hop or #[transitive]
  • Index
    • requires: Base: Index or (via = <Type>) and Type: Index
  • IndexMut
    • requires: Base: IndexMut or (via = <Type>) and Type: IndexMut
  • DerefMut
    • requires: Base: DerefMut or (via = <Type>) and Type: DerefMut
• hash
  • Hash
    • requires: Base: Hash or (via = <Type>) and Type: Hash
• serde
  • Serialize
    • requires: Base: Serialize or (via = <Type>) and Type: Serialize
  • Deserialize
    • requires: Base: Deserialize or (via = <Type>) and Type: Deserialize
• convert
  • AsRef
  • AsMut
  • FromIterator
    • requires: (via: <ItemType>)
  • IntoIterator
    • requires: Base: IntoIterator or (via: <Type>), Type: IntoIterator
  • Into
    • requires: Base: Into<Underlying>
    • limitations: one hop or #[transitive]
  • From
    • limitations: one hop or #[transitive]
  • TryFrom
    • requires: Base: From<Underlying>
    • limitations: one hop or #[transitive]
  • FromStr
    • requires: Base: From<Underlying>
    • limitations: one hop or #[transitive]
• impls
  • Iter
    • requires: Base: IntoIterator and Base dereferenceable to slice or (via: <Type>), Type: IntoIterator and Type dereferenceable to slice

Caveat

DerivingVia using transitive case of Type Coercion. According to rumours, transitive Type Coercion is not fully supported yet.

See: https://doc.rust-lang.org/reference/type-coercions.html#coercion-types