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);

• Display
• Eq
• Ord
• Hash
• serde::Serialize
• serde::Deserialize
• Into
  • additional requirements: Base: Into<Underlying>
  • limitations: one hop or #[transitive]
• From
  • additional requirements: Base: From<Underlying>
  • limitations: one hop or #[transitive]
• TryFrom
  • additional requirements: Base: From<Underlying>
  • limitations: one hop or #[transitive]
• FromStr
  • additional requirements: Base: From<Underlying>
  • limitations: one hop or #[transitive]
• Add-lile (Add, Sub)
  • additional requirements: Base: From<Underlying>
  • limitations: one hop or #[transitive]
• Mul-like (Mul, Div)
  • additional requirements: Base: From<Underlying>
  • limitations: one hop or #[transitive]
• Arithmetic (Add, Sub, Mul, Div)
  • additional requirements: Base: From<Underlying>
  • limitations: one hop or #[transitive]

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