deriving_via

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

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.

```rust

[derive(DerivingVia)]

[deriving(From, Display)]

pub struct D(i32); ```

with generics

```rust

[derive(DerivingVia)]

[deriving(From, Display)]

pub struct D(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.

```rust

[derive(DerivingVia)]

[deriving(From, Display)]

pub struct Test(#[underlying] i32, std::marker::PhantomData); ```

Syntax

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

```rust

[derive(DerivingVia)]

[deriving(...)]

struct Target(Base); ```

The syntax of <Derive> is defined as follows.

text 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

```rust 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

```rust 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

```rust struct Base(Underlying);

[derive(DerivingVia)]

[deriving(...)]

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. Note that this is not fully supported yet.

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