Crate transitive

This crate provides a derive macro that takes care of boilerplate code to make transitive conversions in Rust using From and TryFrom traits.

It’s not magic and it’s completely static. The derive here merely implements From or TryFrom between types by relying on existent impls between items in the path.

The path taken for transitions must be annotated (correctly) for transitions to work. Additonally, there can only be one transition between a source and a target type, as otherwise there would be duplicate trait implementations.

The path is provided in the [#[transitive]] attribute along with a direction:

#[derive(Transitive)]
#[transitive(from(D, C, B))] // Results in `impl From<D> for A` as `D -> C -> B -> A`
#[transitive(into(B, C, D))] // Results in `impl From<A> for D` as `A -> B -> C -> D`
struct A;

Conversions table:

Derived Type	Annotation	Will impl	Conditions
A	#[transitive(into(B, C, D))]	From<A> for D	From<A> for B; From<B> for C; From<C> for D
A	#[transitive(from(D, C, B))]	From<D> for A	From<D> for C; From<C> for B; From<B> for A
A	#[transitive(try_into(B, C, D))]	TryFrom<A> for D	TryFrom<A> for B; TryFrom<B> for C; TryFrom<C> for D; errors must impl From<ErrType> for <D as TryFrom<C>>::Error
A	#[transitive(try_from(D, C, B))]	TryFrom<D> for A	TryFrom<D> for C; TryFrom<C> for B; TryFrom<B> for A; errors must impl From<ErrType> for <A as TryFrom<B>>::Error

Custom error type:

For try_from and try_into annotations, the macro attribute can accept an error = MyError argument as the last element, like so: #[transitive(try_into(A, B, C, error = MyError))]. This overrides the default behavior and allows specifying a custom error type, but all the error types resulting from conversions must be convertible to this type.

Examples:

use transitive::Transitive;

#[derive(Transitive)]
#[transitive(into(B, C, D))] // impl From<A> for D by doing A -> B -> C -> D
struct A;

#[derive(Transitive)]
#[transitive(into(C, D))] // impl From<B> for D by doing B -> C -> D
struct B;
struct C;
struct D;

impl From<A> for B {
    fn from(val: A) -> Self {
        Self
    }
};

impl From<B> for C {
    fn from(val: B) -> Self {
        Self
    }
};

impl From<C> for D {
    fn from(val: C) -> Self {
        Self
    }
};

D::from(A);
D::from(B);

Note that the macro does nothing for types in the middle:

use transitive::Transitive;

#[derive(Transitive)]
#[transitive(into(B, C, D))] // impl From<A> for D by doing A -> B -> C -> D
struct A;
struct B;
struct C;
struct D;

impl From<A> for B {
    fn from(val: A) -> Self {
        Self
    }
};

impl From<B> for C {
    fn from(val: B) -> Self {
        Self
    }
};

impl From<C> for D {
    fn from(val: C) -> Self {
        Self
    }
};

D::from(A); // works
C::from(A); // does not compile

use transitive::Transitive;

#[derive(Transitive)]
#[transitive(into(B, C))] // impl From<A> for C by doing A -> B -> C
#[transitive(into(C, D))] // impl From<A> for D by doing A -> C -> D
struct A;
struct B;
struct C;
struct D;

impl From<A> for B {
    fn from(val: A) -> Self {
        Self
    }
};

impl From<B> for C {
    fn from(val: B) -> Self {
        Self
    }
};

impl From<C> for D {
    fn from(val: C) -> Self {
        Self
    }
};

D::from(A);

Let’s see an example on how to use Transitive when combining the “reversed” nature of the from and try_from attribute modifiers and the error transitions constraints:

#![allow(non_camel_case_types)]
use transitive::Transitive;

// Note how the annotation now considers `A` as target type
// and `D`, the first element in the type list, as source.
#[derive(Transitive)]
#[transitive(try_from(D, C, B))] // impl TryFrom<D> for A
struct A;

#[derive(Transitive)]
#[transitive(try_from(D, C, error = ConvErr))] // impl TryFrom<D> for B, with custom error
struct B;
struct C;
struct D;

struct ConvErr;

struct ErrD_C;
struct ErrC_B;

#[derive(Transitive)]
#[transitive(from(ErrD_C, ErrC_B))] // impl From<ErrD_C> for ErrB_A
struct ErrB_A;

impl From<ErrD_C> for ErrC_B {
    fn from(val: ErrD_C) -> Self {
        Self
    }
};

impl From<ErrC_B> for ErrB_A {
    fn from(val: ErrC_B) -> Self {
        Self
    }
};

impl From<ErrD_C> for ConvErr {
    fn from(val: ErrD_C) -> Self {
        Self
    }
};

impl From<ErrC_B> for ConvErr {
    fn from(val: ErrC_B) -> Self {
        Self
    }
};

impl TryFrom<D> for C {
    type Error = ErrD_C;

    fn try_from(val: D) -> Result<Self, Self::Error> {
        Ok(Self)
    }
};

impl TryFrom<C> for B {
    type Error = ErrC_B;

    fn try_from(val: C) -> Result<Self, Self::Error> {
        Ok(Self)
    }
};

impl TryFrom<B> for A {
    type Error = ErrB_A;

    fn try_from(val: B) -> Result<Self, Self::Error> {
        Ok(Self)
    }
};

A::try_from(D);
B::try_from(D);

Derive Macros

Transitive