Convert Trait

Published on

  • Feature Name: convert_trait
  • Start Date: 2018-12-02
  • RFC PR: (leave this empty)
  • Rust Issue: (leave this empty)

Summary

This is a partner trait to From and Into that allows using a conversion method in the middle of a chain.

Motivation

Currently, Rust code cannot write

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struct A; struct B; struct C;
impl From<A> for B { /* … */ }

let a: A = A::new();
let c: C = a.into().b_to_c();

This is because the expression a.into() does not specify a final type, and there is an infinite set of possible types that may have a method with the signature fn b_to_c(self) -> C. The compiler is incapable of determining which specific Into<?> for A impl to use, and fails compilation.

Because the Into trait defines fn into(self) -> T as a non-generic function, the expression .into::<T>() is ill-formed, and attempts to select a method that does not exist.

Currently, the only way to write this expression without temporaries is to use UFCS:

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let c: C = <A as Into<B>>::into(a).b_to_c();

This can be shortened as Into<B>::into(a), because a has a known type and thus fully specifies the trait impl to use. I will use fully specified trait functions in this RFC for clarity.

or to use the companion trait, From:

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let c: C = <B as From<A>>::from(a).b_to_c();

This can be shortened as B::from(a).

Both of these require significantly rearranging the expression, using a leading type and function instead of method and trailing type, and introduce a lot of noise. This loses all the advantages of method syntax, both syntactic and semantic.

This trait supports the use case of type conversions in a method chain where map is unavailable. The expected outcome is that a.into().use() chains will require a much smaller insertion of type information, rather than a large rewrite, making writing these expressions more smooth.

Guide-level explanation

Rust offers the std::convert module as a defined set of interfaces for converting values from one type to another. The From trait is the idiomatic standard for moving values between types, and the companion traits Into and Convert provide automatic conveniences for invoking it.

A type conversion between an existing source type, Source, and a target type, Target, is established by writing an impl From<Source> for Target block with the conversion function. Once this block exists, the universal implementations in the standard library make Into<Target> and Convert methods available on Source.

Conversion from an instance of the source type to the target type can be achieved with the From free function or with the Into or Convert methods.

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#[derive(Clone, Copy)]
struct Source;
struct Target;

impl From<Source> for Target {
    fn from(src: Source) -> Self {
        Target
    }
}

fn main() {
    let a = Source;
    let b = Target::from(a);
    let c: Target = a.into();
    let d = a.convert::<Target>();
}

Into and Convert are complements of each other. Into<Target> is the right choice for use in trait bounds, while .convert::<Target>() is the right choice for use in method calls. They both perform the same underlying thing: <Target as From<Self>>::from(self).

Reference-level explanation

In the std::convert module, define a new trait:

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pub trait Convert : Sized {
    fn convert<T: Sized + From<Self>>(self) -> T {
        <T as From<Self>>::from(self)
    }
}

and blanket-implement it on all sized types:

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impl<T: Sized> Convert for T {}

Drawbacks

Why should we not do this?

The standard library should not be a slowly-accumulating pile of idioms.

Rationale and alternatives

  • Why is this design the best in the space of possible designs?

    It's a trivial wrap around From, just like how Into is written

  • What other designs have been considered and what is the rationale for not choosing them?

    Type ascription : apparently difficult, has not made much progress, requires language-level support

  • What is the impact of not doing this?

    Method chains are less ergonomic than possible when they require type conversions.

Prior art

The Into trait shows willingness in the standard library to define companion traits that function only as a reshaping of how the base trait is invoked.

Unresolved questions

  • What parts of the design do you expect to resolve through the RFC process before this gets merged?

    What should it be named?

    • Convert is the full word
    • Conv is four letters, matching From and Into
    • other?
  • What parts of the design do you expect to resolve through the implementation of this feature before stabilization?

    None

  • What related issues do you consider out of scope for this RFC that could be addressed in the future independently of the solution that comes out of this RFC?

    None

Future possibilities

None