#traits #dyn #object #safe #type


Implement object-unsafe traits for trait objects

4 releases (2 breaking)

0.3.1 Aug 17, 2023
0.3.0 Aug 8, 2023
0.2.0 Aug 4, 2023
0.1.0 Aug 3, 2023

#995 in Rust patterns

Used in 2 crates


304 lines

Object Safe

Do you need trait objects, in the form dyn MyTrait, that implement traits which are not object-safe?

This crate solves the problem by providing object-safe traits which are analogous to some commonly used traits that are not object-safe, and auto-implementing both for a wide range of types. Currently, the following traits are supported:

  • Hash
  • PartialEq
  • Eq

I plan to extend this support to other traits, and offer macros to simplify the process for custom traits.

Learn about object safety here: https://doc.rust-lang.org/reference/items/traits.html#object-safety


Let's take the Hash trait as an example. Hash is not object-safe. This means that dyn Hash is not a valid type in rust. Now, imagine you define this custom trait:

pub trait MyTrait: Hash {}

Since MyTrait extends Hash, it is not object safe either, and dyn MyTrait is not a valid type. This crate offers a way to work around this limitation, so you can have object-safe traits whose objects implement object-unsafe traits such as Hash.

Instead of expressing Hash as the trait bound, express HashObj as the trait bound.

pub trait MyTrait: HashObj {}

You do not need to implement HashObj. It is automatically implemented for any type that implements Hash. Now, dyn MyTrait is object-safe. Add one line of code if you want dyn MyTrait to implement Hash:

impl_hash(dyn MyTrait);

Here are all the characteristics that come with HashObj:

  • anything implementing Hash automatically implements HashObj
  • dyn HashObj implements Hash.
  • Obj<T> implements Hash for any T that derefs to something implementing HashObj.
  • impl_hash can implement Hash for any type that implements HashObj, for example a trait object dyn MyTrait where MyTrait is a trait extending HashObj.
impl_hash! {
    // typical use, where MyTrait: HashObj
    dyn MyTrait,
    dyn AnotherTrait,

    // structs and enums are supported if they deref to
    // a target that implements HashObj or Hash.

    // special syntax for generics.
    MySimpleGeneric<T> where <T>,
    MyGenericType<T, F> where <T, F: HashObj>,
    dyn MyGenericTrait<T> where <T: SomeTraitBound>,

    // the actual impl for Obj
    Obj<T> where <T: Deref<Target=X>, X: HashObj + ?Sized>,

No runtime deps