Crate auto_impl

A proc-macro attribute for automatically implementing a trait for references, some common smart pointers and closures.

Simple example

use auto_impl::auto_impl;

// This will generate two additional impl blocks: one `for &T` and one
// `for Box<T>` where `T: Foo`.
#[auto_impl(&, Box)]
trait Foo {
    fn foo(&self);
}

impl Foo for i32 {
    fn foo(&self) {}
}

fn requires_foo(_: impl Foo) {}


requires_foo(0i32);  // works: through the impl we defined above
requires_foo(&0i32); // works: through the generated impl
requires_foo(Box::new(0i32)); // works: through the generated impl

Basic syntax and supported types

You can annotate your trait with the #[auto_impl(...)] attribute. That attribute can only be used on traits and not on structs, enums or anything else.

In the attribute, you have to specify all so called proxy types (the types you want to generate impls for) as a comma separated list. Each proxy type has a short abbreviation that you have to list there.

Currently the following proxy types are supported:

Abbreviation	Example generated impl
&	impl<T: Trait> Trait for &T
&mut	impl<T: Trait> Trait for &mut T
Box	impl<T: Trait> Trait for Box<T>
Rc	impl<T: Trait> Trait for Rc<T>
Arc	impl<T: Trait> Trait for Arc<T>
Fn	impl<T: Fn()> Trait for T
FnMut	impl<T: FnMut()> Trait for T
FnOnce	impl<T: FnOnce()> Trait for T

More examples

More examples can be found in the examples folder. In particular, the greet_closure example shows how to use the Fn* proxy types.

The following example shows that a trait can contain associated consts, associated types and complex methods (with generics, bounds, …).

use auto_impl::auto_impl;
use std::{fmt, rc::Rc};


#[auto_impl(&, &mut, Box, Rc)]
trait Animal {
    const NUMBER_OF_LEGS: u8;

    type Name: fmt::Display;
    fn name(&self) -> Self::Name;

    fn select_favorite<'a, I>(&self, toys: I) -> &'a str
    where
        I: Iterator<Item = &'a str>;
}

struct Dog(String);

impl Animal for Dog {
    const NUMBER_OF_LEGS: u8 = 4;

    type Name = String;
    fn name(&self) -> Self::Name {
        self.0.clone()
    }

    fn select_favorite<'a, I>(&self, mut toys: I) -> &'a str
    where
        I: Iterator<Item = &'a str>
    {
        toys.next().unwrap()
    }
}

fn require_animal(_: impl Animal) {}

// All these calls work, as the `#[auto_impl]` attribute generated four
// impls for all those proxy types
require_animal(Dog("Doggo".into()));
require_animal(&Dog("Doggo".into()));
require_animal(&mut Dog("Doggo".into()));
require_animal(Box::new(Dog("Doggo".into())));
require_animal(Rc::new(Dog("Doggo".into())));

Restriction of references and smart pointers

Not every trait can be implemented for every proxy type. As an easy example, consider this trait:

trait Bar {
    fn bar(&mut self);
}

If we try to implement it for immutable references via #[auto_impl(&)] the following impl would be generated:

impl<T: Bar> Bar for &T {
    fn bar(&mut self) {
        T::bar(*self)  // fails to compile
    }
}

As you can easily see, this won’t work because we can’t call bar through an immutable reference. There are similar restrictions for many other smart pointers and references.

In the following table you can see which methods can be implemented for which proxy type. If a trait contains at least one method that cannot be implemented for a proxy type, you cannot implement the trait for that proxy type.

Trait contains method with…	&	&mut	Box	Rc	Arc
&self receiver	✔	✔	✔	✔	✔
&mut self receiver	✗	✔	✔	✗	✗
self receiver	✗	✗	✔	✗	✗
no self receiver	✔	✔	✔	✔	✔

References and smart pointers have no restriction in regard to associated types and associated consts! Meaning: traits with associated types/consts can always be implemented for references and smart pointers as long as the methods of that trait can be implemented.

Restriction of closure types (Fn* traits)

The Fn* proxy types have a lot more restrictions than references and smart pointer:

• the trait must not define any associated types or consts
• the trait must define exactly one method
  • the method must have a self receiver
  • the method must not return anything borrowed from self
  • the method must not have generic type or const parameters

Additionally, some Fn* traits cannot be implemented for all self receiver types:

self Receiver	Fn	FnMut	FnOnce
&self	✔	✗	✗
&mut self	✔	✔	✗
self	✔	✔	✔

Lastly, the impls generated for the Fn* proxy types contain for T. This is the most general blanket impl. So just be aware of the problems with coherence and orphan rules that can emerge due to this impl.

The keep_default_for attribute for methods

By default, the impls generated by auto_impl will overwrite all methods of the trait, even those with default implementation. Sometimes, you want to not overwrite default methods and instead use the default implementation. You can do that by adding the #[auto_impl(keep_default_for(...))] attribute to a default method. In the parenthesis you need to list all proxy types for which the default method should be kept.

From the keep_default_for example:

#[auto_impl(&, Box)]
trait Foo {
    fn required(&self) -> String;

    // The generated impl for `&T` will not override this method.
    #[auto_impl(keep_default_for(&))]
    fn provided(&self) {
        println!("Hello {}", self.required());
    }
}

Attribute Macros

• auto_impl
  See crate documentation for more information.