Impl-tools

A set of helper macros

Macros

Autoimpl

#[autoimpl] is a partial replacement for #[derive], supporting:

Explicit where clause on generic parameters
No implicit bounds on generic parameters beyond those required by the type
Traits like Deref by using a named field
Traits like Debug may ignore named fields

#[autoimpl] may also be used on trait definitions to impl for specified types supporting Deref.

Unlike alternatives, #[autoimpl] has minimal and intuitive syntax.

use impl_tools::autoimpl;
use std::fmt::Debug;

#[autoimpl(for<'a, T: trait + ?Sized> Box<T>)]
// Generates: impl<'a, T: Animal + ?Sized> Animal for Box<T> { .. }
trait Animal {
    fn number_of_legs(&self) -> u32;
}

#[autoimpl(Debug ignore self.animal where T: Debug)]
// Generates: impl<T, A: Animal> std::fmt::Debug for Named<A> where T: Debug { .. }
#[autoimpl(Deref, DerefMut using self.animal)]
// Generates: impl<T, A: Animal> std::ops::Deref for Named<A> { .. }
// Generates: impl<T, A: Animal> std::ops::DerefMut for Named<A> { .. }
struct Named<T, A: Animal> {
    name: T,
    animal: A,
}

fn main() {
    struct Fish;
    impl Animal for Fish {
        fn number_of_legs(&self) -> u32 {
            0
        }
    }

    let my_fish = Named {
        name: "Nemo",
        animal: Box::new(Fish),
    };

    assert_eq!(
        format!("{my_fish:?} has {} legs!", my_fish.number_of_legs()),
        r#"Named { name: "Nemo", .. } has 0 legs!"#
    );
}

Impl Default

#[impl_default] implements std::default::Default:

use impl_tools::{impl_default, impl_scope};

#[impl_default(Tree::Ash)]
enum Tree { Ash, Beech, Birch, Willow }

impl_scope! {
    #[impl_default]
    struct Copse {
        tree_type: Tree,
        number: u32 = 7,
    }
}

Impl Scope

impl_scope! is a function-like macro used to define a type plus its implementations. It supports impl Self syntax:

use impl_tools::impl_scope;
use std::fmt::Display;

impl_scope! {
    /// I don't know why this exists
    pub struct NamedThing<T: Display, F> {
        name: T,
        func: F,
    }

    // Repeats generic parameters of type
    impl Self {
        fn format_name(&self) -> String {
            format!("{}", self.name)
        }
    }

    // Merges generic parameters of type
    impl<O> Self where F: Fn(&str) -> O {
        fn invoke(&self) -> O {
            (self.func)(&self.format_name())
        }
    }
}

Caveat: rustfmt won't currently touch the contents. Hopefully that can be fixed!

Extensibility

Rust's #[derive] macro is extensible via #[proc_macro_derive] in a proc-macro crate. Our macros cannot be extended in the same way, but they can be extended via a new front-end:

Create a copy of the impl-tools crate to create a new "front-end" (proc-macro crate). This crate is contains only a little code over the impl-tools-lib crate.
To extend #[autoimpl], write an impl of ImplTrait and add it to the attribute's definition. To extend impl_scope!, write an impl of ScopeAttr and add it to the macro's definition.
Depend on your new front end crate instead of impl-tools.

Supported Rust Versions

The MSRV is 1.56.0 for no particular reason other than that it is the first to support Edition 2021.

Alternatives

Both Educe and Derivative have similar functionality: the ability to implement various traits with more flexibility than libstd's #[derive]. They also support more functionality such as tweaking the output of Debug. Both have less clean syntax, requiring a minimum of two attributes to do anything, with further attributes to customise implementations (e.g. to ignore a field).

derive_more isn't exactly an "alternative", simply supporting #[derive] for more standard traits. Possible functionality overlap in the future (though for now #[autoimpl] doesn't support half the traits supported by #[derive]).

auto_impl allows implementing a trait for reference types (&, &mut, Box, Rc, Arc) as well as function types.

Copyright and Licence

The COPYRIGHT file includes a list of contributors who claim copyright on this project. This list may be incomplete; new contributors may optionally add themselves to this list.

The impl-tools library is published under the terms of the Apache License, Version 2.0. You may obtain a copy of this licence from the LICENSE file or on the following webpage: https://www.apache.org/licenses/LICENSE-2.0

impl-tools 0.4.0