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;

// Impl Animal for Box<T> where T: Animal + ?Sized
#[autoimpl(for<T: trait + ?Sized> Box<T>)]
trait Animal {
    fn number_of_legs(&self) -> u32;
}

// Impl Debug for Named<T, A: Animal> omitting field animal from output
#[autoimpl(Debug ignore self.animal where T: Debug)]
// Impl Deref and DerefMut to field animal for Named<T, A: Animal>
#[autoimpl(Deref, DerefMut using self.animal)]
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!"#
    );
}

New-type wrappers

A combination of Deref on the new-type and trait-reimplementation on the trait allows succinct new-type patterns:

use impl_tools::autoimpl;
use std::sync::Arc;

// Impl Foo for &T, &mut T and Arc<T>
#[autoimpl(for<T: trait + ?Sized> &T, &mut T, Arc<T>)]
// Optional: impl Foo for NewFoo (requires NewFoo: Deref<Target = T>)
#[autoimpl(for<T: trait> NewFoo<T>)]
pub trait Foo {
    fn success(&self) -> bool;
}

// Impl Deref and DerefMut to a Target which itself supports Foo
#[autoimpl(Deref<Target = T>, DerefMut using self.0)]
pub struct NewFoo<T: Foo>(T);

// Impl Deref and DerefMut to a Target which itself supports Foo
#[autoimpl(Deref<Target = dyn Foo>, DerefMut using self.0)]
pub struct ArcDynFoo(Arc<dyn Foo>);

#[test]
fn test_foo_newtypes() {
    struct Success;
    impl Foo for Success {
        fn success(&self) -> bool { true }
    }

    // We can now directly call Foo's methods on the wrapper:
    assert!(NewFoo(Success).success());
    assert!(ArcDynFoo(Arc::new(Success)).success());
}

See tests/newtype.rs for more variants of this pattern.

Impl Default

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

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

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

Note: #[impl_default] is matched within an impl_scope! regardless of imports.

Impl Self

#[impl_self] provides impl Self syntax, avoiding the need to repeat generics when writing impls on a local type definition. This supercedes impl_scope! (except regarding macro@impl_default).

use std::fmt::Display;

#[impl_tools::impl_self]
mod NamedThing {
    /// 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())
        }
    }
}

Note that struct NamedThing is defined directly within the outer namespace, not within the mod NamedThing. This is a hack required to ensure the contents use valid Rust syntax and are thus formattable using cargo fmt.

Impl Anon

impl_anon! is a function-like macro to construct a single-use struct with custom implementations (similar: RFC#2604).

Example:

use std::fmt;
fn main() {
    let world = "world";
    let says_hello_world = impl_tools::impl_anon! {
        struct(&'static str = world);
        impl fmt::Display for Self {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                write!(f, "hello {}", self.0)
            }
        }
    };
    assert_eq!(format!("{}", says_hello_world), "hello world");
}

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:

1. 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.
2. To extend #[autoimpl], write an impl of ImplTrait and add it to the attribute's definition. To extend #[impl_self], write an impl of ScopeAttr and add it to the macro's definition.
3. Depend on your new front end crate instead of impl-tools.

For an example of this approach, see kas-macros.

Alternatives

Derive alternatives

Both Educe and Derivative have similar functionality: the ability to implement standard traits with more flexibility than libstd's #[derive].

In comparison, impl-tools' #[autoimpl] has cleaner syntax but is less flexible:

#[derive(Derivative)]
#[derivative(PartialEq, Eq)]
struct Foo<S, T: ?Sized> {
    foo: S,
    #[derivative(PartialEq="ignore")]
    bar: u8,
    #[derivative(PartialEq(bound=""), Eq(bound=""))]
    ptr: *const T,
}

#[derive(Educe)]
#[educe(PartialEq(bound = "S: PartialEq"), Eq(bound = "S: Eq"))]
struct Foo<S, T: ?Sized> {
    foo: S,
    #[educe(PartialEq(ignore))]
    bar: u8,
    ptr: *const T,
}

// impl-tools:
#[autoimpl(PartialEq, Eq ignore self.bar where S: trait)]
struct Foo<S, T: ?Sized> {
    foo: S,
    bar: u8,
    ptr: *const T,
}

Note: #[derive] and Derivative add bounds like S: PartialEq, T: PartialEq on generic parameters by default; Educe and impl-tools do not.

Derive extensions

derive_more isn't exactly an "alternative", simply supporting #[derive] for more standard traits such as Add and From. This is not (currently) supported by #[autoimpl] (or, to my knowledge, any alternative).

auto_impl allows implementing a trait for reference types (&, &mut, Box, Rc, Arc) as well as function types. The former (reference types) is supported by #[autoimpl] (and is slightly more general):

// auto_impl:
#[auto_impl(&, Box)]
trait Foo {
    fn foo(&self);
}

// impl-tools:
#[autoimpl(for<T: trait + ?Sized> &T, Box<T>)]
trait Foo {
    fn foo(&self);
}

derive-where is a variant of the standard #[derive] macro supporting custom generic bounds. (This offers a subset of the functionality of #[autoimpl]).

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