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.

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

[autoimpl(for<'a, T: trait + ?Sized> Box)]

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

[autoimpl(Debug ignore self.animal where T: Debug)]

// Generates: impl std::fmt::Debug for Named where T: Debug { .. }

[autoimpl(Deref, DerefMut using self.animal)]

// Generates: impl std::ops::Deref for Named { .. } // Generates: impl std::ops::DerefMut for Named { .. } struct Named { name: T, animal: A, }

fn main() { struct Fish; impl Animal for Fish { fn numberoflegs(&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:

```rust use impltools::{impldefault, impl_scope};

[impl_default(Tree::Ash)]

enum Tree { Ash, Beech, Birch, Willow }

implscope! { #[impldefault] 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:

```rust use impltools::implscope; use std::fmt::Display;

impl_scope! { /// I don't know why this exists pub struct NamedThing { 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