Impl-tools
=======
[](https://github.com/kas-gui/impl-tools/actions)
[](https://crates.io/crates/impl-tools)
[](https://docs.rs/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](#alternatives), `#[autoimpl]` has minimal and intuitive syntax.
```rust
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:
```rust
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`](https://github.com/kas-gui/impl-tools/blob/master/tests/newtype.rs) for more variants of this pattern.
### Impl Default
`#[impl_default]` implements `std::default::Default`:
```rust
#[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.
### Split Impl
Sometimes a trait is used only (or primarily) to provide an interface over a single object. In such cases, writing out the method prototypes twice (in both the trait and its implementation) should be unnecessary.
Example:
```rust
#[impl_tools::split_impl(for str)]
trait Greet {
/// Introduce yourself
fn greet(&self) {
println!("Hello world, I am {self}!");
}
}
fn main() {
"Ferris".greet();
}
```
### 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`).
```rust
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](https://github.com/rust-lang/rfcs/pull/2604)).
Example:
```rust
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](https://github.com/kas-gui/kas/tree/master/crates/kas-macros).
[`impl-tools-lib`]: https://docs.rs/impl-tools-lib/
[`ImplTrait`]: https://docs.rs/impl-tools-lib/latest/impl_tools_lib/autoimpl/trait.ImplTrait.html
[`ScopeAttr`]: https://docs.rs/impl-tools-lib/latest/impl_tools_lib/trait.ScopeAttr.html
Alternatives
------------
### Derive alternatives
Both [Educe](https://crates.io/crates/educe) and [Derivative](https://crates.io/crates/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:
```rust,ignore
#[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](https://crates.io/crates/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](https://crates.io/crates/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):
```rust,ignore
// 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](https://crates.io/crates/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](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](LICENSE) file or on
the following webpage: <https://www.apache.org/licenses/LICENSE-2.0>