# 🧣 `blanket` [![Star me](https://img.shields.io/github/stars/althonos/blanket.svg?style=social&label=Star&maxAge=3600)](https://github.com/althonos/blanket/stargazers)

*A simple macro to derive blanket implementations for your traits.*

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## 🔍 Overview

The Rust standard library has plenty of traits, but they shine in how well
they integrate with new types. Declare an implementation of
[`std::io::Write`] for
a type `W`, and you also get it for [`&mut W`] and [`Box<W>`]! This however
translates into a [lot of boilerplate code]
that can be hard to maintain, which is why many crates don't bother
providing the same convenience implementations.

[`std::io::Write`]: https://doc.rust-lang.org/std/io/trait.Write.html
[lot of boilerplate code]: https://doc.rust-lang.org/src/std/io/impls.rs.html#49-79
[`&mut W`]: https://doc.rust-lang.org/std/io/trait.Write.html#impl-Write-17
[`Box<W>`]: https://doc.rust-lang.org/std/io/trait.Write.html#impl-Write-19

This is where `blanket` comes in! This crate helps you build the same kind
of blanket implementations for your own traits with as least additional code
as possible: in fact, this is as close as what a `derive` macro would look
like for a `trait` item.

## 🔌 Usage

`blanket` exports a single eponymous attribute macro, which can be imported
simply after the crate has been added to the `Cargo.toml` dependencies:

```rust
extern crate blanket;
use blanket::blanket;
```

### `#[blanket(derive(...))]`

Use this macro attribute to derive a blanket implementation for a trait,
provided the trait methods fit the constraints for that derive, such as
only declaring methods with `&self` of `&mut self` as their receiver.
The following derives are available:

| Derive | Impl block                                 | `fn (&self)` | `fn (&mut self)` | `fn (self)` |
|--------|--------------------------------------------|--------------|------------------|-------------|
| Ref    | `impl<T: Trait + ?Sized> Trait for &T`     | ✔️            |                  |             |
| Rc     | `impl<T: Trait + ?Sized> Trait for Rc<T>`  | ✔️            |                  |             |
| Arc    | `impl<T: Trait + ?Sized> Trait for Arc<T>` | ✔️            |                  |             |
| Mut    | `impl<T: Trait + ?Sized> Trait for &mut T` | ✔️            | ✔️                |             |
| Box    | `impl<T: Trait> Trait for Box<T>`          | ✔️            | ✔️                | ✔️           |

For instance, with our own version of `std::fmt::Write`, we can provide
an implementation for `Box<impl Write>` and `&mut impl Write`:

```rust
extern crate blanket;
use blanket::blanket;

#[blanket(derive(Mut, Box))]
pub trait Write {
    fn write_str(&mut self, s: &str) -> std::fmt::Result;
    fn write_char(&mut self, c: char) -> std::fmt::Result {
         self.write_str(c.encode_utf8(&mut [0; 4]))
    }
}
```

Note that we can't derive `Ref` because the `Write` trait we declared expects
mutable references, which we can't provide from an immutable reference. If we
were to try, the compiler would warn us:

```rustc
---- src/lib.rs - (line 55) stdout ----
error: cannot derive `Ref` for a trait declaring `&mut self` methods
 --> src/lib.rs:61:18
  |
8 |     fn write_str(&mut self, s: &str) -> std::fmt::Result;
  |                  ^^^^^^^^^
```


### `#[blanket(default = "...")]`

`blanket` can delegate default implementations of trait methods to functions
of another module. This can be useful for some traits such as
[visitors](https://github.com/rust-unofficial/patterns/blob/master/patterns/visitor.md)
to provide a default behaviour as an external function, such as what
[`syn::visit`](https://docs.rs/syn/latest/syn/visit/index.html) is doing.

The following example implements a very simple visitor trait for types
able to process a `&str` char-by-char.

```rust
extern crate blanket;
use blanket::blanket;

#[blanket(default = "visitor")]
trait Visitor {
    fn visit_string(&self, s: &str);
    fn visit_char(&self, c: char);
}

mod visitor {
    use super::Visitor;

    pub fn visit_string<V: Visitor + ?Sized>(v: &V, s: &str) {
        for c in s.chars() {
            v.visit_char(c);
        }
    }

    pub fn visit_char<V: Visitor + ?Sized>(v: &V, c: char) {}
}
```

`blanket` will check that all methods are declared without a default block,
and then create a default implementation for all of the declared methods,
generating the following code:

```rust
trait Visitor {
    fn visit_string(&self, s: &str) {
      visitor::visit_string(self, s)
    }
    fn visit_char(&self, c: char) {
      visitor::visit_char(self, c)
    }
}
```

## 📝 To-Do

- ✓ Delegation of default method to external functions.
- ✓ Support for traits with generic arguments.
- ✓ `#[derive(Ref)]`
- ✓ `#[derive(Mut)]`
- ✓ `#[derive(Box)]`
- ✓ `#[derive(Rc)]`
- ✓ `#[derive(Arc)]`
- ✗ Update `Box` derive to allow unsized types if possible.
- ✗ `#[derive(Cow)]`

## 🤝 Credits

`blanket` is developed and maintained by:
- [Martin Larralde](https://github.com/althonos)

The following people contributed to the project:
- [Alexander Linne](https://github.com/alexanderlinne)
- [Naja Melan](https://github.com/najamelan)
- [Justin Lovinger](https://github.com/JustinLovinger)

## 📋 Changelog

This project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html)
and provides a [changelog](https://github.com/althonos/blanket/blob/master/CHANGELOG.md)
in the [Keep a Changelog](http://keepachangelog.com/en/1.0.0/) format.


## 📜 License

This library is provided under the open-source
[MIT license](https://choosealicense.com/licenses/mit/).
