derivative 0.3.0

A set of alternative `derive` attributes for Rust
Documentation 
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# Custom attributes
The `Hash` trait supports the following attributes:

* **Container attributes**
    * [`Hash(bound="<where-clause or empty>")`](#custom-bound)
* **Field attributes**
    * [`Hash(bound="<where-clause or empty>")`](#custom-bound)
    * [`Hash(format_with="<path>")`](#hash-with)
    * [`Hash="ignore"`](#ignoring-a-field)

# Ignoring a field

You can use *derivative* to ignore fields from a `Hash` implementation:

```rust
#[derive(Derivative)]
#[derivative(Hash)]
struct Foo {
    foo: u8,
    #[derivative(Hash="ignore")]
    bar: i32,
}

#[derive(Hash)]
struct Bar {
    foo: u8,
}

assert_eq!(hash(Foo { foo: 42, bar: -1337 }), hash(Bar { foo: 42 }));
```

# Hash with

You can pass a field to a hash function:

```rust
#[derive(Derivative)]
#[derivative(Hash)]
struct Foo {
    foo: u32,
    #[derivative(Hash(hash_with="path::to::my_hash_fn"))]
    bar: SomeTypeThatMightNotBeHash,
}
```

The field `bar` will be hashed with `path::to::my_hash_fn(&bar, &mut state)`
where `state` is the current [`Hasher`].

The function must the following prototype:

```rust
fn my_hash_fn<H>(&T, state: &mut H) where H: Hasher;
```

# Limitations

On structure, `derivative(Hash)` will produce the same hash as `derive(Hash)`.
On unions however, it will produces the same hashes *only for unitary
variants*!

# Custom bound
As most other traits, `Hash` supports a custom bound on container and fields.
See [`Debug`'s documentation](Debug.md#custom-bound) for more information.

[`Hasher`]: https://doc.rust-lang.org/std/hash/trait.Hasher.html