# Objective
Bevy code tends to make heavy use of the [newtype](
https://doc.rust-lang.org/rust-by-example/generics/new_types.html)
pattern, which is why we have a dedicated derive for
[`Deref`](https://doc.rust-lang.org/std/ops/trait.Deref.html) and
[`DerefMut`](https://doc.rust-lang.org/std/ops/trait.DerefMut.html).
This derive works for any struct with a single field:
```rust
#[derive(Component, Deref, DerefMut)]
struct MyNewtype(usize);
```
One reason for the single-field limitation is to prevent confusion and
footguns related that would arise from allowing multi-field structs:
<table align="center">
<tr>
<th colspan="2">
Similar structs, different derefs
</th>
</tr>
<tr>
<td>
```rust
#[derive(Deref, DerefMut)]
struct MyStruct {
foo: usize, // <- Derefs usize
bar: String,
}
```
</td>
<td>
```rust
#[derive(Deref, DerefMut)]
struct MyStruct {
bar: String, // <- Derefs String
foo: usize,
}
```
</td>
</tr>
<tr>
<th colspan="2">
Why `.1`?
</th>
</tr>
<tr>
<td colspan="2">
```rust
#[derive(Deref, DerefMut)]
struct MyStruct(Vec<usize>, Vec<f32>);
let mut foo = MyStruct(vec![123], vec![1.23]);
// Why can we skip the `.0` here?
foo.push(456);
// But not here?
foo.1.push(4.56);
```
</td>
</tr>
</table>
However, there are certainly cases where it's useful to allow for
structs with multiple fields. Such as for structs with one "real" field
and one `PhantomData` to allow for generics:
```rust
#[derive(Deref, DerefMut)]
struct MyStruct<T>(
// We want use this field for the `Deref`/`DerefMut` impls
String,
// But we need this field so that we can make this struct generic
PhantomData<T>
);
// ERROR: Deref can only be derived for structs with a single field
// ERROR: DerefMut can only be derived for structs with a single field
```
Additionally, the possible confusion and footguns are mainly an issue
for newer Rust/Bevy users. Those familiar with `Deref` and `DerefMut`
understand what adding the derive really means and can anticipate its
behavior.
## Solution
Allow users to opt into multi-field `Deref`/`DerefMut` derives using a
`#[deref]` attribute:
```rust
#[derive(Deref, DerefMut)]
struct MyStruct<T>(
// Use this field for the `Deref`/`DerefMut` impls
#[deref] String,
// We can freely include any other field without a compile error
PhantomData<T>
);
```
This prevents the footgun pointed out in the first issue described in
the previous section, but it still leaves the possible confusion
surrounding `.0`-vs-`.#`. However, the idea is that by making this
behavior explicit with an attribute, users will be more aware of it and
can adapt appropriately.
---
## Changelog
- Added `#[deref]` attribute to `Deref` and `DerefMut` derives
