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Gino Valente 56686a8962
bevy_derive: Add #[deref] attribute (#8552) 
# 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
2023-05-16 18:29:09 +00:00
..
src bevy_derive: Add #[deref] attribute (#8552) 2023-05-16 18:29:09 +00:00
tests bevy_derive: Add #[deref] attribute (#8552) 2023-05-16 18:29:09 +00:00
Cargo.toml bevy_derive: Add #[deref] attribute (#8552) 2023-05-16 18:29:09 +00:00
README.md bevy_derive: Add #[deref] attribute (#8552) 2023-05-16 18:29:09 +00:00

README.md

Compile fail tests for Bevy macros

This crate is not part of the Bevy workspace in order to not fail crater tests for Bevy. The tests assert on the exact compiler errors and can easily fail for new Rust versions due to updated compiler errors (e.g. changes in spans).

The CI workflow executes these tests on the stable rust toolchain (see tools/ci).