# Objective
- Follow-up of #13184 :)
- We use `ui_test` to test compiler errors for our custom macros.
- There are four crates related to compile fail tests
- `bevy_ecs_compile_fail_tests`, `bevy_macros_compile_fail_tests`, and
`bevy_reflect_compile_fail_tests`, which actually test the macros.
-
[`bevy_compile_test_utils`](64c1c65783/crates/bevy_compile_test_utils),
which provides helpers and common patterns for these tests.
- All of these crates reside within the `crates` directory.
- This can be confusing, especially for newcomers. All of the other
folders in `crates` are actual published libraries, except for these 4.
## Solution
- Move all compile fail tests to a `compile_fail` folder under their
corresponding crate.
- E.g. `crates/bevy_ecs_compile_fail_tests` would be moved to
`crates/bevy_ecs/compile_fail`.
- Move `bevy_compile_test_utils` to `tools/compile_fail_utils`.
There are a few benefits to this approach:
1. An internal testing detail is less intrusive (and confusing) for
those who just want to browse the public Bevy interface.
2. Follows a pre-existing approach of organizing related crates inside a
larger crate's folder.
- See `bevy_gizmos/macros` for an example.
4. Makes consistent the terms `compile_test`, `compile_fail`, and
`compile_fail_test` in code. It's all just `compile_fail` now, because
we are specifically testing the error messages on compiler failures.
- To be clear it can still be referred to by these terms in comments and
speech, just the names of the crates and the CI command are now
consistent.
## Testing
Run the compile fail CI command:
```shell
cargo run -p ci -- compile-fail
```
If it still passes, then my refactor was successful.
# 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
