bevyengine/bevy

bevy_derive

Small, general-purpose derive macros that aren't tied to a specific subsystem.

Purpose

Three derives that show up across the workspace:

#[derive(Deref)] — implement Deref to the first field of a tuple struct.
#[derive(DerefMut)] — implement DerefMut similarly.
#[derive(EnumVariantMeta)] — generate a method that returns variant indices/names.

Plus a few smaller helpers used by Bevy's macros internally.

Directory layout

crates/bevy_derive/src/
├── lib.rs                # Re-exports
├── derefs.rs             # Deref / DerefMut
├── enum_variant_meta.rs  # EnumVariantMeta
└── bevy_main.rs          # The #[bevy_main] attribute (Android entry point)

Key abstractions

Macro	File	Description
`Deref`	`crates/bevy_derive/src/derefs.rs`	Auto-impl `Deref` for a newtype wrapping a single field.
`DerefMut`	`crates/bevy_derive/src/derefs.rs`	Auto-impl `DerefMut` similarly.
`EnumVariantMeta`	`crates/bevy_derive/src/enum_variant_meta.rs`	Adds `enum_variant_index` / `enum_variant_name` methods.
`#[bevy_main]`	`crates/bevy_derive/src/bevy_main.rs`	Attribute on `main` to register the Android entry point.

How it's used

#[derive(Deref, DerefMut)]
struct Velocity(Vec3);

let v = Velocity(Vec3::ZERO);
let _len = v.length(); // Vec3::length, accessed through Deref

Most Bevy newtypes use this pattern — RenderLayers, Time<Fixed>, many resource types. It avoids hand-writing Deref/DerefMut impls and keeps the wrapped type accessible.

Integration points

Depends on: proc-macro2, quote, syn, bevy_macro_utils.
Depended on by: Many crates that wrap a single field. Re-exported through bevy::prelude::*.

Entry points for modification

The crate is small; it is unlikely to need many changes. New general-purpose derives can be added here.
Compile-fail tests under crates/bevy_derive/compile_fail/ lock in error messages — update them when changing the macros.