bevy/crates/bevy_render/src/render_resource/resource_macros.rs

// structs containing wgpu types take a long time to compile. this is particularly bad for generic
// structs containing wgpu structs. we avoid that in debug builds (and for cargo check and rust analyzer)
// by type-erasing with the `render_resource_wrapper` macro. The resulting type behaves like Arc<$wgpu_type>,
// but avoids explicitly storing an Arc<$wgpu_type> member.
// analysis from https://github.com/bevyengine/bevy/pull/5950#issuecomment-1243473071 indicates this is
// due to `evaluate_obligations`. we should check if this can be removed after a fix lands for
// https://github.com/rust-lang/rust/issues/99188 (and after other `evaluate_obligations`-related changes).
#[cfg(debug_assertions)]
#[macro_export]
macro_rules! render_resource_wrapper {
    ($wrapper_type:ident, $wgpu_type:ty) => {
        #[cfg(not(all(target_arch = "wasm32", target_feature = "atomics")))]
        #[derive(Debug)]
        // SAFETY: while self is live, self.0 comes from `into_raw` of an Arc<$wgpu_type> with a strong ref.
        pub struct $wrapper_type(*const ());

        #[cfg(all(target_arch = "wasm32", target_feature = "atomics"))]
        #[derive(Debug)]
        pub struct $wrapper_type(send_wrapper::SendWrapper<*const ()>);

        impl $wrapper_type {
            pub fn new(value: $wgpu_type) -> Self {
                let arc = alloc::sync::Arc::new(value);
                let value_ptr = alloc::sync::Arc::into_raw(arc);
                let unit_ptr = value_ptr.cast::<()>();

                #[cfg(not(all(target_arch = "wasm32", target_feature = "atomics")))]
                return Self(unit_ptr);
                #[cfg(all(target_arch = "wasm32", target_feature = "atomics"))]
                return Self(send_wrapper::SendWrapper::new(unit_ptr));
            }

            pub fn try_unwrap(self) -> Option<$wgpu_type> {
                let value_ptr = self.0.cast::<$wgpu_type>();
                // SAFETY: pointer refers to a valid Arc, and was created from Arc::into_raw.
                let arc = unsafe { alloc::sync::Arc::from_raw(value_ptr) };

                // we forget ourselves here since the reconstructed arc will be dropped/decremented within this scope
                core::mem::forget(self);

                alloc::sync::Arc::try_unwrap(arc).ok()
            }
        }

        impl core::ops::Deref for $wrapper_type {
            type Target = $wgpu_type;

            fn deref(&self) -> &Self::Target {
                let value_ptr = self.0.cast::<$wgpu_type>();
                // SAFETY: the arc lives for 'self, so the ref lives for 'self
                let value_ref = unsafe { value_ptr.as_ref() };
                value_ref.unwrap()
            }
        }

        impl Drop for $wrapper_type {
            fn drop(&mut self) {
                let value_ptr = self.0.cast::<$wgpu_type>();
                // SAFETY: pointer refers to a valid Arc, and was created from Arc::into_raw.
                // this reconstructed arc is dropped/decremented within this scope.
                unsafe { alloc::sync::Arc::from_raw(value_ptr) };
            }
        }

        #[cfg(not(all(target_arch = "wasm32", target_feature = "atomics")))]
        // SAFETY: We manually implement Send and Sync, which is valid for Arc<T> when T: Send + Sync.
        // We ensure correctness by checking that $wgpu_type does implement Send and Sync.
        // If in future there is a case where a wrapper is required for a non-send/sync type
        // we can implement a macro variant that omits these manual Send + Sync impls
        unsafe impl Send for $wrapper_type {}
        #[cfg(not(all(target_arch = "wasm32", target_feature = "atomics")))]
        // SAFETY: As explained above, we ensure correctness by checking that $wgpu_type implements Send and Sync.
        unsafe impl Sync for $wrapper_type {}
        #[cfg(not(all(target_arch = "wasm32", target_feature = "atomics")))]
        const _: () = {
            trait AssertSendSyncBound: Send + Sync {}
            impl AssertSendSyncBound for $wgpu_type {}
        };

        impl Clone for $wrapper_type {
            fn clone(&self) -> Self {
                let value_ptr = self.0.cast::<$wgpu_type>();
                // SAFETY: pointer refers to a valid Arc, and was created from Arc::into_raw.
                let arc = unsafe { alloc::sync::Arc::from_raw(value_ptr.cast::<$wgpu_type>()) };
                let cloned = alloc::sync::Arc::clone(&arc);
                // we forget the reconstructed Arc to avoid decrementing the ref counter, as self is still live.
                core::mem::forget(arc);
                let cloned_value_ptr = alloc::sync::Arc::into_raw(cloned);
                let cloned_unit_ptr = cloned_value_ptr.cast::<()>();

                #[cfg(not(all(target_arch = "wasm32", target_feature = "atomics")))]
                return Self(cloned_unit_ptr);

                // Note: this implementation means that this Clone will panic
                // when called off the wgpu thread.
                #[cfg(all(target_arch = "wasm32", target_feature = "atomics"))]
                return Self(send_wrapper::SendWrapper::new(cloned_unit_ptr));
            }
        }
    };
}

#[cfg(not(debug_assertions))]
#[macro_export]
macro_rules! render_resource_wrapper {
    ($wrapper_type:ident, $wgpu_type:ty) => {
        #[cfg(not(all(target_arch = "wasm32", target_feature = "atomics")))]
        #[derive(Clone, Debug)]
        pub struct $wrapper_type(std::sync::Arc<$wgpu_type>);
        #[cfg(all(target_arch = "wasm32", target_feature = "atomics"))]
        #[derive(Clone, Debug)]
        pub struct $wrapper_type(std::sync::Arc<send_wrapper::SendWrapper<$wgpu_type>>);

        impl $wrapper_type {
            pub fn new(value: $wgpu_type) -> Self {
                #[cfg(not(all(target_arch = "wasm32", target_feature = "atomics")))]
                return Self(std::sync::Arc::new(value));

                #[cfg(all(target_arch = "wasm32", target_feature = "atomics"))]
                return Self(std::sync::Arc::new(send_wrapper::SendWrapper::new(value)));
            }

            pub fn try_unwrap(self) -> Option<$wgpu_type> {
                #[cfg(not(all(target_arch = "wasm32", target_feature = "atomics")))]
                return std::sync::Arc::try_unwrap(self.0).ok();

                #[cfg(all(target_arch = "wasm32", target_feature = "atomics"))]
                return std::sync::Arc::try_unwrap(self.0).ok().map(|p| p.take());
            }
        }

        impl core::ops::Deref for $wrapper_type {
            type Target = $wgpu_type;

            fn deref(&self) -> &Self::Target {
                self.0.as_ref()
            }
        }

        #[cfg(not(all(target_arch = "wasm32", target_feature = "atomics")))]
        const _: () = {
            trait AssertSendSyncBound: Send + Sync {}
            impl AssertSendSyncBound for $wgpu_type {}
        };
    };
}

#[macro_export]
macro_rules! define_atomic_id {
    ($atomic_id_type:ident) => {
        #[derive(Copy, Clone, Hash, Eq, PartialEq, PartialOrd, Ord, Debug)]
        pub struct $atomic_id_type(core::num::NonZero<u32>);

        // We use new instead of default to indicate that each ID created will be unique.
        #[allow(clippy::new_without_default)]
        impl $atomic_id_type {
            pub fn new() -> Self {
                use core::sync::atomic::{AtomicU32, Ordering};

                static COUNTER: AtomicU32 = AtomicU32::new(1);

                let counter = COUNTER.fetch_add(1, Ordering::Relaxed);
                Self(core::num::NonZero::<u32>::new(counter).unwrap_or_else(|| {
                    panic!(
                        "The system ran out of unique `{}`s.",
                        stringify!($atomic_id_type)
                    );
                }))
            }
        }

        impl From<$atomic_id_type> for core::num::NonZero<u32> {
            fn from(value: $atomic_id_type) -> Self {
                value.0
            }
        }

        impl From<core::num::NonZero<u32>> for $atomic_id_type {
            fn from(value: core::num::NonZero<u32>) -> Self {
                Self(value)
            }
        }
    };
}

pub use render_resource_wrapper;