Feature flag to add repr(c) to bevy types

[ScottKane]

What problem does this solve or what need does it fill?

I am developing bindings to bevy in C# using the new QueryBuilder/SytemBuilder API's. I am at the point where I can spawn entities with componenets and have systems run against them. This currently only works for custom types definied on the C# side as there is no nice way to pass a bevy Transform or math/bundle types over FFI.

What solution would you like?

Add a repr-c feature which would tag relevant structs in components/math/bundles with a repr(c) allowing them to be passed over FFI without duplication/hacky solutions.

Something like:

#[derive(Debug, PartialEq, Clone, Copy)]
#[cfg_attr(feature = "serialize", derive(serde::Serialize, serde::Deserialize))]
#[cfg_attr(
    feature = "bevy-support",
    derive(Component, Reflect),
    reflect(Component, Default, PartialEq)
)]
#[cfg_attr(feature = "repr-c", repr(C))]
pub struct Transform {
    /// Position of the entity. In 2d, the last value of the `Vec3` is used for z-ordering.
    ///
    /// See the [`translations`] example for usage.
    ///
    /// [`translations`]: https://github.com/bevyengine/bevy/blob/latest/examples/transforms/translation.rs
    pub translation: Vec3,
    /// Rotation of the entity.
    ///
    /// See the [`3d_rotation`] example for usage.
    ///
    /// [`3d_rotation`]: https://github.com/bevyengine/bevy/blob/latest/examples/transforms/3d_rotation.rs
    pub rotation: Quat,
    /// Scale of the entity.
    ///
    /// See the [`scale`] example for usage.
    ///
    /// [`scale`]: https://github.com/bevyengine/bevy/blob/latest/examples/transforms/scale.rs
    pub scale: Vec3,
}

What alternative(s) have you considered?

I would either have to duplicate all of the component/math/bundle types into my rust library or devise some way to calculate the struct layout so I can match it from C#.

[ScottKane]

So far the biggest problem I can see with implementing this is that some types that come from glam aren't repr(C) so would need some work to get this feature working. Does anyone have any suggestions?

[ScottKane]

Could be an issue as glam is using simd vector types which right now dont work properly over FFI:

rust-lang/rust#53346