Const functions for initializing transforms

[alice-i-cecile]

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

Transforms are common configuration points, but there's no current way to initialize them as constants that are easily referred to and shared.

What solution would you like?

Follow the example set by glam and provide const fn and macros to initialize const Transform structs.

What alternative(s) have you considered?

Currently, I'm just storing these values as local non-const variables within my systems.

You could instead use resources as pseudo-consts?

In heavier games, you could also store this information as part of a scene.

Additional context

Inspired by work in #2094, but out of scope for that.

See bitshifter/glam-rs#76 for more discussion on the challenges involved in doing so, and why they can't just use simple const fn.

For those familiar with Rust's macro system, this should be a simple enough first issue and require very little knowledge of Bevy's internals.

[cart]

Defining const Transforms is already possible using glam's macros. I think this is clearer than most equivalent macro impls as its explicit about each field:

use bevy_math::*;
pub const IDENTITY: Transform = Transform {
    translation: const_vec3!([0.0, 0.0, 0.0]),
    rotation: const_quat!([0.0, 0.0, 0.0, 1.0]),
    scale: const_vec3!([0.0, 0.0, 0.0]),
};

Compare that to some theoretical Transform macro:

pub const IDENTITY: Transform = const_transform!(
    [0.0, 0.0, 0.0],
    [0.0, 0.0, 0.0, 1.0],
    [0.0, 0.0, 0.0],
);

Its less boilerplate, but much harder to tell whats going on.

[alice-i-cecile]

Oh cool! Thanks @cart; I'll close this and add it to the Breakout example :3