Compile-time checks to ensure structs conform to WGSL's memory layout rules

The core of this crate is the [shader_layout], [shader_layout_compat] macros and the [ShaderLayout], [ShaderLayoutCompat] traits.

ShaderLayout is corresponding to https://www.w3.org/TR/WGSL/#alignment-and-size.

ShaderLayoutCompat is a subset of ShaderLayout and is corresponding to address space layout constraints of uniform without uniform_buffer_standard_layout language extension.

use const_shader_layout::{shader_layout, shader_layout_compat, ShaderLayout, ShaderLayoutCompat};
use glam::{Vec2, Vec3, Vec4};

shader_layout! {
    pub struct MyStorage {
        a1: f32,
        a2: [f32; 2],
        a3: [f32; 1],
        a4: Vec3,
        a5: f32,
        a6: Vec3,
        p1: f32, // Padding needed otherwise struct size (44) won't match shader size (48).
    }
}
const {
    assert!(<MyStorage as ShaderLayout>::ALIGN.get() == 16);
    assert!(size_of::<MyStorage>() == 48);
}

shader_layout_compat! {
    pub struct Nested {
        a1: [Vec4; 2],
        a2: Vec3,
        a3: f32
    }
}
shader_layout_compat! {
    pub struct MyUniform {
        a1: Nested,
        a2: Vec3,
        // Padding is not needed, because struct size (64) aligned to 16 in `repr(C)`, so it matches shader size (64).
    }
}
const {
    assert!(<Nested as ShaderLayoutCompat>::ALIGN_COMPAT.get() == 16);
    assert!(<Nested as ShaderLayoutCompat>::SIZE_COMPAT.get() == 48);
    assert!(size_of::<Nested>() == 48);
    assert!(<MyUniform as ShaderLayoutCompat>::ALIGN_COMPAT.get() == 16);
    assert!(<MyUniform as ShaderLayoutCompat>::SIZE_COMPAT.get() == 64);
    assert!(size_of::<MyUniform>() == 64);
}

See https://github.com/beicause/const_shader_layout/tree/master/tests for what this supports and checks.

This doesn't provide any byte conversion methods. Instead, it's intended to be used with other libraries such as bytemuck.