Suggestion: Use for_ macro instead of manual while loops

[Boscop]

Suggestion: Use this for_ macro instead of manual while loops to make the intention clearer:

pub macro for_($start:expr, $end:expr, $(step: $step:expr,)? $($label:lifetime:)? |$var:pat| $body:block) {
	{
		let _step = if false { $start } else { 1 as _ }; // type inference for default step
		$(
			let _step = $step; // custom step
		)?
		let mut i = $start;
		$($label:)? while i < $end {
			let $var = i;
			i += _step; // before body, to make `continue` work
			$body;
		}
	}
}

You can use it like:

fn ray_march(ro: Vec3, rd: Vec3) -> f32 {
	let mut dist_from_origin = 0.;

	for_!(0, MAX_STEPS, |_| {
		let here = ro + rd * dist_from_origin;
		let dist_from_here = scene_dist(here);
		dist_from_origin += dist_from_here;
		if dist_from_origin > MAX_DIST || dist_from_here < SURF_DIST {
			break;
		}
	});

	dist_from_origin
}

It also works for floats and supports a custom step and label arg, e.g.:

fn ray_march(ro: Vec3, rd: Vec3) -> f32 {
	let mut dist_from_origin = 0.;

	for_!(0., MAX_STEPS as f32, step: 1., 'for_: |_| {
		let here = ro + rd * dist_from_origin;
		let dist_from_here = scene_dist(here);
		dist_from_origin += dist_from_here;
		if dist_from_origin > MAX_DIST || dist_from_here < SURF_DIST {
			break 'for_;
		}
	});

	dist_from_origin
}

I think it makes the purpose clearer (no need to declare an extra variable just for looping), it makes looping "automatic" (no need to remember to put i += 1; at the end) and reduces the number of mutable vars, thus making it easier to reason about the code / makes the purpose clearer.