Fragment shader can be silently compiled out

[LegNeato]

This works fine:

#[spirv(fragment)]
pub fn main_fs(
 in_color: Vec3,
 out_frag_color: &mut Vec4,
) {
 *out_frag_color = vec4(in_color.x, in_color.y, in_color.z, 1.0);
}

This silently does not output a fragment shader:

#[spirv(fragment)]
pub fn main_fs(
 in_color: Vec3,
 out_frag_color: &mut Vec4,
) {
 out_frag_color.x = in_color.x;
 out_frag_color.y = in_color.y;
 out_frag_color.z = in_color.z;
}

Rather than failing silently, it should probably be an error when an entire shader is dead code / compiled out and does not produce an artifact.

[Firestar99]

Another shader that does not emit anything, potentially for very different reasons:

#[spirv(fragment)]
pub fn main_fs(
 in_color: Vec4,
 out_frag_color: &mut Vec4,
) {
 *out_frag_color = in_color;
}

It wouldn't surprise me if that's rustc optimizing out the function before the codegen backend get's to see it.

[dphfox]

Encountered this myself. For any future people who stumble upon this, you can get around this by consuming frag_coord or something else bogus like that.