Fragment shader can be silently compiled out

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.

Start by compiling the two Rust fragment-shader examples from the issue and compare their outputs. Trace the compiler path for a shader that produces no artifact, then determine where an error should be reported. Done means the second example fails explicitly instead of being silently compiled out, while the first continues to produce a fragment shader.