I have this code below in both my vertex and fragment shader, however when I request glGetUniformLocation("Lights[0].diffuse") or "Lights[0].attenuation", it returns -1. It will only give me a valid uniform location if I actually use the diffuse/attenuation variables in the VERTEX shader. Because I use position in the vertex shader, it always returns a valid uniform location. I've read that I can share uniforms across both vertex and fragment, but I'm confused what this is even compiling to if this is the case.

#define NUM_LIGHTS 2

struct Light
{
    vec3 position;
    vec3 diffuse;
    float attenuation;
};
uniform Light Lights[NUM_LIGHTS];

That's correct. Using it in the fragment shader seems to not effect anything. Copy/pasted the structure declarations, they are identical.

I must have been smoking crack. It all works now with no changes. I think I was editing the wrong fragment shader for the vertex shader. And therefore it was bad testing.