Efficient flat shading

[eqdiag]

I'm currently writing a tool for manipulating meshes and am using bevy to visualize the meshes. One problem though is it seems like bevy duplicates vertex data for each face that contains a single vertex. For example in https://github.com/bevyengine/bevy/blob/main/crates/bevy_render/src/mesh/shape/mod.rs, a cube is specified with 24 pieces of vertex data instead of just having 6 in combination with a smaller index buffer.

I want to try a different approach with a custom shader that computes the face normal in the fragment shader instead of interpolating it from the vertices.

What's the easiest way to do this right now? I still want PBR shading, I basically just want to compute my surface normals differently.