Function offset_faces 

pub fn offset_faces(
    model: &mut Model,
    solid: &Shape,
    faces: &[Shape],
    distance: f64,
    tol: Tolerances,
) -> OgeomResult<Built>

Offset some faces of a solid by distance along their outward normals (negative moves them into the material), the faces around them following: a plane moves parallel, a cylinder, cone, sphere or torus changes radius about its own axis or centre, and each neighbour stays on its own surface, its edges re-derived where the moved faces now meet it. The topology is kept one for one, so the history maps every face to the face it became.

Errors

As offset_shape, and additionally if distance moves nothing or a face is not a face of solid. A move that would need a face the solid does not have (a step where a moved face runs past a neighbour) or would collapse one fails by name, as the rebuild’s vertices and edges do.