All of the vector operations return new vectors instead of mutating.

    func v3_add (a, b v3) v3 {
     return v3{x: a.x + b.x, y: a.y + b.y, z: a.z + b.z}
    }

A sensible approach for maintainable code but without knowing if a and b are used elsewhere the compiler can't reuse the space they occupy. If C can't figure it out, it can just stick a new one on the stack which doesn't cost too much.

This is, of course, Rust's bread and butter which is probably why it takes the top spot.

Those are copied on the stack, not heap allocated, so the GC wouldn't come into play.