From/Into traits and mem::transmute

[raizam]

I need to convert a Vec3 type to another Vec3 from a different library.
They have the exact same memory layout, and I have written this:

impl From<cgmath::Point3<FLOAT>> for Vec3 {
    fn from(val: cgmath::Point3<FLOAT>) -> Self {
        use std::mem;
        unsafe { mem::transmute::<cgmath::Point3<FLOAT>, Vec3>(val) }
    }
}

I'd like a zero cost conversion, is this the right approach?

[Hyeonu]

No, this code is unsound.

Unless both types are #[repr(C)], Rust DOES NOT guarantee the two different types with same type of fields in same order on definition to have same memory layout.

[alice]

This should compile down to the most efficient version anyway, so if Rust happens to not decide to reorder the fields, it should be equivalent to the transmute.

impl From<cgmath::Point3<FLOAT>> for Vec3 {
    fn from(val: cgmath::Point3<FLOAT>) -> Self {
        Vec3 { x: val.x, y: val.y, z: val.z }
    }
}