Function build 

pub unsafe fn build(
    slot: &'static mut Slot,
    property: Property<'_>,
    thunks: (*const c_void, *const c_void, *mut *const c_void, *mut *const c_void),
) -> u64

Fill slot in and hand back a WritableKeyPath for property.

The returned object is retained for the process’s lifetime: SwiftUI keeps the key path inside whatever modifier it was handed to.

Safety

slot must be a static mut in this image, used by one property only, and thunks must be the pair below whose target words this call also sets.