passing a pointer to a field in a struct, the Go memory in question is
the memory occupied by the field, not the entire struct. When passing a
pointer to an element in an array or slice, the Go memory in question is
the entire array or the entire backing array of the slice.

C code may keep a copy of a Go pointer only as long as the memory it
points to is pinned.

C code may not keep a copy of a Go pointer after the call returns,