points to is pinned.

C code may not keep a copy of a Go pointer after the call returns,
unless the memory it points to is pinned with [runtime.Pinner] and the
Pinner is not unpinned while the Go pointer is stored in C memory.
This implies that C code may not keep a copy of a string, slice,
channel, and so forth, because they cannot be pinned with
[runtime.Pinner].