[runtime.Pinner].

The _GoString_ type also may not be pinned with [runtime.Pinner].
Because it includes a Go pointer, the memory it points to is only pinned
for the duration of the call; _GoString_ values may not be retained by C
code.

A Go function called by C code may return a Go pointer to pinned memory
(which implies that it may not return a string, slice, channel, and so
forth). A Go function called by C code may take C pointers as arguments,