Pointer being freed was not allocated

[yuyichao]

@ihnorton

This has been a soft rule-of-thumb for a long time, because struct/immutablewill basically just work like a C struct

For layout, kind of, but in general, it depends. Since immutable are, well, not mutable, it is actually preferable to use mutable struct in this case so that the C library can easily mutate the struct.