GTY GC question

[Andrew MacLeod <amacleod at redhat dot com>]

So I've got a structure which needs to be garbage collected, and it
contains a pointer to a chunk of memory which is *not* to be garbage
collected. like so:

typedef struct X 
{
   int num;
   tree **vec;
} X_t;

and this structure is used inside inside a garbage colected structure:

struct P GTY(())
{
  X_t x;
  X_t y;
}

gengtype complains that it doesn't know about X_t, so I change it to:

typedef struct X GTY(())
{
   int num;
   tree **vec;
} X_t;

then it complains that vec is of an "unimplemented type".

Now, vec points to memory which is managed in a different way. It won't
get lost, but I dont want the garbage collector to do any more than
simply collect the P structure, never touching what vec points to.
struct P is the only hunk of memory which is actually GC allocaed. How
do I do that?

Andrew