How to write a Raku declaration for a C function returning a whole struct?

For that you need a CStruct. The P5localtime module contains a more elaborate example.

The problem

Some C APIs work with structs using a three-phase approach, passing around structs by reference, like this:

struct mystruct *init_mystruct(arguments, ...);
double compute(struct mystruct *);
void clean_mystruct(struct mystruct *);

This way the implementation hides the data structure, but this comes with a price: the final users have to keep track of their pointers and remember to clean up after themselves, or the program will leak memory.
Another approach is the one that the library I was interfacing used: return the data on the stack, so it can be assigned to an auto variable and automatically discarded when it goes out of scope.
In this case the API is modeled as a two-phase operation:

struct mystruct init_mystruct(arguments, ...);
double compute(struct mystruct);

The structure is passed on the stack, by value and there's no need to clean up afterwards.
But Raku's NativeCall interface is only able to use C structs passing them by reference, hence the problem.

The solution

This is not a clean solution, because it steps back into the first approach described, the three-phase one, but it's the only one I have been able to devise so far.
Here I consider two C functions from the library's API: the first creates a complex number as a struct, the second adds up two numbers.
First I wrote a tiny C code interface, the file complex.c:

#include <gsl/gsl_complex.h>
#include <gsl/gsl_complex_math.h>
#include <stdlib.h>

gsl_complex *alloc_gsl_complex(void)
{
  gsl_complex *c = malloc(sizeof(gsl_complex));
  return c;
}

void free_gsl_complex(gsl_complex *c)
{
  free(c);
}

void mgsl_complex_rect(double x, double y, gsl_complex *res)
{
  gsl_complex ret = gsl_complex_rect(x, y);
  *res = ret;
}

void mgsl_complex_add(gsl_complex *a, gsl_complex *b, gsl_complex *res)
{
  *res = gsl_complex_add(*a, *b);
}

I compiled it this way:
gcc -c -fPIC complex.c
gcc -shared -o libcomplex.so complex.o -lgsl
Note the final -lgsl used to link the libgsl C library I am interfacing to.
Then I wrote the Raku low-level interface:

#!/usr/bin/env raku

use NativeCall;

constant LIB  = ('/mydir/libcomplex.so');

class gsl_complex is repr('CStruct') {
  HAS num64 @.dat[2] is CArray;
}

sub mgsl_complex_rect(num64 $x, num64 $y, gsl_complex $c) is native(LIB) { * }
sub mgsl_complex_add(gsl_complex $a, gsl_complex $b, gsl_complex $res) is native(LIB) { * }
sub alloc_gsl_complex(--> gsl_complex) is native(LIB) { * }
sub free_gsl_complex(gsl_complex $c) is native(LIB) { * }

my gsl_complex $c1 = alloc_gsl_complex;
mgsl_complex_rect(1e0, 2e0, $c1);
say "{$c1.dat[0], $c1.dat[1]}";          # output: 1 2
my gsl_complex $c2 = alloc_gsl_complex;
mgsl_complex_rect(1e0, 2e0, $c2);
say "{$c2.dat[0], $c2.dat[1]}";          # output: 1 2
my gsl_complex $res = alloc_gsl_complex;
mgsl_complex_add($c1, $c2, $res);
say "{$res.dat[0], $res.dat[1]}";        # output: 2 4
free_gsl_complex($c1);
free_gsl_complex($c2);
free_gsl_complex($res);

Note that I had to free explicitly the three data structures I created, spoiling the original C API careful design.