With file and stream operations it is common to use uint8[] as a buffer. For example see GLib.FileStream and GIO's InputStream and OutputStream.

The following code works:

struct Foo {
    public double x;
    public double y;
    public double z;
}

void main () {
    Foo bar = { 200.0, 3.0, 10.1 };
    print ("Field from original struct, bar.x: %f\n", bar.x);
    unowned uint8[] buffer = (uint8[])&bar;
    unowned Foo[] result = (Foo[]) buffer;
    print ("Value after struct buffered and unbuffered: %f\n", (result[0].x));
}

but the line:

unowned uint8[] buffer = (uint8[])&bar;

is not very Vala like because of the use of the address-of operator. A more Vala like syntax would be:

unowned uint8[] buffer = (uint8[])bar;

This change line currently produces an error from the C compiler: cannot convert to a pointer type
  buffer = (guint8*) _tmp3_;

Vala should either produce an error that a value type cannot be cast to an array without the address-of operator or cast the value type properly in the resulting C code.