Flexible Array Member convenience method

[marler8997]

Win32 will sometimes use a "flexible array member" to copy data to/from the application. The GetInterfaceInfo function is an example:

https://docs.microsoft.com/en-us/windows/win32/api/iphlpapi/nf-iphlpapi-getinterfaceinfo

//...

    dwRetVal = GetInterfaceInfo(NULL, &ulOutBufLen);
    if (dwRetVal == ERROR_INSUFFICIENT_BUFFER) {
        pInfo = (IP_INTERFACE_INFO *) MALLOC(ulOutBufLen);
        if (pInfo == NULL) {
            printf
                ("Unable to allocate memory needed to call GetInterfaceInfo\n");
            return 1;
        }
    }

// Make a second call to GetInterfaceInfo to get
// the actual data we need
    dwRetVal = GetInterfaceInfo(pInfo, &ulOutBufLen);
    if (dwRetVal == NO_ERROR) {
        printf("Number of Adapters: %ld\n\n", pInfo->NumAdapters);
        for (i = 0; i < pInfo->NumAdapters; i++) {
            printf("Adapter Index[%d]: %ld\n", i,
                   pInfo->Adapter[i].Index);
            printf("Adapter Name[%d]: %ws\n\n", i,
                   pInfo->Adapter[i].Name);
        }
//...

Zig could make this easier to do with a convenience method, i.e.

pub const IP_INTERFACE_INFO = extern struct {
    NumAdapters: i32,
    Adapter: [1]IP_ADAPTER_INDEX_MAP,
    pub fn AdapterSlice(self: *IP_INTERFACE_INFO) []IP_ADAPTER_INDEX_MAP {
        return @ptrCast([*]IP_ADAPTER_INDEX_MAP, &self.Adapter)[0 .. self.NumAdapters];
    }
};

The tricky part here would be knowing that NumAdapters is the length of the slice, but I think that info is available in the metadata.