Add in a RtlGenRandom fallback for non-UWP Windows

src/windows.rs

@@ -21,6 +21,14 @@ extern "system" {
     ) -> u32;
 }
 
+// Forbidden when targetting UWP
+#[cfg(not(target_vendor = "uwp"))]
+#[link(name = "advapi32")]
+extern "system" {
+    #[link_name = "SystemFunction036"]
+    fn RtlGenRandom(RandomBuffer: *mut c_void, RandomBufferLength: u32) -> u8;
+}
+
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     // Prevent overflow of u32
     for chunk in dest.chunks_mut(u32::max_value() as usize) {
@@ -35,6 +43,15 @@ pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
         };
         // NTSTATUS codes use the two highest bits for severity status.
         if ret >> 30 == 0b11 {
+            // Failed. Try RtlGenRandom as a fallback.
+            #[cfg(not(target_vendor = "uwp"))]
+            {
+                if unsafe { RtlGenRandom(chunk.as_mut_ptr() as *mut c_void, chunk.len() as u32) }
+                    != 0
+                {
+                    continue;
+                }
+            }
             // We zeroize the highest bit, so the error code will reside
             // inside the range designated for OS codes.
             let code = ret ^ (1 << 31);