fix(servo): gate vulnerable RSA private operations

third_party/rsa/src/dummy_rng.rs

@@ -0,0 +1,24 @@
+use core::convert::Infallible;
+use rand_core::{TryCryptoRng, TryRng};
+
+/// This is a dummy RNG for cases when we need a concrete RNG type
+/// which does not get used.
+#[derive(Copy, Clone)]
+pub(crate) struct DummyRng;
+
+impl TryRng for DummyRng {
+    type Error = Infallible;
+    fn try_next_u32(&mut self) -> Result<u32, Self::Error> {
+        unimplemented!();
+    }
+
+    fn try_next_u64(&mut self) -> Result<u64, Self::Error> {
+        unimplemented!();
+    }
+
+    fn try_fill_bytes(&mut self, _: &mut [u8]) -> Result<(), Self::Error> {
+        unimplemented!();
+    }
+}
+
+impl TryCryptoRng for DummyRng {}