Add a trait conversion test

Author: fjarri

rand_core/src/lib.rs

@@ -639,4 +639,37 @@ mod test {
         // value-breakage test:
         assert_eq!(results[0], 5029875928683246316);
     }
+
+    #[test]
+    fn dyn_trait_conversion() {
+        // Illustrates the need for `+ ?Sized` bound in `impl<R: RngCore> TryRngCore for R`.
+
+        // A method in another crate taking a fallible RNG
+        fn third_party_api(_rng: &mut (impl TryRngCore + ?Sized)) -> bool {
+            true
+        }
+
+        // A method in our crate requiring an infallible RNG
+        fn my_api(rng: &mut dyn RngCore) -> bool {
+            // We want to call the method above
+            third_party_api(rng)
+        }
+
+        // A stub RNG.
+        struct SomeRng;
+
+        impl RngCore for SomeRng {
+            fn next_u32(&mut self) -> u32 {
+                unimplemented!()
+            }
+            fn next_u64(&mut self) -> u64 {
+                unimplemented!()
+            }
+            fn fill_bytes(&mut self, _: &mut [u8]) {
+                unimplemented!()
+            }
+        }
+
+        assert!(my_api(&mut SomeRng));
+    }
 }