Add testcase for issue rust-lang#729 (rust-lang#730)

Add a fixme testcase that can be used to reproduce the issue rust-lang#729.

src/test/rmc/Invariants/fixme_niche_opt.rs

@@ -0,0 +1,37 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+/// This testcase is currently failing with the following error:
+///
+/// [assertion.2] Reached assignment statement with unequal types Pointer { typ:
+/// StructTag("tag-Unit") } Pointer { typ: StructTag("tag-_3943305294634710273") }: FAILURE
+///
+/// If you run:
+/// ```
+/// RUSTFLAGS="--cfg=ok" rmc fixme_niche_opt.rs
+/// ```
+/// This test will succeed.
+///
+/// Issue: https://github.com/model-checking/rmc/issues/729
+
+enum Error {
+    Error1,
+    Error2,
+}
+
+/// This version fails.
+#[cfg(not(ok))]
+fn to_option<T: Copy, E>(result: &Result<T, E>) -> Option<T> {
+    if let Ok(v) = result { Some(*v) } else { None }
+}
+
+/// This version succeeds.
+#[cfg(ok)]
+fn to_option<T: Copy, E>(result: &Result<T, E>) -> Option<T> {
+    if let Ok(v) = *result { Some(v) } else { None }
+}
+
+fn main() {
+    let result: Result<(), Error> = Ok(());
+    assert!(to_option(&result).is_some());
+}