1 files changed, 52 insertions, 0 deletions
diff --git a/src/test/ui/transmutability/visibility/should_reject_if_dst_has_tricky_unreachable_field.rs b/src/test/ui/transmutability/visibility/should_reject_if_dst_has_tricky_unreachable_field.rs
new file mode 100644
index 000000000..35fff5966
--- /dev/null
+++ b/src/test/ui/transmutability/visibility/should_reject_if_dst_has_tricky_unreachable_field.rs
@@ -0,0 +1,52 @@
+// check-pass
+//! NOTE: This test documents a known-bug in the implementation of the
+//! transmutability trait. Once fixed, the above "check-pass" header should be
+//! removed, and an "ERROR cannot be safely transmuted" annotation should be added at the end
+//! of the line starting with `assert::is_transmutable`.
+//!
+//! Unless visibility is assumed, a transmutation should be rejected if the
+//! destination type contains an unreachable field (e.g., a public field with a
+//! private type). (This rule is distinct from type privacy, which still may
+//! forbid naming such types.)
+//!
+//! This test exercises a tricky-to-implement instance of this principle: the
+//! "pub-in-priv trick". In the below example, the type `dst::private::Zst` is
+//! unreachable from `Context`. Consequently, the transmute from `Src` to `Dst`
+//! SHOULD be rejected.
+
+#![crate_type = "lib"]
+#![feature(transmutability)]
+#![allow(dead_code)]
+
+mod assert {
+    use std::mem::BikeshedIntrinsicFrom;
+
+    pub fn is_transmutable<Src, Dst, Context>()
+    where
+        Dst: BikeshedIntrinsicFrom<Src, Context, false, false, false, false>
+        // visibility is NOT assumed ---------------------------------^^^^^
+    {}
+}
+
+mod src {
+    #[repr(C)] pub(in super) struct Zst;
+
+    #[repr(C)] pub(in super) struct Src {
+        pub(in super) field: Zst,
+    }
+}
+
+mod dst {
+    mod private {
+        #[repr(C)] pub struct Zst; // <- unreachable type
+    }
+
+    #[repr(C)] pub(in super) struct Dst {
+        pub(in super) field: private::Zst,
+    }
+}
+
+fn test() {
+    struct Context;
+    assert::is_transmutable::<src::Src, dst::Dst, Context>();
+}