1 files changed, 40 insertions, 0 deletions
diff --git a/src/test/ui/transmutability/unions/should_pad_variants.rs b/src/test/ui/transmutability/unions/should_pad_variants.rs
new file mode 100644
index 000000000..c4757900f
--- /dev/null
+++ b/src/test/ui/transmutability/unions/should_pad_variants.rs
@@ -0,0 +1,40 @@
+//! The variants of a union must be padded with uninit bytes such that they have
+//! the same length (in bytes).
+
+#![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, true, true, true, true>
+    {}
+}
+
+#[derive(Clone, Copy)]
+#[repr(C)] struct Zst;
+
+#[derive(Clone, Copy)]
+#[repr(u8)] enum V0 { V = 0 }
+
+#[derive(Clone, Copy)]
+#[repr(u8)] enum V2 { V = 2 }
+
+#[repr(C)]
+union Lopsided {
+    smol: Zst,
+    lorg: V0,
+}
+
+#[repr(C)] struct Src(V0, Zst, V2);
+#[repr(C)] struct Dst(V0, Lopsided, V2);
+
+fn should_pad_variants() {
+    struct Context;
+    // If the implementation (incorrectly) fails to pad `Lopsided::smol` with
+    // an uninitialized byte, this transmutation might be (wrongly) accepted:
+    assert::is_transmutable::<Src, Dst, Context>(); //~ ERROR cannot be safely transmuted
+}