Foo<T> != Foo<U> under layout randomization

previously field ordering was using the same seed for all instances of Foo,
now we pass seed values through the layout tree so that not only
the struct itself affects layout but also its fields

1 files changed, 29 insertions, 0 deletions

diff --git a/tests/ui/layout/randomize.rs b/tests/ui/layout/randomize.rs
new file mode 100644
index 00000000000..be895252a1b
--- /dev/null
+++ b/tests/ui/layout/randomize.rs
@@ -0,0 +1,29 @@
+//@ build-pass
+//@ revisions: normal randomize-layout
+//@ [randomize-layout]compile-flags: -Zrandomize-layout
+
+#![crate_type = "lib"]
+
+struct Foo<T>(u32, T, u8);
+
+struct Wrapper<T>(T);
+
+#[repr(transparent)]
+struct TransparentWrapper(u16);
+
+const _: () = {
+    // behavior of the current implementation, not guaranteed
+    #[cfg(not(randomize_layout))]
+    assert!(std::mem::offset_of!(Foo::<u16>, 1) == std::mem::offset_of!(Foo::<Wrapper<u16>>, 1));
+
+    // under randomization Foo<T> != Foo<U>
+    #[cfg(randomize_layout)]
+    assert!(std::mem::offset_of!(Foo::<u16>, 1) != std::mem::offset_of!(Foo::<Wrapper<u16>>, 1));
+
+    // but repr(transparent) should make them the same again.
+    // maybe not strictly guaranteed? but UCG has been leaning in that direction at least
+    #[cfg(randomize_layout)]
+    assert!(
+        std::mem::offset_of!(Foo::<u16>, 1) == std::mem::offset_of!(Foo::<TransparentWrapper>, 1)
+    );
+};