test that coercions still work under randomization

tests/ui/layout/randomize.rs

@@ -1,17 +1,23 @@
-//@ build-pass
+//@ run-pass
 //@ revisions: normal randomize-layout
-//@ [randomize-layout]compile-flags: -Zrandomize-layout
+//@ [randomize-layout]compile-flags: -Zrandomize-layout -Zlayout-seed=1
 
-#![crate_type = "lib"]
+use std::ptr;
 
-struct Foo<T>(u32, T, u8);
-
-struct Wrapper<T>(T);
 
+// these types only have their field offsets taken, they're never constructed
+#[allow(dead_code)]
+pub struct Foo<T>(u32, T, u8);
+#[allow(dead_code)]
+pub struct Wrapper<T>(T);
 #[repr(transparent)]
-struct TransparentWrapper(u16);
+#[allow(dead_code)]
+pub struct TransparentWrapper(u16);
 
 const _: () = {
+    // fn pointers are treated interchangably and don't affect layout
+    assert!(std::mem::offset_of!(Foo::<fn(u32)>, 1) == std::mem::offset_of!(Foo::<fn() -> usize>, 1));
+
     // 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));
@@ -28,3 +34,16 @@ const _: () = {
         std::mem::offset_of!(Foo::<u16>, 1) != std::mem::offset_of!(Foo::<TransparentWrapper>, 1)
     );
 };
+
+#[allow(dead_code)]
+struct Unsizable<T: ?Sized>(usize, T);
+
+fn main() {
+    // offset_of doesn't let us probe the unsized field, check at runtime.
+    let x = &Unsizable::<[u32; 4]>(0, [0; 4]);
+    let y: &Unsizable::<[u32]> = x;
+
+    // type coercion must not change the layout.
+    assert_eq!(ptr::from_ref(&x.1).addr(), ptr::from_ref(&y.1).addr());
+
+}