Make `ThinBox<T>` covariant in `T`

Just like `Box<T>`, we want `ThinBox<T>` to be covariant in `T`, but the
projection in `WithHeader<<T as Pointee>::Metadata>` was making it
invariant. This is now hidden as `WithOpaqueHeader`, which we type-cast
whenever the real `WithHeader<H>` type is needed.

1 files changed, 7 insertions, 0 deletions

diff --git a/library/alloc/tests/thin_box.rs b/library/alloc/tests/thin_box.rs
index 70d1db8b457..368aa564f94 100644
--- a/library/alloc/tests/thin_box.rs
+++ b/library/alloc/tests/thin_box.rs
@@ -26,6 +26,13 @@ fn want_thin() {
     assert!(is_thin::<i32>());
 }
 
+#[allow(dead_code)]
+fn assert_covariance() {
+    fn thin_box<'new>(b: ThinBox<[&'static str]>) -> ThinBox<[&'new str]> {
+        b
+    }
+}
+
 #[track_caller]
 fn verify_aligned<T>(ptr: *const T) {
     // Use `black_box` to attempt to obscure the fact that we're calling this