From 7b21c1a4578a7a705f3d104e886ba5192a80db54 Mon Sep 17 00:00:00 2001
From: Lukas Markeffsky <@>
Date: Tue, 19 Mar 2024 13:57:31 +0100
Subject: [PATCH] add test for casting pointer to union with unsized tail

---
 tests/ui/cast/unsized-union-ice.rs     | 14 ++++++++++++++
 tests/ui/cast/unsized-union-ice.stderr | 23 +++++++++++++++++++++++
 2 files changed, 37 insertions(+)
 create mode 100644 tests/ui/cast/unsized-union-ice.rs
 create mode 100644 tests/ui/cast/unsized-union-ice.stderr

diff --git a/tests/ui/cast/unsized-union-ice.rs b/tests/ui/cast/unsized-union-ice.rs
new file mode 100644
index 0000000000000..11aefe57f1dfb
--- /dev/null
+++ b/tests/ui/cast/unsized-union-ice.rs
@@ -0,0 +1,14 @@
+// Regression test for https://github.com/rust-lang/rust/issues/122581
+// This used to ICE, because the union was unsized and the pointer casting code
+// assumed that non-struct ADTs must be sized.
+
+union Union {
+    val: std::mem::ManuallyDrop<[u8]>,
+    //~^ ERROR the size for values of type `[u8]` cannot be known at compilation time
+}
+
+fn cast(ptr: *const ()) -> *const Union {
+    ptr as _
+}
+
+fn main() {}
diff --git a/tests/ui/cast/unsized-union-ice.stderr b/tests/ui/cast/unsized-union-ice.stderr
new file mode 100644
index 0000000000000..05f8645782979
--- /dev/null
+++ b/tests/ui/cast/unsized-union-ice.stderr
@@ -0,0 +1,23 @@
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/unsized-union-ice.rs:6:10
+   |
+LL |     val: std::mem::ManuallyDrop<[u8]>,
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: within `ManuallyDrop<[u8]>`, the trait `Sized` is not implemented for `[u8]`, which is required by `ManuallyDrop<[u8]>: Sized`
+note: required because it appears within the type `ManuallyDrop<[u8]>`
+  --> $SRC_DIR/core/src/mem/manually_drop.rs:LL:COL
+   = note: no field of a union may have a dynamically sized type
+   = help: change the field's type to have a statically known size
+help: borrowed types always have a statically known size
+   |
+LL |     val: &std::mem::ManuallyDrop<[u8]>,
+   |          +
+help: the `Box` type always has a statically known size and allocates its contents in the heap
+   |
+LL |     val: Box<std::mem::ManuallyDrop<[u8]>>,
+   |          ++++                            +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.