Detect trait upcasting through struct tail unsizing

3 files changed, 40 insertions, 0 deletions

diff --git a/tests/ui/traits/trait-upcasting/upcast-through-struct-tail.current.stderr b/tests/ui/traits/trait-upcasting/upcast-through-struct-tail.current.stderr
new file mode 100644
index 00000000000..9f0993d65a7
--- /dev/null
+++ b/tests/ui/traits/trait-upcasting/upcast-through-struct-tail.current.stderr
@@ -0,0 +1,13 @@
+error[E0658]: cannot cast `dyn A` to `dyn B`, trait upcasting coercion is experimental
+  --> $DIR/upcast-through-struct-tail.rs:10:5
+   |
+LL |     x
+   |     ^
+   |
+   = note: see issue #65991 <https://github.com/rust-lang/rust/issues/65991> for more information
+   = help: add `#![feature(trait_upcasting)]` to the crate attributes to enable
+   = note: required when coercing `Box<Wrapper<(dyn A + 'a)>>` into `Box<Wrapper<(dyn B + 'a)>>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/traits/trait-upcasting/upcast-through-struct-tail.next.stderr b/tests/ui/traits/trait-upcasting/upcast-through-struct-tail.next.stderr
new file mode 100644
index 00000000000..9f0993d65a7
--- /dev/null
+++ b/tests/ui/traits/trait-upcasting/upcast-through-struct-tail.next.stderr
@@ -0,0 +1,13 @@
+error[E0658]: cannot cast `dyn A` to `dyn B`, trait upcasting coercion is experimental
+  --> $DIR/upcast-through-struct-tail.rs:10:5
+   |
+LL |     x
+   |     ^
+   |
+   = note: see issue #65991 <https://github.com/rust-lang/rust/issues/65991> for more information
+   = help: add `#![feature(trait_upcasting)]` to the crate attributes to enable
+   = note: required when coercing `Box<Wrapper<(dyn A + 'a)>>` into `Box<Wrapper<(dyn B + 'a)>>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/traits/trait-upcasting/upcast-through-struct-tail.rs b/tests/ui/traits/trait-upcasting/upcast-through-struct-tail.rs
new file mode 100644
index 00000000000..42495f45f8a
--- /dev/null
+++ b/tests/ui/traits/trait-upcasting/upcast-through-struct-tail.rs
@@ -0,0 +1,14 @@
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
+
+struct Wrapper<T: ?Sized>(T);
+
+trait A: B {}
+trait B {}
+
+fn test<'a>(x: Box<Wrapper<dyn A + 'a>>) -> Box<Wrapper<dyn B + 'a>> {
+    x
+    //~^ ERROR cannot cast `dyn A` to `dyn B`, trait upcasting coercion is experimental
+}
+
+fn main() {}