Add downcast_trait ui tests

Author: ivarflakstad

tests/ui/any/downcast_trait.rs

@@ -0,0 +1,29 @@
+//@ run-pass
+#![feature(downcast_trait)]
+
+use std::fmt::Debug;
+
+// Look ma, no `T: Debug`
+fn downcast_debug_format<T: 'static>(t: &T) -> String {
+    match std::any::downcast_trait::<_, dyn Debug>(t) {
+        Some(d) => format!("{d:?}"),
+        None => "default".to_string()
+    }
+}
+
+// Test that downcasting to a dyn trait works as expected
+fn main() {
+    #[allow(dead_code)]
+    #[derive(Debug)]
+    struct A {
+        index: usize
+    }
+    let a = A { index: 42 };
+    let result = downcast_debug_format(&a);
+    assert_eq!("A { index: 42 }", result);
+
+    struct B {}
+    let b = B {};
+    let result = downcast_debug_format(&b);
+    assert_eq!("default", result);
+}