Add downcast_trait_mut ui tests

Author: ivarflakstad

tests/ui/any/downcast_trait_mut.rs

@@ -0,0 +1,20 @@
+//@ run-pass
+#![feature(downcast_trait)]
+
+use std::fmt::{Error, Write};
+
+// Look ma, no `T: Write`
+fn downcast_mut_write<T: 'static>(t: &mut T, s: &str) -> Result<(), Error> {
+    match std::any::downcast_trait_mut::<_, dyn Write>(t) {
+        Some(w) => w.write_str(s),
+        None => Ok(())
+    }
+}
+
+// Test that downcasting to a mut dyn trait works as expected
+fn main() {
+    let mut buf = "Hello".to_string();
+
+    downcast_mut_write(&mut buf, " world!").unwrap();
+    assert_eq!(buf, "Hello world!");
+}