add regression test

Author: Lukas Markeffsky

src/test/ui/array-slice-vec/slice_is_sorted_by_borrow.rs

@@ -0,0 +1,20 @@
+// check-pass
+// regression test for https://github.com/rust-lang/rust/issues/53485#issuecomment-885393452
+
+#![feature(is_sorted)]
+
+struct A {
+    name: String,
+}
+
+fn main() {
+    let a = &[
+        A {
+            name: "1".to_string(),
+        },
+        A {
+            name: "2".to_string(),
+        },
+    ];
+    assert!(a.is_sorted_by_key(|a| a.name.as_str()));
+}