Add test for the known case that doesn't work

tests/ui/pattern/bindings-after-at/bind-by-copy-or-pat.rs

@@ -0,0 +1,17 @@
+//@ known-bug: unknown
+#![allow(unused)]
+
+struct A(u32);
+
+pub fn main() {
+    // The or-pattern bindings are lowered after `x`, which triggers the error.
+    let x @ (A(a) | A(a)) = A(10);
+    // ERROR: use of moved value
+    assert!(x.0 == 10);
+    assert!(a == 10);
+
+    // This works.
+    let (x @ A(a) | x @ A(a)) = A(10);
+    assert!(x.0 == 10);
+    assert!(a == 10);
+}

tests/ui/pattern/bindings-after-at/bind-by-copy-or-pat.stderr

@@ -0,0 +1,31 @@
+error[E0382]: use of moved value
+  --> $DIR/bind-by-copy-or-pat.rs:8:16
+   |
+LL |     let x @ (A(a) | A(a)) = A(10);
+   |         -      ^            ----- move occurs because value has type `A`, which does not implement the `Copy` trait
+   |         |      |
+   |         |      value used here after move
+   |         value moved here
+   |
+help: borrow this binding in the pattern to avoid moving the value
+   |
+LL |     let ref x @ (A(a) | A(a)) = A(10);
+   |         +++
+
+error[E0382]: use of moved value
+  --> $DIR/bind-by-copy-or-pat.rs:8:23
+   |
+LL |     let x @ (A(a) | A(a)) = A(10);
+   |         -             ^     ----- move occurs because value has type `A`, which does not implement the `Copy` trait
+   |         |             |
+   |         |             value used here after move
+   |         value moved here
+   |
+help: borrow this binding in the pattern to avoid moving the value
+   |
+LL |     let ref x @ (A(a) | A(a)) = A(10);
+   |         +++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0382`.