Add a test to prevent regression

The way it defines implementations is unrealistic though.

src/test/rustdoc/issue-53812.rs

@@ -0,0 +1,26 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+pub trait MyIterator {
+}
+
+pub struct MyStruct<T>(T);
+
+macro_rules! array_impls {
+    ($($N:expr)+) => {
+        $(
+            impl<'a, T> MyIterator for &'a MyStruct<[T; $N]> {
+            }
+        )+
+    }
+}
+
+// @has issue_53812/trait.MyIterator.html '//*[@id="implementors-list"]//h3[1]' 'MyStruct<[T; 0]>'
+array_impls! { 10 3 2 1 0 }