diff --git a/src/librustdoc/doctest.rs b/src/librustdoc/doctest.rs
index f9d4d1af1140f..828c72b28407d 100644
--- a/src/librustdoc/doctest.rs
+++ b/src/librustdoc/doctest.rs
@@ -1208,6 +1208,13 @@ impl Tester for Collector {
     }
 }
 
+#[cfg(test)] // used in tests
+impl Tester for Vec<usize> {
+    fn add_test(&mut self, _test: String, _config: LangString, line: usize) {
+        self.push(line);
+    }
+}
+
 struct HirCollector<'a, 'hir, 'tcx> {
     sess: &'a Session,
     collector: &'a mut Collector,
diff --git a/src/librustdoc/html/markdown/tests.rs b/src/librustdoc/html/markdown/tests.rs
index 4dd176b3a692a..1de97e49b8307 100644
--- a/src/librustdoc/html/markdown/tests.rs
+++ b/src/librustdoc/html/markdown/tests.rs
@@ -480,11 +480,6 @@ fn test_markdown_html_escape() {
 #[test]
 fn test_find_testable_code_line() {
     fn t(input: &str, expect: &[usize]) {
-        impl crate::doctest::Tester for Vec<usize> {
-            fn add_test(&mut self, _test: String, _config: LangString, line: usize) {
-                self.push(line);
-            }
-        }
         let mut lines = Vec::<usize>::new();
         find_testable_code(input, &mut lines, ErrorCodes::No, false, None, true);
         assert_eq!(lines, expect);