diff --git a/src/test/rustdoc-ui/cfg-test.rs b/src/test/rustdoc-ui/cfg-test.rs
index e26034371f44..6112e9b30e83 100644
--- a/src/test/rustdoc-ui/cfg-test.rs
+++ b/src/test/rustdoc-ui/cfg-test.rs
@@ -2,12 +2,15 @@
 // compile-flags:--test
 // normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
 
+// Crates like core have doctests gated on `cfg(not(test))` so we need to make
+// sure `cfg(test)` is not active when running `rustdoc --test`.
+
 /// this doctest will be ignored:
 ///
 /// ```
 /// assert!(false);
 /// ```
-#[cfg(not(test))]
+#[cfg(test)]
 pub struct Foo;
 
 /// this doctest will be tested:
@@ -15,5 +18,5 @@ pub struct Foo;
 /// ```
 /// assert!(true);
 /// ```
-#[cfg(test)]
+#[cfg(not(test))]
 pub struct Foo;
diff --git a/src/test/rustdoc-ui/cfg-test.stdout b/src/test/rustdoc-ui/cfg-test.stdout
index 30bb0038d1b8..67873870e89b 100644
--- a/src/test/rustdoc-ui/cfg-test.stdout
+++ b/src/test/rustdoc-ui/cfg-test.stdout
@@ -1,6 +1,6 @@
 
 running 1 test
-test $DIR/cfg-test.rs - Foo (line 15) ... ok
+test $DIR/cfg-test.rs - Foo (line 18) ... ok
 
 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out