diff --git a/compiler/rustc_lint/src/let_underscore.rs b/compiler/rustc_lint/src/let_underscore.rs
index b51c1679f9f..c59f56756c6 100644
--- a/compiler/rustc_lint/src/let_underscore.rs
+++ b/compiler/rustc_lint/src/let_underscore.rs
@@ -11,7 +11,7 @@ declare_lint! {
     /// scope.
     ///
     /// ### Example
-    /// ```compile_fail
+    /// ```rust,compile_fail
     /// struct SomeStruct;
     /// impl Drop for SomeStruct {
     ///     fn drop(&mut self) {