1 files changed, 15 insertions, 1 deletions
diff --git a/src/tools/clippy/tests/ui/map_identity.rs b/src/tools/clippy/tests/ui/map_identity.rs
index 59dcfcda3b6..fc6e018f924 100644
--- a/src/tools/clippy/tests/ui/map_identity.rs
+++ b/src/tools/clippy/tests/ui/map_identity.rs
@@ -1,3 +1,4 @@
+//@require-annotations-for-level: ERROR
 #![warn(clippy::map_identity)]
 #![allow(clippy::needless_return, clippy::disallowed_names)]
 
@@ -78,20 +79,33 @@ fn issue11764() {
 }
 
 fn issue13904() {
-    // don't lint: `it.next()` would not be legal as `it` is immutable
+    // lint, but there's a catch:
+    // when we remove the `.map()`, `it.next()` would require `it` to be mutable
+    // therefore, include that in the suggestion as well
     let it = [1, 2, 3].into_iter();
     let _ = it.map(|x| x).next();
+    //~^ map_identity
+    //~| HELP: remove the call to `map`, and make `it` mutable
+
+    // lint
+    let mut index = [1, 2, 3].into_iter();
+    let subindex = (index.by_ref().take(3), 42);
+    let _ = subindex.0.map(|n| n).next();
+    //~^ map_identity
+    //~| HELP: remove the call to `map`, and make `subindex` mutable
 
     // lint
     #[allow(unused_mut)]
     let mut it = [1, 2, 3].into_iter();
     let _ = it.map(|x| x).next();
     //~^ map_identity
+    //~| HELP: remove the call to `map`
 
     // lint
     let it = [1, 2, 3].into_iter();
     let _ = { it }.map(|x| x).next();
     //~^ map_identity
+    //~| HELP: remove the call to `map`
 }
 
 // same as `issue11764`, but for arrays