Add suggestion for iterators in iterators

src/test/ui/issues/issue-81584.fixed

@@ -0,0 +1,8 @@
+// run-rustfix
+fn main() {
+        let _ = vec![vec![0, 1], vec![2]]
+            .into_iter()
+            .map(|y| y.iter().map(|x| x + 1).collect::<Vec<_>>())
+                  //~^ ERROR cannot return value referencing function parameter `y`
+            .collect::<Vec<_>>();
+}

src/test/ui/issues/issue-81584.rs

@@ -0,0 +1,8 @@
+// run-rustfix
+fn main() {
+        let _ = vec![vec![0, 1], vec![2]]
+            .into_iter()
+            .map(|y| y.iter().map(|x| x + 1))
+                  //~^ ERROR cannot return value referencing function parameter `y`
+            .collect::<Vec<_>>();
+}

src/test/ui/issues/issue-81584.stderr

@@ -0,0 +1,14 @@
+error[E0515]: cannot return value referencing function parameter `y`
+  --> $DIR/issue-81584.rs:5:22
+   |
+LL |             .map(|y| y.iter().map(|x| x + 1))
+   |                      -^^^^^^^^^^^^^^^^^^^^^^
+   |                      |
+   |                      returns a value referencing data owned by the current function
+   |                      `y` is borrowed here
+   |
+   = help: use `.collect()` to allocate the iterator
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0515`.

src/test/ui/static/static-reference-to-fn-2.stderr

@@ -40,6 +40,8 @@ LL | |         statefn: &id(state1 as StateMachineFunc)
    | |                   ------------------------------ temporary value created here
 LL | |     }
    | |_____^ returns a value referencing data owned by the current function
+   |
+   = help: use `.collect()` to allocate the iterator
 
 error: aborting due to 4 previous errors