Add regression test for issue 1246

    error[E0505]: cannot move out of `p` because it is borrowed
      --> tests/test_iterators.rs:56:18
       |
    54 |     for element in &p {
       |                    --
       |                    |
       |                    borrow of `p` occurs here
       |                    a temporary with access to the borrow is created here ...
    55 |         if *element == 2 {
    56 |             drop(p);
       |                  ^ move out of `p` occurs here
    ...
    59 |     }
       |     - ... and the borrow might be used here, when that temporary is dropped and runs the destructor for type `syn::punctuated::Iter<'_, i32>`

    error[E0505]: cannot move out of `p` because it is borrowed
      --> tests/test_iterators.rs:64:18
       |
    62 |     for element in &mut p {
       |                    ------
       |                    |
       |                    borrow of `p` occurs here
       |                    a temporary with access to the borrow is created here ...
    63 |         if *element == 2 {
    64 |             drop(p);
       |                  ^ move out of `p` occurs here
    ...
    67 |     }
       |     - ... and the borrow might be used here, when that temporary is dropped and runs the destructor for type `syn::punctuated::IterMut<'_, i32>`

tests/test_iterators.rs

@@ -47,3 +47,22 @@ fn iter() {
     assert_eq!(p.iter_mut().next_back(), Some(&mut 4));
     assert_eq!(p.into_iter().next_back(), Some(4));
 }
+
+#[test]
+fn may_dangle() {
+    let p: Punctuated<_, Token![,]> = punctuated!(2, 3, 4);
+    for element in &p {
+        if *element == 2 {
+            drop(p);
+            break;
+        }
+    }
+
+    let mut p: Punctuated<_, Token![,]> = punctuated!(2, 3, 4);
+    for element in &mut p {
+        if *element == 2 {
+            drop(p);
+            break;
+        }
+    }
+}