Fix invalid while_let_on_iterator suggestion

Slice patterns without .. are refutable.

Author: jfirebaugh

clippy_lints/src/utils/mod.rs

@@ -958,7 +958,11 @@ pub fn is_refutable(cx: &LateContext<'_, '_>, pat: &Pat) -> bool {
             }
         },
         PatKind::Slice(ref head, ref middle, ref tail) => {
-            are_refutable(cx, head.iter().chain(middle).chain(tail.iter()).map(|pat| &**pat))
+            if middle.is_none() {
+                true
+            } else {
+                are_refutable(cx, head.iter().chain(middle).chain(tail.iter()).map(|pat| &**pat))
+            }
         },
     }
 }

tests/ui/while_loop.rs

@@ -226,4 +226,14 @@ fn refutable() {
             let _ = elem.or_else(|| *iter.next()?);
         }
     }
+
+    // Issue 3780
+    {
+        let v = vec![1, 2, 3];
+        let mut it = v.windows(2);
+        while let Some([x, y]) = it.next() {
+            println!("x: {}", x);
+            println!("y: {}", y);
+        }
+    }
 }