eplace usages of vec![].into_iter with [].into_iter

compiler/rustc_graphviz/src/tests.rs

@@ -56,7 +56,7 @@ impl NodeLabels<&'static str> {
         match self {
             UnlabelledNodes(len) => vec![None; len],
             AllNodesLabelled(lbls) => lbls.into_iter().map(Some).collect(),
-            SomeNodesLabelled(lbls) => lbls.into_iter().collect(),
+            SomeNodesLabelled(lbls) => lbls,
         }
     }