Use drain to avoid moving elements of the vector twice

Author: bugadani

src/librustdoc/passes/collect_trait_impls.rs

@@ -132,25 +132,28 @@ crate fn collect_trait_impls(krate: Crate, cx: &DocContext<'_>) -> Crate {
         }
     }
 
-    new_items.retain(|it| {
-        if let ImplItem(Impl { ref for_, ref trait_, ref blanket_impl, .. }) = *it.kind {
-            cleaner.keep_impl(for_)
-                || trait_.as_ref().map_or(false, |t| cleaner.keep_impl(t))
-                || blanket_impl.is_some()
-        } else {
-            true
-        }
-    });
-
-    if let Some(ref mut it) = krate.module {
+    let items = if let Some(ref mut it) = krate.module {
         if let ModuleItem(Module { ref mut items, .. }) = *it.kind {
-            items.extend(synth.impls);
-            items.extend(new_items);
+            items
         } else {
             panic!("collect-trait-impls can't run");
         }
     } else {
         panic!("collect-trait-impls can't run");
+    };
+
+    items.extend(synth.impls);
+    for it in new_items.drain(..) {
+        if let ImplItem(Impl { ref for_, ref trait_, ref blanket_impl, .. }) = *it.kind {
+            if !(cleaner.keep_impl(for_)
+                || trait_.as_ref().map_or(false, |t| cleaner.keep_impl(t))
+                || blanket_impl.is_some())
+            {
+                continue;
+            }
+        }
+
+        items.push(it);
     }
 
     krate