Avoid the Box in TyCtxt::associated_items.

This reduces instruction counts on packed_simd by 2%.

src/librustc/traits/specialize/specialization_graph.rs

@@ -366,7 +366,7 @@ impl<'a, 'gcx, 'tcx> Node {
     pub fn items(
         &self,
         tcx: TyCtxt<'a, 'gcx, 'tcx>,
-    ) -> impl Iterator<Item = ty::AssociatedItem> + 'a {
+    ) -> ty::AssociatedItemsIterator<'a, 'gcx, 'tcx> {
         tcx.associated_items(self.def_id())
     }
 

src/librustc/ty/mod.rs

@@ -2674,10 +2674,17 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     pub fn associated_items(
         self,
         def_id: DefId,
-    ) -> impl Iterator<Item = AssociatedItem> + 'a {
-        let def_ids = self.associated_item_def_ids(def_id);
-        Box::new((0..def_ids.len()).map(move |i| self.associated_item(def_ids[i])))
-            as Box<dyn Iterator<Item = AssociatedItem> + 'a>
+    ) -> AssociatedItemsIterator<'a, 'gcx, 'tcx> {
+        // Ideally, we would use `-> impl Iterator` here, but it falls
+        // afoul of the conservative "capture [restrictions]" we put
+        // in place, so we use a hand-written iterator.
+        //
+        // [restrictions]: https://github.com/rust-lang/rust/issues/34511#issuecomment-373423999
+        AssociatedItemsIterator {
+            tcx: self,
+            def_ids: self.associated_item_def_ids(def_id),
+            next_index: 0,
+        }
     }
 
     /// Returns `true` if the impls are the same polarity and the trait either
@@ -2874,6 +2881,22 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     }
 }
 
+pub struct AssociatedItemsIterator<'a, 'gcx: 'tcx, 'tcx: 'a> {
+    tcx: TyCtxt<'a, 'gcx, 'tcx>,
+    def_ids: Lrc<Vec<DefId>>,
+    next_index: usize,
+}
+
+impl Iterator for AssociatedItemsIterator<'_, '_, '_> {
+    type Item = AssociatedItem;
+
+    fn next(&mut self) -> Option<AssociatedItem> {
+        let def_id = self.def_ids.get(self.next_index)?;
+        self.next_index += 1;
+        Some(self.tcx.associated_item(*def_id))
+    }
+}
+
 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     pub fn with_freevars<T, F>(self, fid: NodeId, f: F) -> T where
         F: FnOnce(&[hir::Freevar]) -> T,