Deny specializing items not in the parent impl

src/librustc/traits/project.rs

@@ -1505,8 +1505,8 @@ fn assoc_ty_def(
 
     if let Some(assoc_item) = trait_def
         .ancestors(tcx, impl_def_id)
-        .defs(tcx, assoc_ty_name, ty::AssocKind::Type, trait_def_id)
-        .next() {
+        .leaf_def(tcx, assoc_ty_name, ty::AssocKind::Type) {
+
         assoc_item
     } else {
         // This is saying that neither the trait nor

src/librustc/traits/specialize/mod.rs

@@ -125,7 +125,7 @@ pub fn find_associated_item<'tcx>(
     let trait_def = tcx.trait_def(trait_def_id);
 
     let ancestors = trait_def.ancestors(tcx, impl_data.impl_def_id);
-    match ancestors.defs(tcx, item.ident, item.kind, trait_def_id).next() {
+    match ancestors.leaf_def(tcx, item.ident, item.kind) {
         Some(node_item) => {
             let substs = tcx.infer_ctxt().enter(|infcx| {
                 let param_env = param_env.with_reveal_all();

src/librustc/traits/specialize/specialization_graph.rs

@@ -7,7 +7,6 @@ use crate::traits;
 use crate::ty::{self, TyCtxt, TypeFoldable};
 use crate::ty::fast_reject::{self, SimplifiedType};
 use syntax::ast::Ident;
-use crate::util::captures::Captures;
 use crate::util::nodemap::{DefIdMap, FxHashMap};
 
 /// A per-trait graph of impls in specialization order. At the moment, this
@@ -419,6 +418,35 @@ impl<'tcx> Node {
         tcx.associated_items(self.def_id())
     }
 
+    /// Finds an associated item defined in this node.
+    ///
+    /// If this returns `None`, the item can potentially still be found in
+    /// parents of this node.
+    pub fn item(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        trait_item_name: Ident,
+        trait_item_kind: ty::AssocKind,
+        trait_def_id: DefId,
+    ) -> Option<ty::AssocItem> {
+        use crate::ty::AssocKind::*;
+
+        tcx.associated_items(self.def_id())
+            .find(move |impl_item| match (trait_item_kind, impl_item.kind) {
+                | (Const, Const)
+                | (Method, Method)
+                | (Type, Type)
+                | (Type, OpaqueTy)
+                => tcx.hygienic_eq(impl_item.ident, trait_item_name, trait_def_id),
+
+                | (Const, _)
+                | (Method, _)
+                | (Type, _)
+                | (OpaqueTy, _)
+                => false,
+            })
+    }
+
     pub fn def_id(&self) -> DefId {
         match *self {
             Node::Impl(did) => did,
@@ -427,6 +455,7 @@ impl<'tcx> Node {
     }
 }
 
+#[derive(Copy, Clone)]
 pub struct Ancestors<'tcx> {
     trait_def_id: DefId,
     specialization_graph: &'tcx Graph,
@@ -465,32 +494,18 @@ impl<T> NodeItem<T> {
 }
 
 impl<'tcx> Ancestors<'tcx> {
-    /// Search the items from the given ancestors, returning each definition
-    /// with the given name and the given kind.
-    // FIXME(#35870): avoid closures being unexported due to `impl Trait`.
-    #[inline]
-    pub fn defs(
-        self,
+    /// Finds the bottom-most (ie. most specialized) definition of an associated
+    /// item.
+    pub fn leaf_def(
+        mut self,
         tcx: TyCtxt<'tcx>,
         trait_item_name: Ident,
         trait_item_kind: ty::AssocKind,
-        trait_def_id: DefId,
-    ) -> impl Iterator<Item = NodeItem<ty::AssocItem>> + Captures<'tcx> + 'tcx {
-        self.flat_map(move |node| {
-            use crate::ty::AssocKind::*;
-            node.items(tcx).filter(move |impl_item| match (trait_item_kind, impl_item.kind) {
-                | (Const, Const)
-                | (Method, Method)
-                | (Type, Type)
-                | (Type, OpaqueTy)
-                => tcx.hygienic_eq(impl_item.ident, trait_item_name, trait_def_id),
-
-                | (Const, _)
-                | (Method, _)
-                | (Type, _)
-                | (OpaqueTy, _)
-                => false,
-            }).map(move |item| NodeItem { node: node, item: item })
+    ) -> Option<NodeItem<ty::AssocItem>> {
+        let trait_def_id = self.trait_def_id;
+        self.find_map(|node| {
+            node.item(tcx, trait_item_name, trait_item_kind, trait_def_id)
+                .map(|item| NodeItem { node, item })
         })
     }
 }

src/librustc/traits/util.rs

@@ -4,7 +4,6 @@ use syntax_pos::Span;
 
 use crate::hir;
 use crate::hir::def_id::DefId;
-use crate::traits::specialize::specialization_graph::NodeItem;
 use crate::ty::{self, Ty, TyCtxt, ToPredicate, ToPolyTraitRef};
 use crate::ty::outlives::Component;
 use crate::ty::subst::{GenericArg, Subst, SubstsRef};
@@ -667,8 +666,8 @@ impl<'tcx> TyCtxt<'tcx> {
         }
     }
 
-    pub fn impl_item_is_final(self, node_item: &NodeItem<hir::Defaultness>) -> bool {
-        node_item.item.is_final() && !self.impl_is_default(node_item.node.def_id())
+    pub fn impl_item_is_final(self, assoc_item: &ty::AssocItem) -> bool {
+        assoc_item.defaultness.is_final() && !self.impl_is_default(assoc_item.container.id())
     }
 }