Fix an infinite loop in the stability check that was the result of

various bugs in `trait_id_of_impl`. The end result was that looking up
the "trait_id_of_impl" with a trait's def-id yielded the same trait
again, even though it ought to have yielded None.

src/librustc/metadata/decoder.rs

@@ -441,9 +441,15 @@ pub fn get_impl_trait<'tcx>(cdata: Cmd,
                             -> Option<Rc<ty::TraitRef<'tcx>>>
 {
     let item_doc = lookup_item(id, cdata.data());
-    reader::maybe_get_doc(item_doc, tag_item_trait_ref).map(|tp| {
-        doc_trait_ref(tp, tcx, cdata)
-    })
+    let fam = item_family(item_doc);
+    match fam {
+        Family::Impl => {
+            reader::maybe_get_doc(item_doc, tag_item_trait_ref).map(|tp| {
+                doc_trait_ref(tp, tcx, cdata)
+            })
+        }
+        _ => None
+    }
 }
 
 pub fn get_impl_vtables<'tcx>(cdata: Cmd,

src/librustc/middle/stability.rs

@@ -11,7 +11,8 @@
 //! A pass that annotates every item and method with its stability level,
 //! propagating default levels lexically from parent to children ast nodes.
 
-use util::nodemap::{NodeMap, DefIdMap};
+use middle::ty;
+use metadata::csearch;
 use syntax::codemap::Span;
 use syntax::{attr, visit};
 use syntax::ast;
@@ -21,8 +22,8 @@
 use syntax::ast_util::is_local;
 use syntax::attr::Stability;
 use syntax::visit::{FnKind, FkMethod, Visitor};
-use middle::ty;
-use metadata::csearch;
+use util::nodemap::{NodeMap, DefIdMap};
+use util::ppaux::Repr;
 
 use std::mem::replace;
 
@@ -154,10 +155,13 @@ pub fn build(krate: &Crate) -> Index {
 /// Lookup the stability for a node, loading external crate
 /// metadata as necessary.
 pub fn lookup(tcx: &ty::ctxt, id: DefId) -> Option<Stability> {
+    debug!("lookup(id={})",
+           id.repr(tcx));
+
     // is this definition the implementation of a trait method?
     match ty::trait_item_of_item(tcx, id) {
-        Some(ty::MethodTraitItemId(trait_method_id))
-                if trait_method_id != id => {
+        Some(ty::MethodTraitItemId(trait_method_id)) if trait_method_id != id => {
+            debug!("lookup: trait_method_id={}", trait_method_id);
             return lookup(tcx, trait_method_id)
         }
         _ => {}
@@ -178,6 +182,7 @@ pub fn lookup(tcx: &ty::ctxt, id: DefId) -> Option<Stability> {
             // stability of the trait to determine the stability of any
             // unmarked impls for it. See FIXME above for more details.
 
+            debug!("lookup: trait_id={}", trait_id);
             lookup(tcx, trait_id)
         } else {
             None

src/librustc/middle/ty.rs

@@ -2862,8 +2862,6 @@ pub fn maybe_walk_ty<'tcx,F>(ty_root: Ty<'tcx>, mut f: F)
             walker.skip_current_subtree();
         }
     }
-
-    maybe_walk_ty_(ty, &mut f);
 }
 
 // Folds types from the bottom up.
@@ -6071,22 +6069,9 @@ pub fn populate_implementations_for_trait_if_necessary(
 /// Given the def_id of an impl, return the def_id of the trait it implements.
 /// If it implements no trait, return `None`.
 pub fn trait_id_of_impl(tcx: &ctxt,
-                        def_id: ast::DefId) -> Option<ast::DefId> {
-    let node = match tcx.map.find(def_id.node) {
-        Some(node) => node,
-        None => return None
-    };
-    match node {
-        ast_map::NodeItem(item) => {
-            match item.node {
-                ast::ItemImpl(_, _, Some(ref trait_ref), _, _) => {
-                    Some(node_id_to_trait_ref(tcx, trait_ref.ref_id).def_id)
-                }
-                _ => None
-            }
-        }
-        _ => None
-    }
+                        def_id: ast::DefId)
+                        -> Option<ast::DefId> {
+    ty::impl_trait_ref(tcx, def_id).map(|tr| tr.def_id)
 }
 
 /// If the given def ID describes a method belonging to an impl, return the