fix dep-graph checking to account for implitems

Author: nikomatsakis

src/librustc_incremental/assert_dep_graph.rs

@@ -172,8 +172,8 @@ impl<'a, 'tcx> ItemLikeVisitor<'tcx> for IfThisChanged<'a, 'tcx> {
         self.process_attrs(item.id, &item.attrs);
     }
 
-    fn visit_impl_item(&mut self, _impl_item: &'tcx hir::ImplItem) {
-        // handled in `visit_item` above
+    fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem) {
+        self.process_attrs(impl_item.id, &impl_item.attrs);
     }
 }
 

src/test/compile-fail/dep-graph-type-alias.rs

@@ -42,8 +42,9 @@ trait Trait {
 
 struct SomeType;
 
-#[rustc_then_this_would_need(ItemSignature)] //~ ERROR OK
+#[rustc_then_this_would_need(ItemSignature)] //~ ERROR no path
 impl SomeType {
+    #[rustc_then_this_would_need(ItemSignature)] //~ ERROR OK
     fn method(&self, _: TypeAlias) {}
 }