Extend middle::reachable to also consider provided trait methods.

src/librustc/middle/reachable.rs

@@ -323,19 +323,37 @@ impl<'a, 'tcx> ReachableContext<'a, 'tcx> {
 // items of non-exported traits (or maybe all local traits?) unless their respective
 // trait items are used from inlinable code through method call syntax or UFCS, or their
 // trait is a lang item.
-struct CollectPrivateImplItemsVisitor<'a> {
+struct CollectPrivateImplItemsVisitor<'a, 'tcx: 'a> {
+    tcx: TyCtxt<'a, 'tcx, 'tcx>,
     access_levels: &'a privacy::AccessLevels,
     worklist: &'a mut Vec<ast::NodeId>,
 }
 
-impl<'a, 'v> ItemLikeVisitor<'v> for CollectPrivateImplItemsVisitor<'a> {
+impl<'a, 'tcx: 'a> ItemLikeVisitor<'tcx> for CollectPrivateImplItemsVisitor<'a, 'tcx> {
     fn visit_item(&mut self, item: &hir::Item) {
         // We need only trait impls here, not inherent impls, and only non-exported ones
-        if let hir::ItemImpl(.., Some(_), _, ref impl_item_refs) = item.node {
+        if let hir::ItemImpl(.., Some(ref trait_ref), _, ref impl_item_refs) = item.node {
             if !self.access_levels.is_reachable(item.id) {
                 for impl_item_ref in impl_item_refs {
                     self.worklist.push(impl_item_ref.id.node_id);
                 }
+
+                let trait_def_id = match trait_ref.path.def {
+                    Def::Trait(def_id) => def_id,
+                    _ => unreachable!()
+                };
+
+                if !trait_def_id.is_local() {
+                    return
+                }
+
+                for default_method in self.tcx.provided_trait_methods(trait_def_id) {
+                    let node_id = self.tcx
+                                      .map
+                                      .as_local_node_id(default_method.def_id)
+                                      .unwrap();
+                    self.worklist.push(node_id);
+                }
             }
         }
     }
@@ -369,6 +387,7 @@ pub fn find_reachable<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
     {
         let mut collect_private_impl_items = CollectPrivateImplItemsVisitor {
+            tcx: tcx,
             access_levels: access_levels,
             worklist: &mut reachable_context.worklist,
         };

src/test/run-pass/auxiliary/issue_38226_aux.rs

@@ -0,0 +1,33 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![crate_type="rlib"]
+
+#[inline(never)]
+pub fn foo<T>() {
+    let _: Box<SomeTrait> = Box::new(SomeTraitImpl);
+}
+
+pub fn bar() {
+    SomeTraitImpl.bar();
+}
+
+mod submod {
+    pub trait SomeTrait {
+        fn bar(&self) {
+            panic!("NO")
+        }
+    }
+}
+
+use self::submod::SomeTrait;
+
+pub struct SomeTraitImpl;
+impl SomeTrait for SomeTraitImpl {}

src/test/run-pass/issue-38226.rs

@@ -0,0 +1,24 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// This test makes sure that we don't run into a linker error because of the
+// middle::reachable pass missing trait methods with default impls.
+
+// aux-build:issue_38226_aux.rs
+
+// Need -Cno-prepopulate-passes to really disable inlining, otherwise the faulty
+// code gets optimized out:
+// compile-flags: -Cno-prepopulate-passes
+
+extern crate issue_38226_aux;
+
+fn main() {
+    issue_38226_aux::foo::<()>();
+}