Synthesize calls to box_free language item

This gets rid of Drop(Free, _) MIR construct by synthesizing a call to language item which
takes care of dropping instead.

src/librustc/middle/dead.rs

@@ -435,7 +435,7 @@ impl<'a, 'tcx> DeadVisitor<'a, 'tcx> {
         let is_named = node.name().is_some();
         let field_type = self.tcx.node_id_to_type(node.id);
         let is_marker_field = match field_type.ty_to_def_id() {
-            Some(def_id) => self.tcx.lang_items.items().any(|(_, item)| *item == Some(def_id)),
+            Some(def_id) => self.tcx.lang_items.items().iter().any(|item| *item == Some(def_id)),
             _ => false
         };
         is_named

src/librustc/middle/lang_items.rs

@@ -36,9 +36,6 @@ use syntax::parse::token::InternedString;
 use rustc_front::intravisit::Visitor;
 use rustc_front::hir;
 
-use std::iter::Enumerate;
-use std::slice;
-
 // The actual lang items defined come at the end of this file in one handy table.
 // So you probably just want to nip down to the end.
 macro_rules! lets_do_this {
@@ -69,8 +66,8 @@ impl LanguageItems {
         }
     }
 
-    pub fn items<'a>(&'a self) -> Enumerate<slice::Iter<'a, Option<DefId>>> {
-        self.items.iter().enumerate()
+    pub fn items(&self) -> &[Option<DefId>] {
+        &*self.items
     }
 
     pub fn item_name(index: usize) -> &'static str {
@@ -334,6 +331,7 @@ lets_do_this! {
 
     ExchangeMallocFnLangItem,        "exchange_malloc",         exchange_malloc_fn;
     ExchangeFreeFnLangItem,          "exchange_free",           exchange_free_fn;
+    BoxFreeFnLangItem,               "box_free",                box_free_fn;
     StrDupUniqFnLangItem,            "strdup_uniq",             strdup_uniq_fn;
 
     StartFnLangItem,                 "start",                   start_fn;

src/librustc/middle/reachable.rs

@@ -362,7 +362,7 @@ pub fn find_reachable(tcx: &ty::ctxt,
     for (id, _) in &access_levels.map {
         reachable_context.worklist.push(*id);
     }
-    for (_, item) in tcx.lang_items.items() {
+    for item in tcx.lang_items.items().iter() {
         if let Some(did) = *item {
             if let Some(node_id) = tcx.map.as_local_node_id(did) {
                 reachable_context.worklist.push(node_id);