make borrowck more conservative around rvalues.

this will require more temporaries, but is probably less magical.
also, it means that borrowck matches trans better, so fewer crashes.
bonus.

Finally, stop warning about implicit copies when we are actually borrowing.

Also, one test (vec-res-add) stopped failing due to #2587, and hence I
added an xfail-test.

Fixes #3217, #2977, #3067

src/libsyntax/ast_map.rs

@@ -2,7 +2,7 @@ import std::map;
 import std::map::hashmap;
 import ast::*;
 import print::pprust;
-import ast_util::path_to_ident;
+import ast_util::{path_to_ident, stmt_id};
 import diagnostic::span_handler;
 
 enum path_elt { path_mod(ident), path_name(ident) }
@@ -39,6 +39,7 @@ enum ast_node {
     node_method(@method, def_id /* impl did */, @path /* path to the impl */),
     node_variant(variant, @item, @path),
     node_expr(@expr),
+    node_stmt(@stmt),
     node_export(@view_path, @path),
     // Locals are numbered, because the alias analysis needs to know in which
     // order they are introduced.
@@ -65,6 +66,7 @@ fn mk_ast_map_visitor() -> vt {
     return visit::mk_vt(@{
         visit_item: map_item,
         visit_expr: map_expr,
+        visit_stmt: map_stmt,
         visit_fn: map_fn,
         visit_local: map_local,
         visit_arm: map_arm,
@@ -284,6 +286,11 @@ fn map_expr(ex: @expr, cx: ctx, v: vt) {
     visit::visit_expr(ex, cx, v);
 }
 
+fn map_stmt(stmt: @stmt, cx: ctx, v: vt) {
+    cx.map.insert(stmt_id(*stmt), node_stmt(stmt));
+    visit::visit_stmt(stmt, cx, v);
+}
+
 fn node_id_to_str(map: map, id: node_id) -> ~str {
     match map.find(id) {
       none => {
@@ -313,6 +320,10 @@ fn node_id_to_str(map: map, id: node_id) -> ~str {
         fmt!{"expr %s (id=%?)",
              pprust::expr_to_str(expr), id}
       }
+      some(node_stmt(stmt)) => {
+        fmt!{"stmt %s (id=%?)",
+             pprust::stmt_to_str(*stmt), id}
+      }
       // FIXMEs are as per #2410
       some(node_export(_, path)) => {
         fmt!{"export %s (id=%?)", // add more info here