rustc: Run block cleanups on else if blocks

With the scheme used to translate 'else if' currently the if expression is
translated in a new (else) scope context. If that if expression wants to
result in a value that requires refcounting then it will need to drop the
refcount in the cleanups of the else block.

Author: brson

src/comp/middle/trans.rs

@@ -3706,8 +3706,15 @@ fn trans_if(&@block_ctxt cx, &@ast::expr cond,
     alt (els) {
         case (some[@ast::expr](?elexpr)) {
             alt (elexpr.node) {
-                case (ast::expr_if(_, _, _, _)) {
-                    else_res = trans_expr(else_cx, elexpr);
+                case (ast::expr_if(?cond, ?thn, ?els, _)) {
+                    else_res = trans_if(else_cx, cond, thn, els);
+                    // The if expression may need to use the else context to
+                    // drop the refcount of its result so we need to run the
+                    // cleanups
+                    auto bcx = else_res.bcx;
+                    bcx = trans_block_cleanups(bcx,
+                                               find_scope_cx(bcx));
+                    else_res = res(bcx, else_res.val);
                 }
                 case (ast::expr_block(?blk, _)) {
                     // Calling trans_block directly instead of trans_expr

src/test/run-pass/expr-elseif-ref.rs

@@ -0,0 +1,17 @@
+// xfail-stage0
+
+// Make sure we drop the refs of the temporaries needed to return the
+// values from the else if branch
+
+fn main() {
+  let vec[uint] y = [10u];
+  auto x = if (false) {
+    y
+  } else if (true) {
+    y
+  } else {
+    y
+  };
+
+  assert y.(0) == 10u;
+}