Avoid unnecessary codegen in with_cond()

src/librustc_trans/trans/_match.rs

@@ -889,11 +889,13 @@ fn compile_guard<'a, 'p, 'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
         }
     }
 
+    for (_, &binding_info) in &data.bindings_map {
+        bcx.fcx.lllocals.borrow_mut().remove(&binding_info.id);
+    }
+
     with_cond(bcx, Not(bcx, val, guard_expr.debug_loc()), |bcx| {
-        // Guard does not match: remove all bindings from the lllocals table
         for (_, &binding_info) in &data.bindings_map {
             call_lifetime_end(bcx, binding_info.llmatch);
-            bcx.fcx.lllocals.borrow_mut().remove(&binding_info.id);
         }
         match chk {
             // If the default arm is the only one left, move on to the next

src/librustc_trans/trans/base.rs

@@ -1079,6 +1079,12 @@ pub fn with_cond<'blk, 'tcx, F>(bcx: Block<'blk, 'tcx>,
     F: FnOnce(Block<'blk, 'tcx>) -> Block<'blk, 'tcx>,
 {
     let _icx = push_ctxt("with_cond");
+
+    if bcx.unreachable.get() ||
+            (common::is_const(val) && common::const_to_uint(val) == 0) {
+        return bcx;
+    }
+
     let fcx = bcx.fcx;
     let next_cx = fcx.new_temp_block("next");
     let cond_cx = fcx.new_temp_block("cond");