From 9c468f4b65961ec1d3ce3f993c59d250afde00be Mon Sep 17 00:00:00 2001
From: "Felix S. Klock II" <pnkfelix@pnkfx.org>
Date: Fri, 20 May 2016 13:20:33 +0200
Subject: [PATCH] Added comment pointing out somewhat subtle initialization in
 `fn start_block_effect`.

---
 src/librustc_borrowck/borrowck/mir/dataflow/mod.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/librustc_borrowck/borrowck/mir/dataflow/mod.rs b/src/librustc_borrowck/borrowck/mir/dataflow/mod.rs
index cb110e606f2..4bc5a5409ec 100644
--- a/src/librustc_borrowck/borrowck/mir/dataflow/mod.rs
+++ b/src/librustc_borrowck/borrowck/mir/dataflow/mod.rs
@@ -951,6 +951,7 @@ impl<'a, 'tcx> BitDenotation for MaybeUninitializedLvals<'a, 'tcx> {
 
     // sets on_entry bits for Arg lvalues
     fn start_block_effect(&self, ctxt: &Self::Ctxt, sets: &mut BlockSets) {
+        // set all bits to 1 (uninit) before gathering counterevidence
         for e in &mut sets.on_entry[..] { *e = !0; }
 
         super::drop_flag_effects_for_function_entry(