From 1ae131211be24a337877e6dbcd9b6c52a86b9511 Mon Sep 17 00:00:00 2001
From: Oliver Scherer <github35764891676564198441@oli-obk.de>
Date: Tue, 12 Mar 2019 14:43:49 +0100
Subject: [PATCH] Explain the bits of `UndefMask`

---
 src/librustc/mir/interpret/allocation.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/librustc/mir/interpret/allocation.rs b/src/librustc/mir/interpret/allocation.rs
index 004804f7c211e..2ce9a4a0f204d 100644
--- a/src/librustc/mir/interpret/allocation.rs
+++ b/src/librustc/mir/interpret/allocation.rs
@@ -613,6 +613,8 @@ impl<Tag> DerefMut for Relocations<Tag> {
 
 type Block = u64;
 
+/// A bitmask where each bit refers to the byte with the same index. If the bit is `true`, the byte
+/// is defined. If it is `false` the byte is undefined.
 #[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash, RustcEncodable, RustcDecodable)]
 pub struct UndefMask {
     blocks: Vec<Block>,