From dcd80b80ae3fe2f327515e57fdc423a3927e44e6 Mon Sep 17 00:00:00 2001
From: Arthur Silva <arthurprs@gmail.com>
Date: Thu, 10 Nov 2016 22:02:44 +0100
Subject: [PATCH] Remove one bounds check from BufReader

Otherwise the optimizer can't be sure that pos <= cap. Added a paranoid debug_assert to ensure correctness instead.
---
 src/libstd/io/buffered.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/libstd/io/buffered.rs b/src/libstd/io/buffered.rs
index 44dd4e9874ac4..cd7a50d07e268 100644
--- a/src/libstd/io/buffered.rs
+++ b/src/libstd/io/buffered.rs
@@ -187,7 +187,10 @@ impl<R: Read> BufRead for BufReader<R> {
     fn fill_buf(&mut self) -> io::Result<&[u8]> {
         // If we've reached the end of our internal buffer then we need to fetch
         // some more data from the underlying reader.
-        if self.pos == self.cap {
+        // Branch using `>=` instead of the more correct `==`
+        // to tell the compiler that the pos..cap slice is always valid.
+        if self.pos >= self.cap {
+            debug_assert!(self.pos == self.cap);
             self.cap = self.inner.read(&mut self.buf)?;
             self.pos = 0;
         }