From ce5e49f86fc8bff241695f8a62e77f517749bd6d Mon Sep 17 00:00:00 2001
From: lzutao <taolzu@gmail.com>
Date: Mon, 16 Mar 2020 23:43:42 +0700
Subject: [PATCH] Use sublice patterns to avoid computing the len

---
 src/libstd/sys_common/wtf8.rs | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/src/libstd/sys_common/wtf8.rs b/src/libstd/sys_common/wtf8.rs
index 7509e1ee35d..77977e25517 100644
--- a/src/libstd/sys_common/wtf8.rs
+++ b/src/libstd/sys_common/wtf8.rs
@@ -599,24 +599,16 @@ impl Wtf8 {
 
     #[inline]
     fn final_lead_surrogate(&self) -> Option<u16> {
-        let len = self.len();
-        if len < 3 {
-            return None;
-        }
-        match self.bytes[(len - 3)..] {
-            [0xED, b2 @ 0xA0..=0xAF, b3] => Some(decode_surrogate(b2, b3)),
+        match self.bytes {
+            [.., 0xED, b2 @ 0xA0..=0xAF, b3] => Some(decode_surrogate(*b2, *b3)),
             _ => None,
         }
     }
 
     #[inline]
     fn initial_trail_surrogate(&self) -> Option<u16> {
-        let len = self.len();
-        if len < 3 {
-            return None;
-        }
-        match self.bytes[..3] {
-            [0xED, b2 @ 0xB0..=0xBF, b3] => Some(decode_surrogate(b2, b3)),
+        match self.bytes {
+            [0xED, b2 @ 0xB0..=0xBF, b3, ..] => Some(decode_surrogate(*b2, *b3)),
             _ => None,
         }
     }