From 80e7a1be359fc0de4eb17056230ae2fc130b7090 Mon Sep 17 00:00:00 2001
From: Ulrik Sverdrup <bluss@users.noreply.github.com>
Date: Wed, 23 Mar 2016 22:02:36 +0100
Subject: [PATCH 1/2] Mark str::split_at inline

---
 src/libcore/str/mod.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index f9d1902bea7..4584c26bebc 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -1965,6 +1965,7 @@ impl StrExt for str {
         self.find(pat)
     }
 
+    #[inline]
     fn split_at(&self, mid: usize) -> (&str, &str) {
         // is_char_boundary checks that the index is in [0, .len()]
         if self.is_char_boundary(mid) {

From f621193e5ea7ac54fcd37f0e730e955fd9f61200 Mon Sep 17 00:00:00 2001
From: Ulrik Sverdrup <bluss@users.noreply.github.com>
Date: Wed, 23 Mar 2016 21:57:44 +0100
Subject: [PATCH 2/2] Accept 0 as a valid str char boundary

Index 0 must be a valid char boundary (invariant of str that it contains
valid UTF-8 data).

If we check explicitly for index == 0, that removes the need to read the
byte at index 0, so it avoids a trip to the string's memory, and it
optimizes out the slicing index' bounds check whenever it is zero.

With this change, the following examples all change from having a read of
the byte at 0 and a branch to possibly panicing, to having the bounds
checking optimized away.

```rust
pub fn split(s: &str) -> (&str, &str) {
    s.split_at(0)
}

pub fn both(s: &str) -> &str {
    &s[0..s.len()]
}

pub fn first(s: &str) -> &str {
    &s[..0]
}

pub fn last(s: &str) -> &str {
    &s[0..]
}
```
---
 src/libcore/str/mod.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index 4584c26bebc..f033f50e59e 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -1892,7 +1892,10 @@ impl StrExt for str {
 
     #[inline]
     fn is_char_boundary(&self, index: usize) -> bool {
-        if index == self.len() { return true; }
+        // 0 and len are always ok.
+        // Test for 0 explicitly so that it can optimize out the check
+        // easily and skip reading string data for that case.
+        if index == 0 || index == self.len() { return true; }
         match self.as_bytes().get(index) {
             None => false,
             Some(&b) => b < 128 || b >= 192,