diff --git a/src/libcore/str/mod.rs b/src/libcore/str/mod.rs
index 8f21f109e81b7..d5a5e2b47419f 100644
--- a/src/libcore/str/mod.rs
+++ b/src/libcore/str/mod.rs
@@ -1953,7 +1953,10 @@ 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,
@@ -2026,6 +2029,7 @@ 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) {