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,