Improve str prefix/suffix comparison

The comparison can be performed on the raw bytes, as the chars can
only match if their UTF8 encoding matches.

This avoids the `is_char_boundary` checks and translates to a straight
`u8` slice comparison which is optimized to a memcmp or inline
comparison where appropriate.

Author: ranma42

src/libcore/str/pattern.rs

@@ -715,16 +715,13 @@ impl<'a, 'b> Pattern<'a> for &'b str {
     /// Checks whether the pattern matches at the front of the haystack
     #[inline]
     fn is_prefix_of(self, haystack: &'a str) -> bool {
-        haystack.is_char_boundary(self.len()) &&
-            self == &haystack[..self.len()]
+        haystack.as_bytes().starts_with(self.as_bytes())
     }
 
     /// Checks whether the pattern matches at the back of the haystack
     #[inline]
     fn is_suffix_of(self, haystack: &'a str) -> bool {
-        self.len() <= haystack.len() &&
-            haystack.is_char_boundary(haystack.len() - self.len()) &&
-            self == &haystack[haystack.len() - self.len()..]
+        haystack.as_bytes().ends_with(self.as_bytes())
     }
 }