core: rewrite str::byte_index to use vec functions

Author: erickt

src/comp/syntax/codemap.rs

@@ -157,7 +157,8 @@ fn span_to_lines(sp: span, cm: codemap::codemap) -> @file_lines {
 
 fn get_line(fm: filemap, line: int) -> str unsafe {
     let begin: uint = fm.lines[line].byte - fm.start_pos.byte;
-    let end = alt str::byte_index(*fm.src, '\n' as u8, begin) {
+    let end = alt str::byte_index_from(*fm.src, '\n' as u8, begin,
+                                  str::len(*fm.src)) {
       some(e) { e }
       none { str::len(*fm.src) }
     };

src/libcore/str.rs

@@ -71,6 +71,7 @@ export
    // Searching
    index,
    byte_index,
+   byte_index_from,
    rindex,
    find,
    find_bytes,
@@ -859,13 +860,19 @@ fn index(ss: str, cc: char) -> option<uint> {
 //
 // Returns the index of the first matching byte
 // (as option some/none)
-fn byte_index(s: str, b: u8, start: uint) -> option<uint> {
-    let i = start, l = len_bytes(s);
-    while i < l {
-        if s[i] == b { ret some(i); }
-        i += 1u;
-    }
-    ret none;
+fn byte_index(s: str, b: u8) -> option<uint> {
+    byte_index_from(s, b, 0u, len_bytes(s))
+}
+
+// Function: byte_index_from
+//
+// Returns the index of the first matching byte within the range [`start`,
+// `end`).
+// (as option some/none)
+fn byte_index_from(s: str, b: u8, start: uint, end: uint) -> option<uint> {
+    assert end <= len_bytes(s);
+
+    str::as_bytes(s) { |v| vec::position_from(v, start, end) { |x| x == b } }
 }
 
 // Function: rindex