diff --git a/src/libsyntax/source_map.rs b/src/libsyntax/source_map.rs
index 7d0d2392945e5..5e569f9dae378 100644
--- a/src/libsyntax/source_map.rs
+++ b/src/libsyntax/source_map.rs
@@ -878,25 +878,8 @@ impl SourceMap {
 
     // Returns the index of the `SourceFile` (in `self.files`) that contains `pos`.
     pub fn lookup_source_file_idx(&self, pos: BytePos) -> usize {
-        let files = self.files.borrow();
-        let files = &files.source_files;
-        let count = files.len();
-
-        // Binary search for the `SourceFile`.
-        let mut a = 0;
-        let mut b = count;
-        while b - a > 1 {
-            let m = (a + b) / 2;
-            if files[m].start_pos > pos {
-                b = m;
-            } else {
-                a = m;
-            }
-        }
-
-        assert!(a < count, "position {} does not resolve to a source location", pos.to_usize());
-
-        return a;
+        self.files.borrow().source_files.binary_search_by_key(&pos, |key| key.start_pos)
+            .unwrap_or_else(|p| p - 1)
     }
 
     pub fn count_lines(&self) -> usize {