rustc_metadata: Use binary search from standard library

instead of a hand rolled one.

Author: petrochenkov

src/librustc_metadata/rmeta/decoder.rs

@@ -408,20 +408,12 @@ impl<'a, 'tcx> SpecializedDecoder<Span> for DecodeContext<'a, 'tcx> {
             {
                 last_source_file
             } else {
-                let mut a = 0;
-                let mut b = imported_source_files.len();
-
-                while b - a > 1 {
-                    let m = (a + b) / 2;
-                    if imported_source_files[m].original_start_pos > lo {
-                        b = m;
-                    } else {
-                        a = m;
-                    }
-                }
+                let index = imported_source_files
+                    .binary_search_by_key(&lo, |source_file| source_file.original_start_pos)
+                    .unwrap_or_else(|index| index - 1);
 
-                self.last_source_file_index = a;
-                &imported_source_files[a]
+                self.last_source_file_index = index;
+                &imported_source_files[index]
             }
         };