Move relative pos calculation to SourceFile

Author: Rantanen

src/libsyntax/json.rs

@@ -333,24 +333,10 @@ impl DiagnosticSpan {
             })
         });
 
-        let file_np = &start.file.normalized_pos;
-        let np_start_diff = match file_np.binary_search_by(
-                            |np| np.pos.cmp(&span.lo())) {
-            Ok(i) => file_np[i].diff,
-            Err(i) if i == 0 => 0,
-            Err(i) => file_np[i-1].diff,
-        };
-        let np_end_diff = match file_np.binary_search_by(
-                            |np| np.pos.cmp(&span.hi())) {
-            Ok(i) => file_np[i].diff,
-            Err(i) if i == 0 => 0,
-            Err(i) => file_np[i-1].diff,
-        };
-
         DiagnosticSpan {
             file_name: start.file.name.to_string(),
-            byte_start: span.lo().0 - start.file.start_pos.0 + np_start_diff,
-            byte_end: span.hi().0 - start.file.start_pos.0 + np_end_diff,
+            byte_start: start.file.original_relative_byte_pos(span.lo()).0,
+            byte_end: start.file.original_relative_byte_pos(span.hi()).0,
             line_start: start.line,
             line_end: end.line,
             column_start: start.col.0 + 1,

src/libsyntax_pos/lib.rs

@@ -1239,6 +1239,23 @@ impl SourceFile {
     pub fn contains(&self, byte_pos: BytePos) -> bool {
         byte_pos >= self.start_pos && byte_pos <= self.end_pos
     }
+
+    /// Calculates the original byte position relative to the start of the file
+    /// based on the given byte position.
+    pub fn original_relative_byte_pos(&self, pos: BytePos) -> BytePos {
+
+        // Diff before any records is 0. Otherwise use the previously recorded
+        // diff as that applies to the following characters until a new diff
+        // is recorded.
+        let diff = match self.normalized_pos.binary_search_by(
+                            |np| np.pos.cmp(&pos)) {
+            Ok(i) => self.normalized_pos[i].diff,
+            Err(i) if i == 0 => 0,
+            Err(i) => self.normalized_pos[i-1].diff,
+        };
+
+        BytePos::from_u32(pos.0 - self.start_pos.0 + diff)
+    }
 }
 
 /// Normalizes the source code and records the normalizations.