diff --git a/src/librustc_metadata/decoder.rs b/src/librustc_metadata/decoder.rs
index 3e367cbee2e51..7f36fa598305c 100644
--- a/src/librustc_metadata/decoder.rs
+++ b/src/librustc_metadata/decoder.rs
@@ -1207,6 +1207,10 @@ impl<'a, 'tcx> CrateMetadata {
 }
 
 fn are_equal_modulo_startpos(fm1: &syntax_pos::FileMap, fm2: &syntax_pos::FileMap) -> bool {
+    if fm1.byte_length() != fm2.byte_length() {
+        return false;
+    }
+
     if fm1.name != fm2.name {
         return false;
     }
diff --git a/src/libsyntax_pos/lib.rs b/src/libsyntax_pos/lib.rs
index e3feaee5369d1..d83d3a6c5cfac 100644
--- a/src/libsyntax_pos/lib.rs
+++ b/src/libsyntax_pos/lib.rs
@@ -481,6 +481,9 @@ impl FileMap {
         self.src.is_none()
     }
 
+    pub fn byte_length(&self) -> u32 {
+        self.end_pos.0 - self.start_pos.0
+    }
     pub fn count_lines(&self) -> usize {
         self.lines.borrow().len()
     }