prevent trying to read non-existing file when extracting extra info for error messages

Author: zth

compiler/ml/error_message_utils.ml

@@ -41,9 +41,13 @@ end = struct
     String.sub src start_offset (end_offset - start_offset)
 
   let extract_text_at_loc loc =
-    (* TODO: Maybe cache later on *)
-    let src = Ext_io.load_file loc.Location.loc_start.pos_fname in
-    extract_location_string ~src loc
+    if loc.Location.loc_start.pos_fname = "_none_" then ""
+    else
+      try
+        (* TODO: Maybe cache later on *)
+        let src = Ext_io.load_file loc.Location.loc_start.pos_fname in
+        extract_location_string ~src loc
+      with _ -> ""
 
   let parse_expr_at_loc loc =
     let sub_src = extract_text_at_loc loc in

tests/build_tests/super_errors/expected/extract_from_none_file.res.expected

@@ -0,0 +1,6 @@
+
+  We've found a bug for you!
+  /.../fixtures/extract_from_none_file.res
+
+  This has type: RegExp.t
+  But a while loop condition must always be of type: bool

tests/build_tests/super_errors/fixtures/extract_from_none_file.res

@@ -0,0 +1,5 @@
+// This will try and extract the text from a non-existing file.
+// Test is intended to check that this does not crash.
+while /foo/ {
+  ()
+}