Error source highlighting: align using tabs to match tabs in the source

Fixes #9116

parsing/location.ml

@@ -463,21 +463,27 @@ let highlight_quote ppf
         (* Single-line error *)
         Format.fprintf ppf "%s | %s@," line_nb line;
         Format.fprintf ppf "%*s   " (String.length line_nb) "";
-        for pos = line_start_cnum to rightmost.pos_cnum - 1 do
+        String.iteri (fun i c ->
+          let pos = line_start_cnum + i in
           if ISet.is_start iset ~pos <> None then
             Format.fprintf ppf "@{<%s>" highlight_tag;
           if ISet.mem iset ~pos then Format.pp_print_char ppf '^'
-          else Format.pp_print_char ppf ' ';
+          else if pos < rightmost.pos_cnum then begin
+            (* For alignment purposes, align using a tab for each tab in the
+               source code *)
+            if c = '\t' then Format.pp_print_char ppf '\t'
+            else Format.pp_print_char ppf ' '
+          end;
           if ISet.is_end iset ~pos <> None then
             Format.fprintf ppf "@}"
-        done;
+        ) line;
         Format.fprintf ppf "@}@,"
     | _ ->
         (* Multi-line error *)
         Misc.pp_two_columns ~sep:"|" ~max_lines ppf
         @@ List.map (fun (line, line_nb, line_start_cnum) ->
-          let line = String.mapi (fun i car ->
-            if ISet.mem iset ~pos:(line_start_cnum + i) then car else '.'
+          let line = String.mapi (fun i c ->
+            if ISet.mem iset ~pos:(line_start_cnum + i) then c else '.'
           ) line in
           (line_nb, line)
         ) lines

testsuite/tests/messages/highlight_tabs.ml

@@ -7,7 +7,7 @@
 [%%expect{|
 Line 1, characters 10-13:
 1 | 		let x = abc
-              ^^^
+    		        ^^^
 Error: Unbound value abc
 Hint: Did you mean abs?
 |}];;