Fix location of string constants (#1574)

from ocaml/string-lit-loc

Author: voodoos

CHANGES.md

@@ -16,6 +16,7 @@ unreleased
     - Fix `construct` results ordering for sum types sand poly variants (#1603)
     - Fix object method completion not working (#1606, fixes #1575)
     - Improve context detection for package types (#1608, fixes #1607)
+    - Fix incorrect locations for string literals (#1574)
   + editor modes
     - emacs: call the user's configured completion UI in
       `merlin-construct` (#1598)

src/ocaml/preprocess/lexer_raw.mll

@@ -174,14 +174,14 @@ let wrap_string_lexer f state lexbuf =
   state.string_start_loc <- Location.curr lexbuf;
   f state lexbuf >>= fun loc_end ->
   lexbuf.lex_start_p <- state.string_start_loc.Location.loc_start;
-  state.string_start_loc <- Location.none;
   let loc =
     Location.{
       loc_ghost = false;
-      loc_start = state.string_start_loc.Location.loc_start;
+      loc_start = state.string_start_loc.Location.loc_end;
       loc_end;
     }
   in
+  state.string_start_loc <- Location.none;
   return (Buffer.contents state.buffer, loc)
 
 (* to translate escape sequences *)

tests/test-dirs/string-loc.t

@@ -0,0 +1,16 @@
+Ensure the Pexp_constant and Pconst_string nodes have different locations.
+
+  $ echo '    "test"' | $MERLIN single dump -what parsetree -filename test.ml 
+  {
+    "class": "return",
+    "value": "[
+    structure_item (test.ml[1,0+4]..[1,0+10])
+      Pstr_eval
+      expression (test.ml[1,0+4]..[1,0+10])
+        Pexp_constant PConst_string(\"test\",(test.ml[1,0+5]..[1,0+9]),None)
+  ]
+  
+  
+  ",
+    "notifications": []
+  }