fix offset lines calculation

Author: zth

tests/tools_tests/src/expected/FormatDocstringsTestError.res.expected

@@ -1,10 +1,11 @@
 
   Syntax error in code block in docstring
-  /Users/zth/OSS/rescript-compiler/tests/tools_tests/src/docstrings-format/FormatDocstringsTestError.res:2:10-3:3
+  /Users/zth/OSS/rescript-compiler/tests/tools_tests/src/docstrings-format/FormatDocstringsTestError.res:5:10-6:3
 
-  1 │ 
-  2 │ let name= 
-  3 │ let x=12
+  3 │ 
+  4 │ 
+  5 │ let name= 
+  6 │ let x=12
 
   This let-binding misses an expression
 

tools/src/tools.ml

@@ -752,12 +752,12 @@ module FormatDocstrings = struct
       mapRescriptCodeBlocks
         ~colIndent:(payloadLoc.loc_start.pos_cnum - payloadLoc.loc_start.pos_bol)
         ~mapper:(fun code currentLine ->
-          (* TODO: Figure out the line offsets here so the error messages line up as intended. *)
+          let codeLines = String.split_on_char '\n' code in
+          let n = List.length codeLines in
           let newlinesNeeded =
-            payloadLoc.loc_start.pos_lnum + currentLine - 5
+            max 0 (payloadLoc.loc_start.pos_lnum + currentLine - n)
           in
-          let codeOffset = String.make newlinesNeeded '\n' in
-          let codeWithOffset = codeOffset ^ code in
+          let codeWithOffset = String.make newlinesNeeded '\n' ^ code in
           let formatted_code =
             let {Res_driver.parsetree; comments; invalid; diagnostics} =
               Res_driver.parse_implementation_from_source ~for_printer:true