Fix index out of bounds exception in rescript-editor-analysis codeAction (#7523)

* Check offset >= 0 before doing text.[offset]

* Check offsetEnd >= offsetStart >=0 before doing `String.sub text offsetStart (offsetEnd - offsetStart)`

* Ensure Pos.positionToOffset does not return negative offsets

* Add CHANGELOG

Author: mediremi

CHANGELOG.md

@@ -23,6 +23,7 @@
 
 - `rescript-tools doc` no longer includes shadowed bindings in its output. https://github.com/rescript-lang/rescript/pull/7497
 - Treat `throw` like `raise` in analysis. https://github.com/rescript-lang/rescript/pull/7521
+- Fix `index out of bounds` exception thrown in rare cases by `rescript-editor-analysis.exe codeAction` command. https://github.com/rescript-lang/rescript/pull/7523
 
 #### :nail_care: Polish
 

analysis/src/CompletionFrontEnd.ml

@@ -373,7 +373,7 @@ let completionWithParser1 ~currentFile ~debug ~offset ~path ~posCursor
   in
   let posOfDot = Pos.posOfDot text ~pos:posCursor ~offset in
   let charAtCursor =
-    if offset < String.length text then text.[offset] else '\n'
+    if offset >= 0 && offset < String.length text then text.[offset] else '\n'
   in
   let posBeforeCursor = Pos.posBeforeCursor posCursor in
   let charBeforeCursor, blankAfterCursor =
@@ -869,7 +869,8 @@ let completionWithParser1 ~currentFile ~debug ~offset ~path ~posCursor
        match
          (Pos.positionToOffset text posStart, Pos.positionToOffset text posEnd)
        with
-       | Some offsetStart, Some offsetEnd ->
+       | Some offsetStart, Some offsetEnd
+         when offsetStart >= 0 && offsetEnd >= offsetStart ->
          (* Can't trust the parser's location
             E.g. @foo. let x... gives as label @foo.let *)
          let label =

analysis/src/Pos.ml

@@ -22,8 +22,8 @@ let positionToOffset text (line, character) =
   match offsetOfLine text line with
   | None -> None
   | Some bol ->
-    if bol + character <= String.length text then Some (bol + character)
-    else None
+    let offset = bol + character in
+    if offset >= 0 && offset <= String.length text then Some offset else None
 
 let posBeforeCursor pos = (fst pos, max 0 (snd pos - 1))