[3.8] backport #7949 and #7950 (#7975)

* test: reproduce #7905 (#7949)

Signed-off-by: Rudi Grinberg <[email protected]>

* fix: correctly print multi-line excerpts (#7950)

The previous code would pretend that a multi-line excerpt was
single-line whenever the stop character was outside the line.

Fixes #7905

Signed-off-by: Rudi Grinberg <[email protected]>

---------

Signed-off-by: Rudi Grinberg <[email protected]>
Co-authored-by: Rudi Grinberg <[email protected]>

CHANGES.md

@@ -12,6 +12,9 @@
 - Fix RPC buffer corruption issues due to multi threading. This issue was only
   reproducible with large RPC payloads (#7418)
 
+- Fix printing errors from excerpts whenever character offsets span multiple
+  lines (#7950, fixes #7905, @rgrinberg)
+
 3.8.1 (2023-06-05)
 ------------------
 

otherlibs/stdune/src/loc.ml

@@ -57,21 +57,24 @@ let pp_file_excerpt ~context_lines ~max_lines_to_print_in_full { start; stop } :
   let stop_c = stop.pos_cnum - start.pos_bol in
   let file = start.pos_fname in
   let pp_file_excerpt () =
-    let line_num = start.pos_lnum in
-    let line_num_str = string_of_int line_num in
-    let padding_width = String.length line_num_str in
     let open Result.O in
-    let* line =
-      Result.try_with (fun () -> Io.String_path.file_line file line_num)
-    in
-    if stop_c <= String.length line then
+    match
+      if start.pos_lnum <> stop.pos_lnum then `Multiline else `Singleline
+    with
+    | `Singleline ->
+      let line_num = start.pos_lnum in
+      let line_num_str = string_of_int line_num in
+      let padding_width = String.length line_num_str in
+      let* line =
+        Result.try_with (fun () -> Io.String_path.file_line file line_num)
+      in
       let len = stop_c - start_c in
       let open Pp.O in
       Ok
         (pp_line padding_width (line_num_str, line)
         ++ pp_left_pad (stop_c + padding_width + 3) (String.make len '^')
         ++ Pp.newline)
-    else
+    | `Multiline ->
       let get_padding lines =
         let lnum, _ = Option.value_exn (List.last lines) in
         String.length lnum

otherlibs/stdune/test/loc_tests.ml

@@ -0,0 +1,24 @@
+open Stdune
+
+let%expect_test "#7905 - inverted char offsets" =
+  let dir = Temp.create Dir ~prefix:"" ~suffix:"loc" in
+  let file = Path.relative dir "file.ml" in
+  Io.write_file file {|
+type t = A | B
+
+let f () () = function
+  | A -> ()
+|};
+  let pos_fname = Path.to_string file in
+  let start = { Lexing.pos_fname; pos_lnum = 4; pos_bol = 0; pos_cnum = 14 } in
+  let stop = { start with pos_lnum = 5; pos_cnum = 11 } in
+  let loc = { Loc.start; stop } in
+  Format.printf "%a@." Pp.to_fmt (Loc.pp loc);
+  let output =
+    [%expect.output] |> String.split_lines |> List.tl |> String.concat ~sep:"\n"
+  in
+  Temp.destroy Dir file;
+  print_endline output;
+  [%expect {|
+    4 | let f () () = function
+    5 |   | A -> () |}]