Ignore ESC K VT code emitted by GCC

GCC emits coloured output when it's printing to a tty, and coloured
output is disabled e.g., when the output is piped.
When GCC is called by Dune, it doesn't output ANSI sequences.

However, that behaviour can be forced with the
`-fdiagnostics-color=always` flag. GCC has the particularity that
after each color sequence, it will also emit a ESC K "erase from the
cursor the end of line".

Dune did not support this sequence, and this messes up the output. The
sequence does not seem important; clang doesn't use it, and it
clearing the line means that it would already have been written to,
which seems unlikely. So, this patch simply ignores that sequence.

Closes ocaml#3263.

Signed-off-by: Antonin Décimo <antonin@tarides.com>

otherlibs/stdune-unstable/ansi_color.ml

@@ -123,6 +123,12 @@ module Style = struct
     Printf.sprintf "\027[%sm" (String.concat l ~sep:";")
 end
 
+module Codes = struct
+  type t = char
+
+  let erase_from_cursor_to_eol = 'K'
+end
+
 let term_supports_color =
   lazy
     (match Stdlib.Sys.getenv "TERM" with
@@ -205,6 +211,8 @@ let parse_line str styles =
       let seq_start = seq_start + 2 in
       if seq_start >= len || str.[seq_start - 1] <> '[' then
         (styles, acc)
+      else if str.[seq_start] = Codes.erase_from_cursor_to_eol then
+        loop styles (seq_start + 1) acc
       else
         match String.index_from str seq_start 'm' with
         | None -> (styles, acc)

otherlibs/stdune-unstable/ansi_color.mli

@@ -84,6 +84,12 @@ end
 val make_printer :
   bool Lazy.t -> Format.formatter -> (Style.t list Pp.t -> unit) Staged.t
 
+module Codes : sig
+  type t
+
+  val erase_from_cursor_to_eol : t
+end
+
 (** Print to [Format.std_formatter] *)
 val print : Style.t list Pp.t -> unit