Print a new line between error messages

Signed-off-by: Benoît Montagu <benoit.montagu@inria.fr>

bin/common.ml

@@ -522,6 +522,7 @@ module Builder = struct
     ; workspace_config : Dune_rules.Workspace.Clflags.t
     ; cache_debug_flags : Dune_engine.Cache_debug_flags.t
     ; report_errors_config : Dune_engine.Report_errors_config.t
+    ; separate_error_messages : bool
     ; require_dune_project_file : bool
     ; insignificant_changes : [ `React | `Ignore ]
     ; build_dir : string
@@ -805,6 +806,14 @@ module Builder = struct
             ~doc:
               "react to insignificant file system changes; this is only useful \
                for benchmarking dune")
+    and+ separate_error_messages =
+      Arg.(
+        value & flag
+        & info
+            [ "separate-error-messages" ]
+            ~doc:
+              "Separate error messages with a blank line. This feature is \
+               always disabled when watch mode is enabled.")
     in
     { debug_dep_path
     ; debug_findlib
@@ -840,6 +849,11 @@ module Builder = struct
         }
     ; cache_debug_flags
     ; report_errors_config
+    ; separate_error_messages =
+        (* this flag is ALWAYS DISABLED when dune works in watch mode *)
+        (match watch with
+        | Yes _ -> false
+        | No -> separate_error_messages)
     ; require_dune_project_file
     ; insignificant_changes =
         (if react_to_insignificant_changes then `React else `Ignore)
@@ -1031,6 +1045,8 @@ let init ?log_file c =
     [ Pp.textf "Workspace root: %s"
         (Path.to_absolute_filename Path.root |> String.maybe_quoted)
     ];
+  Dune_util.Report_error.separate_error_messages
+    c.builder.separate_error_messages;
   config
 
 let footer =

src/dune_util/report_error.ml

@@ -135,6 +135,37 @@ let format_memo_stack pps =
                    (Pp.seq (Pp.verbatim "-> ")
                       (Pp.seq (Pp.text "required by ") pp))))))
 
+(* flag that controls whether error messages should be separated by a
+   blank line *)
+let separate_error_messages_flag = ref false
+
+let separate_error_messages b = separate_error_messages_flag := b
+
+(* If the flag [separate_error_messages] is off, then [print_separator
+   ()] does nothing. If the flag [separate_error_messages] is on, then
+   [print_separator ()] does nothing the first time it is called,
+   whereas subsequent calls print a new line. *)
+let print_separator =
+  let first_error = ref true
+  and msg =
+    { User_message.paragraphs = [ Pp.cut ]
+    ; hints = []
+    ; annots = User_message.Annots.empty
+    ; loc = None
+    }
+  in
+  fun () ->
+    if !separate_error_messages_flag (* only do something when the flag is on *)
+    then
+      if !first_error then
+        (* do not print anything the first time the function is
+           called, but remember it has been called at least once *)
+        first_error := false
+      else
+        (* if the function has already been called at least once,
+           print a blank line *)
+        Console.print_user_message msg
+
 let report { Exn_with_backtrace.exn; backtrace } =
   let exn, memo_stack =
     match exn with
@@ -194,4 +225,5 @@ let report { Exn_with_backtrace.exn; backtrace } =
       | User -> msg
       | Developer -> append msg (i_must_not_crash ())
     in
+    print_separator ();
     Console.print_user_message msg

src/dune_util/report_error.mli

@@ -1,6 +1,10 @@
 (** Error reporting *)
 open Stdune
 
+(** [separate_error_messages b] changes the behavior of [report], so that it
+    separates error messages with a blank line when [b = true]. *)
+val separate_error_messages : bool -> unit
+
 (** Reports an error.
 
     Because dune doesn't stop at the first error, it might end up reporting the

test/blackbox-tests/test-cases/error_messages_separated.t

@@ -0,0 +1,95 @@
+Tests for how error messages are displayed
+==========================================
+
+The purpose of these tests is to check that errors are displayed with
+a separating blank line (issue #6158, PR #6823).
+
+Test setup
+----------
+
+  $ cat >dune-project <<EOF
+  > (lang dune 2.3)
+  > EOF
+
+  $ cat >dune <<EOF
+  > (library
+  >  (name lib))
+  > EOF
+
+  $ cat >a.ml <<EOF
+  > let f x y z = ()
+  > (* this should produce 3 warnings for unused variables *)
+  > EOF
+
+  $ cat >b.ml <<EOF
+  > let () = 1
+  > (* this should produce a type error *)
+  > EOF
+
+  $ cat >c.ml <<EOF
+  > let x = +
+  > (* this should produce a syntax error *)
+  > EOF
+
+Actual tests
+------------
+
+We check that the errors reported for different files are separated by
+blank lines. If a file generates several errors (which is the case for
+the `a.ml` file, then no blank lines are inserted between them,
+because this is the exact message that is reported by the Ocaml
+compiler, and we do not parse or modify such messages.
+
+Without the --separate-error-messages flag, no blank line is put
+between error messages for different files, as expected.
+
+  $ dune build
+  File "c.ml", line 3, characters 0-0:
+  Error: Syntax error
+  File "a.ml", line 1, characters 6-7:
+  1 | let f x y z = ()
+            ^
+  Error (warning 27 [unused-var-strict]): unused variable x.
+  File "a.ml", line 1, characters 8-9:
+  1 | let f x y z = ()
+              ^
+  Error (warning 27 [unused-var-strict]): unused variable y.
+  File "a.ml", line 1, characters 10-11:
+  1 | let f x y z = ()
+                ^
+  Error (warning 27 [unused-var-strict]): unused variable z.
+  File "b.ml", line 1, characters 9-10:
+  1 | let () = 1
+               ^
+  Error: This expression has type int but an expression was expected of type
+           unit
+  [1]
+
+Wit the --separate-error-messages flag, a blank line is put between
+error messages for different files. No blank line is inserted before
+the forst message, and no blank line is inserted after the last
+message either.
+
+  $ dune build --separate-error-messages
+  File "c.ml", line 3, characters 0-0:
+  Error: Syntax error
+
+  File "a.ml", line 1, characters 6-7:
+  1 | let f x y z = ()
+            ^
+  Error (warning 27 [unused-var-strict]): unused variable x.
+  File "a.ml", line 1, characters 8-9:
+  1 | let f x y z = ()
+              ^
+  Error (warning 27 [unused-var-strict]): unused variable y.
+  File "a.ml", line 1, characters 10-11:
+  1 | let f x y z = ()
+                ^
+  Error (warning 27 [unused-var-strict]): unused variable z.
+
+  File "b.ml", line 1, characters 9-10:
+  1 | let () = 1
+               ^
+  Error: This expression has type int but an expression was expected of type
+           unit
+  [1]