dune watch rpc is sending off by minus one lines inconsistently

[mattiasdrp]

Expected Behavior

Dune RPC should return the same diagnostic as the OCaml compiler

Actual Behavior

Some lines are off by one but it doesn't look like it's consistent

Reproduction

I have this simple file:

module type A = sig
  val a : int
  val b : float
end

module M (A : A) = struct
  let a = A.a + A.a
end

module AI = struct
  let a = 3
  let b = 4
end

module MI = M (AI)

When compiling I have the following error:

File "bin/main.ml", line 15, characters 12-18:
15 | module MI = M (AI)
                 ^^^^^^
Error: Modules do not match: sig val a : int val b : int end
     is not included in A
     Values do not match: val b : int is not included in val b : float
     The type int is not compatible with the type float
     File "bin/main.ml", line 3, characters 2-15: Expected declaration
     File "bin/main.ml", line 12, characters 6-7: Actual declaration

If I replace A.a + A.a with A.a + A.b I have the following compilation error:

File "bin/main.ml", line 7, characters 16-19:
7 |   let a = A.a + A.b
                    ^^^
Error: The value A.b has type float but an expression was expected of type
         int

But if I start an LSP server with a dune watch process I receive the following diagnostic:

("/home/mattias/test_ocaml/bin/main.ml"
 ((:message
   "The value \"A.b\" has type \"float\" but an expression was expected of type\n  \"int\""
   :range (:end (:character 19 :line 6) :start (:character 16 :line 6))
   :severity 1 :source "dune")
  (:message
   "Modules do not match: sig val a : int val b : int end is not included in \nA\nValues do not match: val b : int is not included in val b : float\nThe type int is not compatible with the type float"
   :range (:end (:character 18 :line 14) :start (:character 12 :line 14))
   :relatedInformation
   [(:location
     (:range (:end (:character 15 :line 3) :start (:character 2 :line 3)) :uri
             "file:///home/mattias/test_ocaml/bin/main.ml")
     :message "Expected declaration")
    (:location
     (:range (:end (:character 7 :line 12) :start (:character 6 :line 12)) :uri
             "file:///home/mattias/test_ocaml/bin/main.ml")
     :message "Actual declaration")]
   :severity 1 :source "ocamllsp")))

As you can see

• the first error is located at line 6, off by -1
• the second error is located at line 14, off by -1
  • the related information are located at line 3 and 12

Now if I fix this file and create its corresponding mli file with the following content

module type A = sig
  val a : int 
  val b : int
end

I receive the following diagnostic:

("/home/mattias/test_ocaml/bin/main.ml"
 ((:message
   "The implementation \"bin/main.ml\"\ndoes not match the interface \"bin/main.ml\": \nModule type declarations do not match:\n  module type A = sig val a : int val b : float end\ndoes not match\n  module type A = sig val a : int val b : int end\nAt position \"module type A = <here>\"\nModule types do not match:\n  sig val a : int val b : float end\nis not equal to\n  sig val a : int val b : int end\nAt position \"module type A = <here>\"\nValues do not match: val b : float is not included in val b : int\nThe type \"float\" is not compatible with the type \"int\""
   :range (:end (:character 0 :line 0) :start (:character 0 :line 0))
   :relatedInformation
   [(:location
     (:range (:end (:character 13 :line 2) :start (:character 2 :line 2)) :uri
             "file:///home/mattias/test_ocaml/bin/main.mli")
     :message "Expected declaration")
    (:location
     (:range (:end (:character 15 :line 2) :start (:character 2 :line 2)) :uri
             "file:///home/mattias/test_ocaml/bin/main.ml")
     :message "Actual declaration")]
   :severity 1 :source "dune")))

This time, the related information are located at line 2, off by -1.

This inconsistent behaviour prevents me from saying "increment line by one unless you're formatting a related information part".

Specifications

• Version of dune (output of dune --version): 3.17.2
• Version of ocaml (output of ocamlc --version): 5.3.0
• Operating system (distribution and version):

  OS: cachyos rolling
  Kernel: x86_64 Linux 6.14.2-2-cachyos
  

[maiste]

Thanks for reporting.

Does the problem also occur if you run dune build and launch the lsp without the watch mode running in background?

[mattiasdrp]

No, if I just compile with dune build I have the proper error reporting because they're reported by the ocaml-lsp and not by the dune watch rpc. Here is the same error reported by dune and ocamllsp:

dune (notice line 4 and 13)

(:message "Signature mismatch:
Modules do not match:
  sig val a : int val b : float end
is not included in
  A
Values do not match:
  val b : float
is not included in
  val b :
    int ->
    string -> string -> string -> string -> string -> string -> string
The type \"float\" is not compatible with the type
  \"int ->
  string -> string -> string -> string -> string -> string -> string\""
           :range (:end (:character 3 :line 14) :start (:character 16 :line 11))
           :relatedInformation
           [(:location
             (:range
              (:end (:character 77 :line 4) :start (:character 2 :line 3)) :uri
              "file:///home/mattias/test_ocaml/bin/main.ml")
             :message "Expected declaration")
            (:location
             (:range
              (:end (:character 7 :line 13) :start (:character 6 :line 13)) :uri
              "file:///home/mattias/test_ocaml/bin/main.ml")
             :message "Actual declaration")]
           :severity 1 :source "dune")

ocamllsp (notice line 5 and 14)

 (:message "Signature mismatch:
Modules do not match: sig val a : int val b : float end is not included in A
Values do not match:
  val b : float
is not included in
  val b :
    int -> string -> string -> string -> string -> string -> string -> string
The type float is not compatible with the type
  int -> string -> string -> string -> string -> string -> string -> string"
           :range (:end (:character 3 :line 14) :start (:character 16 :line 11))
           :relatedInformation
           [(:location
             (:range
              (:end (:character 77 :line 5) :start (:character 2 :line 4)) :uri
              "file:///home/mattias/test_ocaml/bin/main.ml")
             :message "Expected declaration")
            (:location
             (:range
              (:end (:character 7 :line 14) :start (:character 6 :line 14)) :uri
              "file:///home/mattias/test_ocaml/bin/main.ml")
             :message "Actual declaration")]
           :severity 1 :source "ocamllsp")