Stop changing the default warning set of the compiler

src/dune_lang/pform.ml

@@ -137,6 +137,7 @@ module Var = struct
     | Toolchain
     | Pkg of Pkg.t
     | Oxcaml_supported
+    | Dune_warnings
 
   let compare : t -> t -> Ordering.t = Poly.compare
 
@@ -191,7 +192,8 @@ module Var = struct
        | Toolchain -> variant "Toolchain" []
        | Os os -> Os.to_dyn os
        | Pkg pkg -> Pkg.to_dyn pkg
-       | Oxcaml_supported -> variant "Oxcaml_supported" [])
+       | Oxcaml_supported -> variant "Oxcaml_supported" []
+       | Dune_warnings -> variant "Dune_warnings" [])
   ;;
 
   let of_opam_global_variable_name name =
@@ -538,6 +540,7 @@ let encode_to_latest_dune_lang_version t =
        | Os os -> Some (Var.Os.to_string os)
        | Pkg pkg -> Some (Var.Pkg.encode_to_latest_dune_lang_version pkg)
        | Oxcaml_supported -> Some "oxcaml_supported"
+       | Dune_warnings -> Some "dune-warnings"
      with
      | None -> Pform_was_deleted
      | Some name -> Success { name; payload = None })
@@ -753,6 +756,7 @@ module Env = struct
         ; "toolchains", since ~version:(3, 0) Var.Toolchain
         ; ( "oxcaml_supported"
           , since ~what:Oxcaml.syntax ~version:(0, 1) Var.Oxcaml_supported )
+        ; "dune-warnings", since ~version:(3, 21) Var.Dune_warnings
         ]
       in
       let os =

src/dune_lang/pform.mli

@@ -91,6 +91,7 @@ module Var : sig
     | Toolchain
     | Pkg of Pkg.t
     | Oxcaml_supported
+    | Dune_warnings
 
   val compare : t -> t -> Ordering.t
   val to_dyn : t -> Dyn.t

src/dune_rules/expander.ml

@@ -572,6 +572,15 @@ let expand_pform_var (context : Context.t) ~dir ~source (var : Pform.Var.t) =
      let ocaml_version = Ocaml_config.version_string ocaml.ocaml_config in
      [ Value.of_bool (Ocaml.Version.supports_oxcaml ocaml_version) ])
     |> static
+  | Dune_warnings ->
+    Need_full_expander
+      (fun { scope; _ } ->
+        Deps.Without
+          (let open Memo.O in
+           let+ scope = scope in
+           let dune_version = Dune_project.dune_version (Scope.project scope) in
+           let profile = Context.profile context in
+           Value.L.strings (Ocaml_flags.dune_warnings ~dune_version ~profile)))
 ;;
 
 let ocaml_config_macro source macro_invocation context =

src/dune_rules/ocaml_flags.ml

@@ -51,19 +51,25 @@ let vendored_warnings = [ "-w"; "-a" ]
 let vendored_alerts = [ "-alert"; "-all" ]
 let default_warnings = "-40"
 
-let default_flags ~dune_version ~profile =
+let dune_warnings ~dune_version ~profile =
   if Profile.is_dev profile
   then
     [ "-w"
     ; dev_mode_warnings ~dune_version ^ default_warnings
     ; "-strict-sequence"
     ; "-strict-formats"
-    ; "-short-paths"
-    ; "-keep-locs"
     ]
   else [ "-w"; default_warnings ]
 ;;
 
+let default_flags ~dune_version ~profile =
+  if dune_version < (3, 21)
+  then dune_warnings ~dune_version ~profile
+  else if Profile.is_dev profile
+  then [ "-short-paths"; "-keep-locs"; "-warn-error"; "+a" ]
+  else []
+;;
+
 type t = string list Action_builder.t Dune_lang.Ocaml_flags.t
 
 let empty =

src/dune_rules/ocaml_flags.mli

@@ -14,6 +14,12 @@ val make
   -> t
 
 val allow_only_melange : t -> t
+
+val dune_warnings
+  :  dune_version:Dune_lang.Syntax.Version.t
+  -> profile:Profile.t
+  -> string list
+
 val default : dune_version:Dune_lang.Syntax.Version.t -> profile:Profile.t -> t
 val empty : t
 val of_list : string list -> t

test/blackbox-tests/test-cases/dune-warnings.t

@@ -0,0 +1,89 @@
+Black-box testing: we check that warning 32 (unused-value-declaration) is
+enabled by default in the dev profile in Dune version 3.20 but not in 3.21.
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.20)
+  > EOF
+
+  $ cat >main.ml <<EOF
+  > let unused = 42
+  > EOF
+
+  $ cat >main.mli <<EOF
+  > EOF
+
+  $ cat >dune <<EOF
+  > (executable (name main))
+  > EOF
+
+  $ dune build 
+  File "main.ml", line 1, characters 4-10:
+  1 | let unused = 42
+          ^^^^^^
+  Error (warning 32 [unused-value-declaration]): unused value unused.
+  [1]
+
+Now in 3.21...
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.21)
+  > EOF
+
+  $ dune build
+
+Version check for %{dune-warnings}.
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.20)
+  > EOF
+
+  $ cat >dune <<EOF
+  > (executable (name main) (flags :standard %{dune-warnings}))
+  > EOF
+
+  $ dune build
+  File "dune", line 1, characters 41-57:
+  1 | (executable (name main) (flags :standard %{dune-warnings}))
+                                               ^^^^^^^^^^^^^^^^
+  Error: %{dune-warnings} is only available since version 3.21 of the dune
+  language. Please update your dune-project file to have (lang dune 3.21).
+  [1]
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.21)
+  > EOF
+
+  $ dune build
+  File "main.ml", line 1, characters 4-10:
+  1 | let unused = 42
+          ^^^^^^
+  Error (warning 32 [unused-value-declaration]): unused value unused.
+  [1]
+
+What _is_ %{dune-warnings} ?
+
+  $ cat >dune <<EOF
+  > (rule (alias show) (action (echo "%{dune-warnings}\n")))
+  > EOF
+
+  $ dune build @show
+  -w @1..3@5..28@31..39@43@46..47@49..57@61..62@67@69-40 -strict-sequence -strict-formats
+
+Warnings are still fatal in dev mode.
+
+  $ cat >main.ml <<EOF
+  > let f = function 0 -> 1
+  > EOF
+
+  $ cat >dune <<EOF
+  > (executable (name main))
+  > EOF
+
+  $ dune build
+  File "main.ml", line 1, characters 8-23:
+  1 | let f = function 0 -> 1
+              ^^^^^^^^^^^^^^^
+  Error (warning 8 [partial-match]): this pattern-matching is not exhaustive.
+  Here is an example of a case that is not matched:
+  1
+  [1]