Make it an error for lang >= 3.0

Signed-off-by: Etienne Millon <me@emillon.org>

src/dune_lang/syntax.ml

@@ -32,7 +32,8 @@ module Version = struct
       match Scanf.sscanf s "%u.%u%s" (fun a b s -> ((a, b), s)) with
       | Ok (v, "") -> v
       | Ok (((a, b) as v), s) ->
-        User_warning.emit ~loc
+        let is_error = v >= (3, 0) in
+        User_warning.emit ~loc ~is_error
           [ Pp.textf "The %S part is ignored here." s
           ; Pp.textf "This version is parsed as just %d.%d." a b
           ];

test/blackbox-tests/test-cases/lang-dune-warning.t

@@ -1,4 +1,5 @@
-If (lang dune) does not use a valid format, a warning is emitted:
+If (lang dune) does not use a valid format on a 2.x project, a warning is
+emitted:
 
   $ cat > dune-project << EOF
   > (lang dune 2.3.0)
@@ -22,10 +23,32 @@ If (lang dune) does not use a valid format, a warning is emitted:
   Warning: The "suffix" part is ignored here.
   This version is parsed as just 2.4.
 
-Of course if the version is valid, no warning is emitted:
+If the version is valid, no warning is emitted:
 
   $ cat > dune-project << EOF
   > (lang dune 2.2)
   > EOF
 
   $ dune build
+
+Starting with lang 3.0, the warning turns into an error.
+
+  $ cat > dune-project << EOF
+  > (lang dune 3.0suffix)
+  > EOF
+
+  $ dune build
+  File "dune-project", line 1, characters 11-20:
+  1 | (lang dune 3.0suffix)
+                 ^^^^^^^^^
+  Error: The "suffix" part is ignored here.
+  This version is parsed as just 3.0.
+  [1]
+
+And without suffix it is accepted.
+
+  $ cat > dune-project << EOF
+  > (lang dune 3.0)
+  > EOF
+
+  $ dune build