feat(slang): when with Nil body is Nil (#13773)

Author: Alizter

src/dune_lang/slang.ml

@@ -305,6 +305,7 @@ let rec simplify = function
          let combined_sw = String_with_vars.make ~quoted loc parts in
          Literal combined_sw)
        else Form (loc, Concat (List.map xs ~f:simplify))
+     | When (_, Nil) -> Nil
      | When (condition, t) ->
        let simplified_condition = simplify_blang condition in
        (match (simplified_condition : blang) with

test/expect-tests/dune_lang/slang_tests.ml

@@ -326,10 +326,9 @@ let%expect_test "when unknown" =
   [%expect {| When (Expr (Literal (template "%{pkg-self:x}")), Literal "hello") |}]
 ;;
 
-(* CR-soon Alizter: when with nil body should reduce to Nil *)
 let%expect_test "when unknown nil" =
   print_slang (Slang.when_ (expr (pform "x")) Slang.Nil);
-  [%expect {| When (Expr (Literal (template "%{pkg-self:x}")), Nil) |}]
+  [%expect {| Nil |}]
 ;;
 
 (* Slang: If *)