Add an -unused-type-warnings flag to the driver

CHANGES.md

@@ -1,6 +1,10 @@
 unreleased
 ----------
 
+- Add `-unused-type-warnings` flag to the driver to allow users to disable
+  the generation of warning 34 silencing structure items when using
+  `[@@deriving ...]` on type declarations. (#493, @NathanReb)
+
 - Fix `Longident.parse` so it also handles indexing operators such as
   `.!()`, `.%(;..)<-`, or `Vec.(.%())` (#494, @octachron)
 

doc/driver.mld

@@ -91,8 +91,22 @@ driver.exe [extra_args] [<files>]
   -no-merge                   Do not merge context free transformations (better for debugging rewriters). As a result, the context-free transformations are not all applied before all impl and intf.
   -cookie NAME=EXPR           Set the cookie NAME to EXPR
   --cookie                    Same as -cookie
-  -help                       Display this list of options
-  --help                      Display this list of options
+  -deriving-keep-w32 {impl|intf|both}
+                               Do not try to disable warning 32 for the generated code
+  -deriving-disable-w32-method {code|attribute}
+                               How to disable warning 32 for the generated code
+  -type-conv-keep-w32 {impl|intf|both}
+                               Deprecated, use -deriving-keep-w32
+  -type-conv-w32 {code|attribute}
+                               Deprecated, use -deriving-disable-w32-method
+  -deriving-keep-w60 {impl|intf|both}
+                               Do not try to disable warning 60 for the generated code
+  -unused-code-warnings _      Allow ppx derivers to enable unused code warnings
+  -unused-type-warnings {true|false}
+                               Allow unused type warnings for types with [@@deriving ...]
+  -help                        Display this list of options
+  --help                       Display this list of options
+
 v}
 {%html: </details>%}
 

src/deriving.ml

@@ -67,6 +67,15 @@ let () =
     ~doc:"_ Allow ppx derivers to enable unused code warnings"
 
 let allow_unused_code_warnings () = !allow_unused_code_warnings
+let allow_unused_type_warnings = ref Options.default_allow_unused_type_warnings
+
+let () =
+  Driver.add_arg "-unused-type-warnings"
+    (Bool (( := ) allow_unused_type_warnings))
+    ~doc:
+      "{true|false} Allow unused type warnings for types with [@@deriving ...]"
+
+let allow_unused_type_warnings () = !allow_unused_type_warnings
 
 module Args = struct
   include (
@@ -701,7 +710,7 @@ let wrap_sig ~loc ~hide list =
    +-----------------------------------------------------------------+ *)
 
 let types_used_by_deriving (tds : type_declaration list) : structure_item list =
-  if keep_w32_impl () then []
+  if keep_w32_impl () || allow_unused_type_warnings () then []
   else
     List.map tds ~f:(fun td ->
         let typ = Common.core_type_of_type_declaration td in

src/options.ml

@@ -1,4 +1,5 @@
 let default_allow_unused_code_warnings = false
+let default_allow_unused_type_warnings = false
 let perform_checks = false
 
 (* The checks on extensions are only to get better error messages

test/deriving_warning/run.t

@@ -6,6 +6,9 @@ triggering warnings, we also added optional arguments to `Deriving.make` so that
 the ppx themselves can declare whether they need the driver to disable warnings
 or not.
 
+The following tests describe the behaviour of flags and features used to control
+the emission of such warning silencing features.
+
 One such flag and optional argument pair is the `-unused-code-warnings` flag and
 `?unused_code_warning` `Deriving.V2.make` argument. Both of those default to
 `false` and control whether we disable warnings 32 and 60 for generated code
@@ -26,7 +29,7 @@ We have a driver with 4 derivers linked in:
 - two_do_warn
 - alias_warn
 
-----------------------------------------
+--------------------------------------------------------------------------------
 
 Let's consider the following ocaml source file using the zero_do_warn deriver
 
@@ -96,7 +99,7 @@ and compare the result of both driver invocations:
                                                                   "@inline"]
   [@@merlin.hide ]
 
------------------------------------------------
+--------------------------------------------------------------------------------
 
 The default value of the -unused-code-warnings should be false:
 
@@ -116,7 +119,7 @@ The default value of the -unused-code-warnings should be false:
 As we can see here, the warnings were disabled by the driver, as is expected
 with -unused-code-warnings=false.
 
------------------------------------------------
+--------------------------------------------------------------------------------
 
 Let's consider the following ocaml source file using the one_no_warn deriver
 
@@ -177,7 +180,7 @@ Same goes for .mli files:
       val one : Zero.t One.t
     end[@@ocaml.doc "@inline"][@@merlin.hide ]
 
--------------------------------------------------
+--------------------------------------------------------------------------------
 
 The alias_warn deriver is in fact an alias for two derivers:
 - alias_do_warn, which is registered with unused_code_warnings=true
@@ -235,3 +238,55 @@ Same goes for .mli:
   include sig [@@@ocaml.warning "-32"] val unit_two : unit end[@@ocaml.doc
                                                                 "@inline"]
   [@@merlin.hide ]
+
+--------------------------------------------------------------------------------
+
+Whenever a set of types has a [@@deriving ...] attached, ppxlib's driver always
+generate structure items meant to disable unused type warnings (warning 34) for
+any of those types.
+
+Let's consider the following piece of OCaml code:
+
+  $ cat > unused_types.ml << EOF
+  > type t = int
+  > and u = string
+  > [@@deriving zero_do_warn]
+  > EOF
+
+If we run the driver:
+
+  $ ./driver.exe -impl unused_types.ml
+  type t = int
+  and u = string[@@deriving zero_do_warn]
+  include struct let _ = fun (_ : t) -> ()
+                 let _ = fun (_ : u) -> () end[@@ocaml.doc "@inline"][@@merlin.hide
+                                                                      ]
+  include
+    struct
+      [@@@ocaml.warning "-60"]
+      module Zero = struct type t =
+                             | T0  end
+      let zero = Zero.T0
+      let _ = zero
+    end[@@ocaml.doc "@inline"][@@merlin.hide ]
+
+We can see that the driver generated two `let _ = fun (_ : ...`, one for each type
+in the set.
+
+We have a flag that disable this behaviour and allows unused type warnings to be
+reported properly. Passing that flag to the driver should remove the two previously
+mentionned items.
+
+  $ ./driver.exe -unused-type-warnings=true -impl unused_types.ml
+  type t = int
+  and u = string[@@deriving zero_do_warn]
+  include
+    struct
+      [@@@ocaml.warning "-60"]
+      module Zero = struct type t =
+                             | T0  end
+      let zero = Zero.T0
+      let _ = zero
+    end[@@ocaml.doc "@inline"][@@merlin.hide ]
+
+As you can see, turning on the flag disabled that behaviour as expected.