PPX v4: mark props type in externals as @live.

Fixes rescript-lang/rescript-vscode#994

Dead code elimination in the editor tooling complains about props never being read in the `props` type record defined by the V4 ppx.
This is because externals don't provide the implementation of the component, and it's in the implementation that props could be read.
This OR marks the `props` type definition as `@live` for external components, so the dead code analysis does not fire.

Author: cristianoc

jscomp/syntax/src/jsx_v4.ml

@@ -327,15 +327,15 @@ let make_label_decls named_type_list =
            Type.field ~loc ~attrs {txt = label; loc}
              (Typ.var @@ safe_type_from_value @@ Labelled label))
 
-let make_type_decls props_name loc named_type_list =
+let make_type_decls ~attrs props_name loc named_type_list =
   let label_decl_list = make_label_decls named_type_list in
   (* 'id, 'className, ... *)
   let params =
     make_props_type_params_tvar named_type_list
     |> List.map (fun core_type -> (core_type, Invariant))
   in
   [
-    Type.mk ~loc ~params {txt = props_name; loc}
+    Type.mk ~attrs ~loc ~params {txt = props_name; loc}
       ~kind:(Ptype_record label_decl_list);
   ]
 
@@ -346,22 +346,26 @@ let make_type_decls_with_core_type props_name loc core_type typ_vars =
       ~manifest:core_type;
   ]
 
+let live_attr = ({txt = "live"; loc = Location.none}, PStr [])
+
 (* type props<'x, 'y, ...> = { x: 'x, y?: 'y, ... } *)
-let make_props_record_type ~core_type_of_attr ~typ_vars_of_core_type props_name
-    loc named_type_list =
+let make_props_record_type ~core_type_of_attr ~external_ ~typ_vars_of_core_type
+    props_name loc named_type_list =
+  let attrs = if external_ then [live_attr] else [] in
   Str.type_ Nonrecursive
     (match core_type_of_attr with
-    | None -> make_type_decls props_name loc named_type_list
+    | None -> make_type_decls ~attrs props_name loc named_type_list
     | Some core_type ->
       make_type_decls_with_core_type props_name loc core_type
         typ_vars_of_core_type)
 
 (* type props<'x, 'y, ...> = { x: 'x, y?: 'y, ... } *)
-let make_props_record_type_sig ~core_type_of_attr ~typ_vars_of_core_type
-    props_name loc named_type_list =
+let make_props_record_type_sig ~core_type_of_attr ~external_
+    ~typ_vars_of_core_type props_name loc named_type_list =
+  let attrs = if external_ then [live_attr] else [] in
   Sig.type_ Nonrecursive
     (match core_type_of_attr with
-    | None -> make_type_decls props_name loc named_type_list
+    | None -> make_type_decls ~attrs props_name loc named_type_list
     | Some core_type ->
       make_type_decls_with_core_type props_name loc core_type
         typ_vars_of_core_type)
@@ -950,8 +954,8 @@ let map_binding ~config ~empty_loc ~pstr_loc ~file_name ~rec_flag binding =
     let named_type_list = List.fold_left arg_to_type [] named_arg_list in
     (* type props = { ... } *)
     let props_record_type =
-      make_props_record_type ~core_type_of_attr ~typ_vars_of_core_type "props"
-        pstr_loc named_type_list
+      make_props_record_type ~core_type_of_attr ~external_:false
+        ~typ_vars_of_core_type "props" pstr_loc named_type_list
     in
     let inner_expression =
       Exp.apply
@@ -1211,8 +1215,8 @@ let transform_structure_item ~config item =
       in
       (* type props<'x, 'y> = { x: 'x, y?: 'y, ... } *)
       let props_record_type =
-        make_props_record_type ~core_type_of_attr ~typ_vars_of_core_type "props"
-          pstr_loc named_type_list
+        make_props_record_type ~core_type_of_attr ~external_:true
+          ~typ_vars_of_core_type "props" pstr_loc named_type_list
       in
       (* can't be an arrow because it will defensively uncurry *)
       let new_external_type =
@@ -1317,9 +1321,10 @@ let transform_signature_item ~config item =
             | [] -> []
             | _ -> [Typ.any ()]))
       in
+      let external_ = psig_desc.pval_prim <> [] in
       let props_record_type =
-        make_props_record_type_sig ~core_type_of_attr ~typ_vars_of_core_type
-          "props" psig_loc named_type_list
+        make_props_record_type_sig ~core_type_of_attr ~external_
+          ~typ_vars_of_core_type "props" psig_loc named_type_list
       in
       (* can't be an arrow because it will defensively uncurry *)
       let new_external_type =

jscomp/syntax/tests/ppx/react/expected/externalWithCustomName.res.txt

@@ -13,6 +13,7 @@ let t = React.createElement(Foo.component, Foo.componentProps(~a=1, ~b={"1"}, ()
 @@jsxConfig({version: 4, mode: "classic"})
 
 module Foo = {
+  @live
   type props<'a, 'b> = {a: 'a, b: 'b}
 
   @module("Foo")
@@ -24,6 +25,7 @@ let t = React.createElement(Foo.component, {a: 1, b: "1"})
 @@jsxConfig({version: 4, mode: "automatic"})
 
 module Foo = {
+  @live
   type props<'a, 'b> = {a: 'a, b: 'b}
 
   @module("Foo")

jscomp/syntax/tests/ppx/react/expected/externalWithRef.res.txt

@@ -18,6 +18,7 @@ module V3 = {
 @@jsxConfig({version: 4, mode: "classic"})
 
 module V4C = {
+  @live
   type props<'x, 'ref> = {
     x: 'x,
     ref?: 'ref,
@@ -28,9 +29,21 @@ module V4C = {
     "component"
 }
 
+module type V4CType = {
+  @live
+  type props<'x, 'y> = {
+    x: 'x,
+    y: 'y,
+  }
+
+  @module("someModule")
+  external make: React.componentLike<props<string, string>, React.element> = "component"
+}
+
 @@jsxConfig({version: 4, mode: "automatic"})
 
 module V4C = {
+  @live
   type props<'x, 'ref> = {
     x: 'x,
     ref?: 'ref,

jscomp/syntax/tests/ppx/react/expected/externalWithTypeVariables.res.txt

@@ -16,6 +16,7 @@ module V3 = {
 @@jsxConfig({version: 4, mode: "classic"})
 
 module V4C = {
+  @live
   type props<'x, 'children> = {
     x: 'x,
     children: 'children,
@@ -28,6 +29,7 @@ module V4C = {
 @@jsxConfig({version: 4, mode: "automatic"})
 
 module V4C = {
+  @live
   type props<'x, 'children> = {
     x: 'x,
     children: 'children,

jscomp/syntax/tests/ppx/react/expected/firstClassModules.res.txt

@@ -88,6 +88,7 @@ module External = {
     type key
     type t
   }
+  @live
   type props<'model, 'selected, 'onChange, 'items> = {
     model: 'model,
     selected: 'selected,

jscomp/syntax/tests/ppx/react/expected/mangleKeyword.res.txt

@@ -47,6 +47,7 @@ module C4C0 = {
   }
 }
 module C4C1 = {
+  @live
   type props<'T_open, 'T_type> = {@as("open") _open: 'T_open, @as("type") _type: 'T_type}
 
   external make: @as("open") React.componentLike<props<string, string>, React.element> = "default"
@@ -68,6 +69,7 @@ module C4A0 = {
   }
 }
 module C4A1 = {
+  @live
   type props<'T_open, 'T_type> = {@as("open") _open: 'T_open, @as("type") _type: 'T_type}
 
   external make: @as("open") React.componentLike<props<string, string>, React.element> = "default"

jscomp/syntax/tests/ppx/react/expected/noPropsWithKey.res.txt

@@ -12,6 +12,7 @@ module V4CA = {
 }
 
 module V4CB = {
+  @live
   type props = {}
 
   @module("c")
@@ -51,6 +52,7 @@ module V4CA = {
 }
 
 module V4CB = {
+  @live
   type props = {}
 
   @module("c")

jscomp/syntax/tests/ppx/react/expected/v4.res.txt

@@ -35,12 +35,14 @@ module type TUncurried = {
 }
 
 module E = {
+  @live
   type props<'x> = {x: 'x}
 
   external make: React.componentLike<props<string>, React.element> = "default"
 }
 
 module EUncurried = {
+  @live
   type props<'x> = {x: 'x}
 
   external make: React.componentLike<props<string>, React.element> = "default"

jscomp/syntax/tests/ppx/react/externalWithRef.res

@@ -18,6 +18,14 @@ module V4C = {
     ) => React.element = "component"
 }
 
+module type V4CType = {
+  @module("someModule") @react.component
+    external make: (
+      ~x: string,
+      ~y: string,
+    ) => React.element = "component"
+}
+
 @@jsxConfig({version: 4, mode: "automatic"})
 
 module V4C = {