[flow] Replace RCustom "providers" with an identifier description

Summary:
We should never show providers as the reason to the user. Instead, in all the call sites, we have access to the identifier name, so we should use that as reason description instead.

In these cases, it doesn't quite matter, since the reason is only accessible from typed ast, but it's still better than RCustom.

Changelog: [internal]

Reviewed By: gkz

Differential Revision: D56714487

fbshipit-source-id: 5496073588df5c08dd21d4d20b7061a58c563abc

src/typing/destructuring.ml

@@ -307,7 +307,7 @@ module Make (Statement : Statement_sig.S) : Destructuring_sig.S = struct
     let f ~use_op ~name_loc name _default t =
       (* TODO destructuring+defaults unsupported in assignment expressions *)
       ignore Type_env.(set_var cx ~use_op name t name_loc);
-      Type_env.constraining_type ~default:t cx name_loc
+      Type_env.constraining_type ~default:t cx name name_loc
     in
     pattern cx ~f acc
 end

src/typing/statement.ml

@@ -2407,7 +2407,7 @@ module Make
             init_var cx ~use_op init_t id_loc
           | None -> ()
         end;
-        let ast_t = Type_env.constraining_type ~default:annot_t cx id_loc in
+        let ast_t = Type_env.constraining_type ~default:annot_t cx name id_loc in
         ( (ploc, ast_t),
           Ast.Pattern.Identifier
             {
@@ -2458,6 +2458,7 @@ module Make
                 Type_env.constraining_type
                   ~default:(Type_env.get_var_declared_type cx (OrdinaryName name) name_loc)
                   cx
+                  name
                   name_loc
               )
             in

src/typing/type_env.ml

@@ -703,7 +703,7 @@ let get_var_declared_type ?(lookup_mode = ForValue) ?(is_declared_function = fal
     let env = Context.environment cx in
     provider_type_for_def_loc cx env ~intersect:is_declared_function loc
 
-let constraining_type ~default cx loc =
+let constraining_type ~default cx name loc =
   let { Loc_env.var_info; _ } = Context.environment cx in
   match EnvMap.find_opt_ordinary loc var_info.Env_api.env_entries with
   | Some Env_api.NonAssigningWrite -> default
@@ -718,7 +718,8 @@ let constraining_type ~default cx loc =
     | Some [] ->
       default
     | Some [t] -> t
-    | Some (t1 :: t2 :: ts) -> UnionT (mk_reason (RCustom "providers") loc, UnionRep.make t1 t2 ts))
+    | Some (t1 :: t2 :: ts) ->
+      UnionT (mk_reason (RIdentifier (OrdinaryName name)) loc, UnionRep.make t1 t2 ts))
 
 (*************)
 (*  Writing  *)

src/typing/type_env.mli

@@ -106,7 +106,7 @@ val get_var_declared_type :
   ALoc.t ->
   Type.t
 
-val constraining_type : default:Type.t -> Context.t -> ALoc.t -> Type.t
+val constraining_type : default:Type.t -> Context.t -> string -> ALoc.t -> Type.t
 
 val read_declared_type : Context.t -> Reason.t -> ALoc.t -> Type.t