[nullsafe] Look at NullsafeType in check_return_annotation instead of looking it up in Annot.Item.t

Summary:
This proceed the work of getting rid of Annot.Item.t.
This diff:
- Moves "check assignment rule" to recently supported NullsafeRules
- Implements their own "check overannotated" (defers consolidating this
check into NullsafeRule for the future diffs).

Note that we don't need PropagatesNullable logic anymore because it is
already ported to NullsafeType (return value will be marked as Nullable
in NullsafeType)!

implicit_nullable (a.k.a. Void types) will require a follow up diff to
model.

Reviewed By: artempyanykh

Differential Revision: D17499246

fbshipit-source-id: 14b473f29

infer/src/nullsafe/eradicateChecks.ml

@@ -262,18 +262,40 @@ let check_constructor_initialization tenv find_canonical_duplicate curr_pname cu
         ()
 
 
+let check_return_not_nullable tenv find_canonical_duplicate loc curr_pname curr_pdesc
+    (ret_signature : AnnotatedSignature.ret_signature) ret_inferred_nullability =
+  if
+    not
+      (NullsafeRules.passes_assignment_rule ~lhs:ret_signature.ret_nullsafe_type.nullability
+         ~rhs:ret_inferred_nullability)
+  then
+    report_error tenv find_canonical_duplicate
+      (TypeErr.Return_annotation_inconsistent
+         (curr_pname, InferredNullability.descr_origin ret_inferred_nullability))
+      None loc curr_pdesc
+
+
+(* TODO(T54308240) Consolidate return over annotated checks in NullsafeRules *)
+let check_return_overrannotated tenv find_canonical_duplicate loc curr_pname curr_pdesc
+    (ret_signature : AnnotatedSignature.ret_signature) ret_inferred_nullability =
+  (* TODO(T54308240) this needs to be changed when we introduce Unknown nullability *)
+  let is_ret_inferred_nonnull = not (InferredNullability.is_nullable ret_inferred_nullability) in
+  let is_ret_annotated_nullable =
+    match ret_signature.ret_nullsafe_type.nullability with
+    | NullsafeType.Nonnull _ ->
+        false
+    | NullsafeType.Nullable _ ->
+        true
+  in
+  if is_ret_inferred_nonnull && is_ret_annotated_nullable then
+    report_error tenv find_canonical_duplicate (TypeErr.Return_over_annotated curr_pname) None loc
+      curr_pdesc
+
+
 (** Check the annotations when returning from a method. *)
 let check_return_annotation tenv find_canonical_duplicate curr_pdesc ret_range
     (annotated_signature : AnnotatedSignature.t) ret_implicitly_nullable loc : unit =
-  let AnnotatedSignature.{ret_annotation_deprecated} = annotated_signature.ret in
   let curr_pname = Procdesc.get_proc_name curr_pdesc in
-  let ret_annotated_nullable =
-    Annotations.ia_is_nullable ret_annotation_deprecated
-    || List.exists
-         ~f:(fun AnnotatedSignature.{param_annotation_deprecated} ->
-           Annotations.ia_is_propagates_nullable param_annotation_deprecated )
-         annotated_signature.params
-  in
   match ret_range with
   (* Disables the warnings since it is not clear how to annotate the return value of lambdas *)
   | Some _
@@ -283,22 +305,14 @@ let check_return_annotation tenv find_canonical_duplicate curr_pdesc ret_range
          | _ ->
              false ->
       ()
-  | Some (_, final_inferred_nullability, _) ->
-      let is_final_nullable = InferredNullability.is_nullable final_inferred_nullability in
-      let origin_descr = InferredNullability.descr_origin final_inferred_nullability in
-      let return_not_nullable =
-        is_final_nullable && (not ret_annotated_nullable) && not ret_implicitly_nullable
-      in
-      let return_over_annotated =
-        (not is_final_nullable) && ret_annotated_nullable && Config.eradicate_return_over_annotated
-      in
-      if return_not_nullable then
-        report_error tenv find_canonical_duplicate
-          (TypeErr.Return_annotation_inconsistent (curr_pname, origin_descr))
-          None loc curr_pdesc ;
-      if return_over_annotated then
-        report_error tenv find_canonical_duplicate (TypeErr.Return_over_annotated curr_pname) None
-          loc curr_pdesc
+  | Some (_, ret_inferred_nullability, _) ->
+      (* TODO(T54308240) Model ret_implicitly_nullable in NullsafeType.t *)
+      if not ret_implicitly_nullable then
+        check_return_not_nullable tenv find_canonical_duplicate loc curr_pname curr_pdesc
+          annotated_signature.ret ret_inferred_nullability ;
+      if Config.eradicate_return_over_annotated then
+        check_return_overrannotated tenv find_canonical_duplicate loc curr_pname curr_pdesc
+          annotated_signature.ret ret_inferred_nullability
   | None ->
       ()