[nullsafe][refactor] More functional-style code for building errors

Summary:
We in fact build NullsafeIssue.t by parts, filling needed details until
all the info is provided.

This is nicely expressed via partial functions.

See the next diff that uses this to add additional information to the created
`NullsafeIssue.t`

Reviewed By: skcho

Differential Revision: D23706674

fbshipit-source-id: ca5fac704

infer/src/nullsafe/AssignmentRule.ml

@@ -59,9 +59,24 @@ module ReportableViolation = struct
     else Format.fprintf fmt "(%a)" MarkupFormatter.pp_monospaced name
 
 
-  let mk_description_for_bad_param_passed
+  (* A slight adapter over [NullsafeIssue.make]: the same signature but additionally accepts an alternative method *)
+  let make_issue_with_recommendation ~alternative_method ~description =
+    (* If there is an alternative method to propose, tell about it at the end of the description *)
+    let alternative_recommendation =
+      Option.value_map alternative_method
+        ~f:
+          (Format.asprintf " If you don't expect null, use %a instead."
+             MarkupFormatter.pp_monospaced)
+        ~default:""
+    in
+    let full_description = Format.sprintf "%s%s" description alternative_recommendation in
+    NullsafeIssue.make ~description:full_description
+
+
+  let mk_issue_for_bad_param_passed
       {kind; param_signature; actual_param_expression; param_position; function_procname}
-      ~param_nullability_kind ~nullability_evidence =
+      ~param_nullability_kind ~nullability_evidence
+      ~(make_issue_fn : description:string -> issue_type:IssueType.t -> NullsafeIssue.t) =
     let nullability_evidence_as_suffix =
       Option.value_map nullability_evidence ~f:(fun evidence -> ": " ^ evidence) ~default:""
     in
@@ -79,6 +94,7 @@ module ReportableViolation = struct
         in
         Format.asprintf "%a is %s" MF.pp_monospaced actual_param_expression nullability_descr
     in
+    let issue_type = IssueType.eradicate_parameter_not_nullable in
     match AnnotatedNullability.get_nullability annotated_param_nullability with
     | Nullability.Null ->
         Logging.die Logging.InternalError "Unexpected param nullability: Null"
@@ -105,12 +121,15 @@ module ReportableViolation = struct
             ~default:"the third party signature storage"
         in
         let procname_str = Procname.Java.to_simplified_string ~withclass:true function_procname in
-        Format.asprintf
-          "Third-party %a is missing a signature that would allow passing a nullable to param \
-           #%d%a. Actual argument %s%s. Consider adding the correct signature of %a to %s."
-          MF.pp_monospaced procname_str param_position pp_param_name param_signature.mangled
-          argument_description nullability_evidence_as_suffix MF.pp_monospaced procname_str
-          where_to_add_signature
+        let description =
+          Format.asprintf
+            "Third-party %a is missing a signature that would allow passing a nullable to param \
+             #%d%a. Actual argument %s%s. Consider adding the correct signature of %a to %s."
+            MF.pp_monospaced procname_str param_position pp_param_name param_signature.mangled
+            argument_description nullability_evidence_as_suffix MF.pp_monospaced procname_str
+            where_to_add_signature
+        in
+        make_issue_fn ~description ~issue_type
     | Nullability.LocallyCheckedNonnull
     | Nullability.LocallyTrustedNonnull
     | Nullability.UncheckedNonnull
@@ -127,24 +146,18 @@ module ReportableViolation = struct
                    ~filename)
                 line_number
         in
-        Format.asprintf "%a: parameter #%d%a is declared non-nullable%s but the argument %s%s."
-          MF.pp_monospaced
-          (Procname.Java.to_simplified_string ~withclass:true function_procname)
-          param_position pp_param_name param_signature.mangled nonnull_evidence argument_description
-          nullability_evidence_as_suffix
-
-
-  let get_issue_type = function
-    | PassingParamToFunction _ ->
-        IssueType.eradicate_parameter_not_nullable
-    | AssigningToField _ ->
-        IssueType.eradicate_field_not_nullable
-    | ReturningFromFunction _ ->
-        IssueType.eradicate_return_not_nullable
+        let description =
+          Format.asprintf "%a: parameter #%d%a is declared non-nullable%s but the argument %s%s."
+            MF.pp_monospaced
+            (Procname.Java.to_simplified_string ~withclass:true function_procname)
+            param_position pp_param_name param_signature.mangled nonnull_evidence
+            argument_description nullability_evidence_as_suffix
+        in
+        make_issue_fn ~description ~issue_type
 
 
-  let mk_nullsafe_issue_for_explicitly_nullable_values ~assignment_type ~rhs_origin
-      ~explicit_rhs_nullable_kind ~assignment_location ~nullsafe_mode =
+  let mk_nullsafe_issue_for_explicitly_nullable_values ~assignment_type ~rhs_origin ~nullsafe_mode
+      ~explicit_rhs_nullable_kind ~assignment_location =
     let nullability_evidence =
       get_origin_opt assignment_type rhs_origin
       |> Option.bind ~f:(fun origin -> TypeOrigin.get_description origin)
@@ -152,51 +165,51 @@ module ReportableViolation = struct
     let nullability_evidence_as_suffix =
       Option.value_map nullability_evidence ~f:(fun evidence -> ": " ^ evidence) ~default:""
     in
-    let module MF = MarkupFormatter in
-    let alternative_method_description =
-      ErrorRenderingUtils.find_alternative_nonnull_method_description rhs_origin
-    in
-    let alternative_recommendation =
-      Option.value_map alternative_method_description
-        ~f:(fun descr ->
-          Format.asprintf " If you don't expect null, use %a instead." MF.pp_monospaced descr )
-        ~default:""
+    (* A functor for creating the nullsafe issue: the alternative method, location,
+       and severity are known at this point.
+       The rest to be filled by the client *)
+    let make_issue_fn =
+      make_issue_with_recommendation
+        ~alternative_method:
+          (ErrorRenderingUtils.find_alternative_nonnull_method_description rhs_origin)
+        ~severity:(NullsafeMode.severity nullsafe_mode)
+        ~loc:assignment_location
     in
-    let description =
-      match assignment_type with
-      | PassingParamToFunction function_info ->
-          Format.sprintf "%s%s"
-            (mk_description_for_bad_param_passed function_info ~nullability_evidence
-               ~param_nullability_kind:explicit_rhs_nullable_kind)
-            alternative_recommendation
-      | AssigningToField field_name ->
-          let rhs_description =
-            match explicit_rhs_nullable_kind with
-            | ErrorRenderingUtils.UserFriendlyNullable.Null ->
-                "`null`"
-            | ErrorRenderingUtils.UserFriendlyNullable.Nullable ->
-                "a nullable"
-          in
-          Format.asprintf "%a is declared non-nullable but is assigned %s%s.%s" MF.pp_monospaced
+    match assignment_type with
+    | PassingParamToFunction function_info ->
+        mk_issue_for_bad_param_passed function_info ~nullability_evidence
+          ~param_nullability_kind:explicit_rhs_nullable_kind ~make_issue_fn
+    | AssigningToField field_name ->
+        let rhs_description =
+          match explicit_rhs_nullable_kind with
+          | ErrorRenderingUtils.UserFriendlyNullable.Null ->
+              "`null`"
+          | ErrorRenderingUtils.UserFriendlyNullable.Nullable ->
+              "a nullable"
+        in
+        let description =
+          Format.asprintf "%a is declared non-nullable but is assigned %s%s."
+            MarkupFormatter.pp_monospaced
             (Fieldname.get_field_name field_name)
-            rhs_description nullability_evidence_as_suffix alternative_recommendation
-      | ReturningFromFunction function_proc_name ->
-          let return_description =
-            match explicit_rhs_nullable_kind with
-            | ErrorRenderingUtils.UserFriendlyNullable.Null ->
-                (* Return `null` in all_whitelisted branches *)
-                "`null`"
-            | ErrorRenderingUtils.UserFriendlyNullable.Nullable ->
-                "a nullable value"
-          in
-          Format.asprintf "%a: return type is declared non-nullable but the method returns %s%s.%s"
-            MF.pp_monospaced
+            rhs_description nullability_evidence_as_suffix
+        in
+        make_issue_fn ~description ~issue_type:IssueType.eradicate_field_not_nullable
+    | ReturningFromFunction function_proc_name ->
+        let return_description =
+          match explicit_rhs_nullable_kind with
+          | ErrorRenderingUtils.UserFriendlyNullable.Null ->
+              (* Return `null` in all_whitelisted branches *)
+              "`null`"
+          | ErrorRenderingUtils.UserFriendlyNullable.Nullable ->
+              "a nullable value"
+        in
+        let description =
+          Format.asprintf "%a: return type is declared non-nullable but the method returns %s%s."
+            MarkupFormatter.pp_monospaced
             (Procname.Java.to_simplified_string ~withclass:false function_proc_name)
-            return_description nullability_evidence_as_suffix alternative_recommendation
-    in
-    let issue_type = get_issue_type assignment_type in
-    NullsafeIssue.make ~description ~issue_type ~loc:assignment_location
-      ~severity:(NullsafeMode.severity nullsafe_mode)
+            return_description nullability_evidence_as_suffix
+        in
+        make_issue_fn ~description ~issue_type:IssueType.eradicate_return_not_nullable
 
 
   let make_nullsafe_issue ~assignment_location assignment_type {nullsafe_mode; violation= {rhs}} =