[flow][errors] Eliminate last usage of ExplanationAlreadyPrinted

Summary:
I want to ensure that `intermediate_error` contains no `Friendly.message_feature`, which contains a lot of strings that are mostly constant, but very wasteful to send between processes. The end goal is that we will send `intermediate_error` to the main process instead, which will contain already decided primary error location.

This diff starts with the easy one: we get rid of `ExplanationAlreadyPrinted`. We just need to add two more variants.

Changelog: [internal]

Reviewed By: panagosg7

Differential Revision: D56164157

fbshipit-source-id: 4e73c5a1a9ea7ffc97288b13b162d4911956f34d

src/typing/errors/error_message.ml

@@ -8,6 +8,7 @@
 open Type
 open Reason
 open Utils_js
+open Flow_intermediate_error_types
 
 exception EDebugThrow of ALoc.t
 
@@ -2228,7 +2229,7 @@ type 'loc friendly_message_recipe =
       loc: 'loc;
       features: Loc.t Flow_errors_utils.Friendly.message_feature list;
       use_op: 'loc Type.virtual_use_op;
-      explanation: Loc.t Flow_errors_utils.Friendly.message_feature list option;
+      explanation: 'loc explanation option;
     }
   | PropPolarityMismatch of {
       prop: string option;
@@ -4923,25 +4924,15 @@ let friendly_message_of_msg loc_of_aloc msg =
         loc;
         features = lower @ [text " but "] @ upper;
         use_op;
-        explanation =
-          Some
-            [
-              text
-                "React hooks and other functions are not compatible with each other, because hooks cannot be called conditionally";
-            ];
+        explanation = Some ExplanationReactHookIncompatibleWithNormalFunctions;
       }
   | EHookUniqueIncompatible { use_op; lower; upper } ->
     UseOp
       {
         loc = loc_of_reason lower;
         features = [ref lower; text " and "; ref upper; text " are different React hooks"];
         use_op;
-        explanation =
-          Some
-            [
-              text
-                "Different React hooks are not compatible with each other, because hooks cannot be called conditionally";
-            ];
+        explanation = Some ExplanationReactHookIncompatibleWithEachOther;
       }
   | EHookNaming _ ->
     Normal { features = [text "Hooks must have names that begin with "; code "use"; text "."] }

src/typing/errors/flow_intermediate_error.ml

@@ -1002,7 +1002,6 @@ let rec make_intermediate_error :
         (Flow_error.code_of_error error)
         (MessageAlreadyFriendlyPrinted features)
     | (None, UseOp { loc; features; use_op; explanation }) ->
-      let explanation = Base.Option.map explanation ~f:(fun e -> ExplanationAlreadyPrinted e) in
       mk_use_op_error loc use_op ?explanation (MessageAlreadyFriendlyPrinted features)
     | (None, PropMissing { loc; prop; reason_obj; use_op; suggestion }) ->
       mk_prop_missing_error loc prop reason_obj use_op suggestion
@@ -1061,7 +1060,6 @@ let to_printable_error :
   let ref = Friendly.ref_map loc_of_aloc in
   let desc = Friendly.desc_of_reason_desc in
   let explanation_to_friendly_msgs = function
-    | ExplanationAlreadyPrinted features -> features
     | ExplanationAbstractEnumCasting ->
       [
         text "You can explicitly cast your enum value to its representation type using ";
@@ -1160,6 +1158,16 @@ let to_printable_error :
         ref (mk_reason (RCustom "hook arguments") props_loc);
         text " and their nested elements cannot be written to";
       ]
+    | ExplanationReactHookIncompatibleWithEachOther ->
+      [
+        text
+          "Different React hooks are not compatible with each other, because hooks cannot be called conditionally";
+      ]
+    | ExplanationReactHookIncompatibleWithNormalFunctions ->
+      [
+        text
+          "React hooks and other functions are not compatible with each other, because hooks cannot be called conditionally";
+      ]
     | ExplanationReactHookReturnDeepReadOnly hook_loc ->
       [
         text "The return value of a ";

src/typing/errors/flow_intermediate_error_types.ml

@@ -9,7 +9,6 @@ open Reason
 
 type 'loc explanation =
   | ExplanationAbstractEnumCasting
-  | ExplanationAlreadyPrinted of Loc.t Flow_errors_utils.Friendly.message_feature list
   | ExplanationArrayInvariantTyping
   | ExplanationConstrainedAssign of {
       name: string;
@@ -26,6 +25,8 @@ type 'loc explanation =
   | ExplanationReactComponentPropsDeepReadOnly of 'loc
   | ExplanationReactComponentRefRequirement
   | ExplanationReactHookArgsDeepReadOnly of 'loc
+  | ExplanationReactHookIncompatibleWithEachOther
+  | ExplanationReactHookIncompatibleWithNormalFunctions
   | ExplanationReactHookReturnDeepReadOnly of 'loc
   | ExplanationRenderTypeRequirement
   | ExplanationTypeGuardCompatibility