[flow][match] Error on invalid match pattern binding kind

Summary:
For `match` expressions, only `const` is allowed, since nothing else makes sense. (When we add support for `match` statements in Flow, those will also allow `let`, like destructuring does.)

Changelog: [internal]

Reviewed By: SamChou19815

Differential Revision: D67502326

fbshipit-source-id: 79e1ce3ff0b0668b2f0d8444764477e79f5ffb9b

src/common/errors/error_codes.ml

@@ -107,6 +107,7 @@ type error_code =
   | InvalidTempType
   | LintSetting
   | MalformedPackage
+  | MatchInvalidPattern
   | MatchNotExhaustive
   | MethodUnbinding
   | MissingLocalAnnot
@@ -320,6 +321,7 @@ let string_of_code : error_code -> string = function
   | InvalidTempType -> "invalid-temp-type"
   | LintSetting -> "lint-setting"
   | MalformedPackage -> "malformed-package"
+  | MatchInvalidPattern -> "match-invalid-pattern"
   | MatchNotExhaustive -> "match-not-exhaustive"
   | MethodUnbinding -> "method-unbinding"
   | MissingLocalAnnot -> "missing-local-annot"

src/typing/debug_js.ml

@@ -1900,6 +1900,11 @@ let dump_error_message =
       spf "ENegativeTypeGuardConsistency (%s)" (dump_reason cx reason)
     | EMatchNotExhaustive { loc; reason } ->
       spf "EMatchNotExhaustive (%s) (%s)" (string_of_aloc loc) (dump_reason cx reason)
+    | EMatchInvalidBindingKind { loc; kind } ->
+      spf
+        "EMatchInvalidBindingKind (%s) (%s)"
+        (string_of_aloc loc)
+        (Flow_ast_utils.string_of_variable_kind kind)
     | EDevOnlyRefinedLocInfo { refined_loc; refining_locs = _ } ->
       spf "EDevOnlyRefinedLocInfo {refined_loc=%s}" (string_of_aloc refined_loc)
     | EDevOnlyInvalidatedRefinementInfo { read_loc; invalidation_info = _ } ->

src/typing/errors/error_message.ml

@@ -618,6 +618,10 @@ and 'loc t' =
       loc: 'loc;
       reason: 'loc virtual_reason;
     }
+  | EMatchInvalidBindingKind of {
+      loc: 'loc;
+      kind: Flow_ast.Variable.kind;
+    }
   (* Dev only *)
   | EDevOnlyRefinedLocInfo of {
       refined_loc: 'loc;
@@ -1421,6 +1425,7 @@ let rec map_loc_of_error_message (f : 'a -> 'b) : 'a t' -> 'b t' =
     EDevOnlyRefinedLocInfo { refined_loc = f refined_loc; refining_locs = List.map f refining_locs }
   | EMatchNotExhaustive { loc; reason } ->
     EMatchNotExhaustive { loc = f loc; reason = map_reason reason }
+  | EMatchInvalidBindingKind { loc; kind } -> EMatchInvalidBindingKind { loc = f loc; kind }
   | EDevOnlyInvalidatedRefinementInfo { read_loc; invalidation_info } ->
     EDevOnlyInvalidatedRefinementInfo
       {
@@ -1718,7 +1723,8 @@ let util_use_op_of_msg nope util = function
   | EUnionOptimization _
   | EUnionOptimizationOnNonUnion _
   | ECannotCallReactComponent _
-  | EMatchNotExhaustive _ ->
+  | EMatchNotExhaustive _
+  | EMatchInvalidBindingKind _ ->
     nope
 
 (* Not all messages (i.e. those whose locations are based on use_ops) have locations that can be
@@ -1920,6 +1926,7 @@ let loc_of_msg : 'loc t' -> 'loc option = function
   | EEmptyArrayNoProvider { loc } -> Some loc
   | EUnusedPromise { loc; _ } -> Some loc
   | EMatchNotExhaustive { loc; _ } -> Some loc
+  | EMatchInvalidBindingKind { loc; _ } -> Some loc
   | EDevOnlyRefinedLocInfo { refined_loc; refining_locs = _ } -> Some refined_loc
   | EDevOnlyInvalidatedRefinementInfo { read_loc; invalidation_info = _ } -> Some read_loc
   | EUnableToSpread _
@@ -2868,6 +2875,7 @@ let friendly_message_of_msg = function
     Normal (MessageInvalidUseOfFlowEnforceOptimized arg)
   | ECannotCallReactComponent { reason } -> Normal (MessageCannotCallReactComponent reason)
   | EMatchNotExhaustive { loc = _; reason } -> Normal (MessageMatchNotExhaustive reason)
+  | EMatchInvalidBindingKind { loc = _; kind } -> Normal (MessageMatchInvalidBindingKind { kind })
 
 let defered_in_speculation = function
   | EUntypedTypeImport _
@@ -3205,3 +3213,4 @@ let error_code_of_message err : error_code option =
   | EUnionOptimizationOnNonUnion _ -> Some UnionUnoptimizable
   | ECannotCallReactComponent _ -> Some ReactRuleCallComponent
   | EMatchNotExhaustive _ -> Some MatchNotExhaustive
+  | EMatchInvalidBindingKind _ -> Some MatchInvalidPattern

src/typing/errors/flow_intermediate_error.ml

@@ -3989,6 +3989,14 @@ let to_printable_error :
         ref reason;
         text " has not been fully checked against by the match patterns below.";
       ]
+    | MessageMatchInvalidBindingKind { kind } ->
+      [
+        text "Cannot use ";
+        code (Flow_ast_utils.string_of_variable_kind kind);
+        text " for match pattern binding. Only ";
+        code "const";
+        text " is allowed.";
+      ]
   in
   let rec convert_error_message { kind; loc; error_code; root; message; misplaced_source_file = _ }
       =

src/typing/errors/flow_intermediate_error_types.ml

@@ -898,6 +898,7 @@ type 'loc message =
       null_loc: 'loc option;
     }
   | MessageMatchNotExhaustive of 'loc virtual_reason
+  | MessageMatchInvalidBindingKind of { kind: Flow_ast.Variable.kind }
 
 type 'loc intermediate_error = {
   kind: Flow_errors_utils.error_kind;

src/typing/match_pattern.ml

@@ -6,6 +6,7 @@
  *)
 
 module Ast = Flow_ast
+module Tast_utils = Typed_ast_utils
 open Reason
 open Type
 
@@ -67,10 +68,17 @@ let binding_identifier cx ~on_binding ~kind acc (loc, { Ast.Identifier.name; com
   let t = binding cx ~on_binding ~kind acc loc name in
   ((loc, t), { Ast.Identifier.name; comments })
 
-let binding_pattern cx ~on_binding acc binding =
+let binding_pattern cx ~on_binding ~loc acc binding =
   let open Ast.MatchPattern.BindingPattern in
   let { kind; id; comments } = binding in
-  let id = binding_identifier cx ~on_binding ~kind acc id in
+  let id =
+    match kind with
+    | Ast.Variable.Var
+    | Ast.Variable.Let ->
+      Flow_js.add_output cx (Error_message.EMatchInvalidBindingKind { loc; kind });
+      Tast_utils.error_mapper#t_identifier id
+    | Ast.Variable.Const -> binding_identifier cx ~on_binding ~kind acc id
+  in
   { kind; id; comments }
 
 let rec member cx ~on_identifier ~on_expression mem =
@@ -113,7 +121,7 @@ let rest_pattern cx ~on_binding acc rest =
         {
           argument =
             Base.Option.map argument ~f:(fun (arg_loc, arg) ->
-                (arg_loc, binding_pattern cx ~on_binding acc arg)
+                (arg_loc, binding_pattern cx ~on_binding ~loc:arg_loc acc arg)
             );
           comments;
         }
@@ -144,15 +152,15 @@ let rec pattern cx ~on_identifier ~on_expression ~on_binding acc (loc, p) :
       let target =
         match target with
         | AsPattern.Binding (loc, binding) ->
-          AsPattern.Binding (loc, binding_pattern cx ~on_binding acc binding)
+          AsPattern.Binding (loc, binding_pattern cx ~on_binding ~loc acc binding)
         | AsPattern.Identifier id ->
           AsPattern.Identifier (binding_identifier cx ~on_binding ~kind:Ast.Variable.Const acc id)
       in
       AsPattern { AsPattern.pattern = p; target; comments }
     | IdentifierPattern (loc, x) ->
       let t = on_identifier cx x loc in
       IdentifierPattern ((loc, t), x)
-    | BindingPattern x -> BindingPattern (binding_pattern cx ~on_binding acc x)
+    | BindingPattern x -> BindingPattern (binding_pattern cx ~on_binding ~loc acc x)
     | WildcardPattern x -> WildcardPattern x
     | ArrayPattern { ArrayPattern.elements; rest; comments } ->
       let rest = rest_pattern cx ~on_binding acc rest in