Add spread element check when the last element is a spread.

src/res_core.ml

@@ -3716,6 +3716,14 @@ and parseSpreadExprRegion p =
   | _ -> None
 
 and parseListExpr ~startPos p =
+  let check_all_non_spread_exp exprs =
+    exprs
+    |> List.map (fun (spread, expr) ->
+           if spread then
+             Parser.err p (Diagnostics.message ErrorMessages.listExprSpread);
+           expr)
+    |> List.rev
+  in
   let listExprs =
     parseCommaDelimitedReversedList p ~grammar:Grammar.ListExpr ~closing:Rbrace
       ~f:parseSpreadExprRegion
@@ -3724,17 +3732,10 @@ and parseListExpr ~startPos p =
   let loc = mkLoc startPos p.prevEndPos in
   match listExprs with
   | (true, expr) :: exprs ->
-    let exprs = exprs |> List.map snd |> List.rev in
+    let exprs = check_all_non_spread_exp exprs in
     makeListExpression loc exprs (Some expr)
   | exprs ->
-    let exprs =
-      exprs
-      |> List.map (fun (spread, expr) ->
-             if spread then
-               Parser.err p (Diagnostics.message ErrorMessages.listExprSpread);
-             expr)
-      |> List.rev
-    in
+    let exprs = check_all_non_spread_exp exprs in
     makeListExpression loc exprs None
 
 (* Overparse ... and give a nice error message *)

tests/parsing/errors/other/expected/spread.res.txt

@@ -49,6 +49,20 @@ Explanation: you can't collect a subset of a record's field into its own record,
 Solution: you need to pull out each field you want explicitly.
 
 
+  Syntax error!
+  tests/parsing/errors/other/spread.res:8:1-3
+
+   6 │ 
+   7 │ let myList = list{...x, ...y}
+   8 │ let list{...x, ...y} = myList
+   9 │ 
+  10 │ type t = {...a}
+
+  Lists can only have one `...` spread, and at the end.
+Explanation: lists are singly-linked list, where a node contains a value and points to the next node. `list[a, ...bc]` efficiently creates a new item and links `bc` as its next nodes. `[...bc, a]` would be expensive, as it'd need to traverse `bc` and prepend each item to `a` one by one. We therefore disallow such syntax sugar.
+Solution: directly use `concat`.
+
+
   Syntax error!
   tests/parsing/errors/other/spread.res:8:13-22