Improve error message for missing label(s) in function application (#6576)

Author: cknitt

CHANGELOG.md

@@ -18,6 +18,10 @@
 - Fixed issue with coercions sometimes raising a `Not_found` instead of giving a proper error message. https://github.com/rescript-lang/rescript-compiler/pull/6574
 - Fix issue with recursive modules and uncurried. https://github.com/rescript-lang/rescript-compiler/pull/6575
 
+#### :nail_care: Polish
+
+- Improve error message for missing label(s) in function application. https://github.com/rescript-lang/rescript-compiler/pull/6576
+
 # 11.0.0
 
 No changes compared to rc.9.

jscomp/build_tests/super_errors/expected/missing_label.res.expected

@@ -0,0 +1,10 @@
+
+  We've found a bug for you!
+  /.../fixtures/missing_label.res:3:9
+
+  1 │ let f = (~a) => a ++ ""
+  2 │ 
+  3 │ let _ = f("")
+  4 │ 
+
+  Label ~a was omitted in the application of this labeled function.

jscomp/build_tests/super_errors/expected/missing_labels.res.expected

@@ -0,0 +1,10 @@
+
+  We've found a bug for you!
+  /.../fixtures/missing_labels.res:3:9
+
+  1 │ let f = (~a, ~b) => a ++ b
+  2 │ 
+  3 │ let _ = f("", "")
+  4 │ 
+
+  Labels ~a, ~b were omitted in the application of this labeled function.

jscomp/build_tests/super_errors/fixtures/missing_label.res

@@ -0,0 +1,3 @@
+let f = (~a) => a ++ ""
+
+let _ = f("")

jscomp/build_tests/super_errors/fixtures/missing_labels.res

@@ -0,0 +1,3 @@
+let f = (~a, ~b) => a ++ b
+
+let _ = f("", "")

jscomp/ml/typecore.ml

@@ -4056,9 +4056,13 @@ let report_error env ppf = function
   | Illegal_letrec_pat ->
       fprintf ppf
         "Only variables are allowed as left-hand side of `let rec'"
+  | Labels_omitted [label] ->
+      fprintf ppf "Label ~%s was omitted in the application of this labeled function." 
+      label  
   | Labels_omitted labels ->
-      fprintf ppf  "For labeled function, labels %s were omitted in the application of this function." 
-      (String.concat ", " labels)  
+      let labelsString = labels |> List.map(fun label -> "~" ^ label) |> String.concat ", " in
+      fprintf ppf "Labels %s were omitted in the application of this labeled function." 
+      labelsString 
   | Empty_record_literal ->
       fprintf ppf  "Empty record literal {} should be type annotated or used in a record context."
   | Uncurried_arity_mismatch (typ, arity, args) ->