Improve diagnostics for missing tilde in labeled parameters.

1) In certain scenarios there was no error: `let f = (x, y=2, z) => x + y + z`. Results in an assertion failure in the typechecker.
2) Streamlines the language in the diagnostic across expressions and types.

Fixes #216

Author: Iwan

src/res_core.ml

@@ -26,7 +26,6 @@ module Recover = struct
   let defaultPattern () =
     let id = Location.mknoloc "rescript.patternhole" in
     Ast_helper.Pat.extension (id, PStr [])
-    (* Ast_helper.Pat.any  () *)
 
   let defaultModuleExpr () = Ast_helper.Mod.structure []
   let defaultModuleType () = Ast_helper.Mty.signature []
@@ -104,6 +103,12 @@ Solution: directly use `concat`."
     "A type declaration's name cannot contain a module access. Did you mean `" ^ (Longident.last longident) ^"`?"
 
   let tupleSingleElement = "A tuple needs at least two elements"
+
+  let missingTildeLabeledParameter name =
+    if name = "" then
+      "A labeled parameter starts with a `~`."
+    else
+      ("A labeled parameter starts with a `~`. Did you mean: `~" ^ name ^ "`?")
 end
 
 
@@ -1595,6 +1600,10 @@ and parseParameter p =
       let lidents = parseLidentList p in
       Some (TypeParameter {uncurried; attrs; locs = lidents; pos = startPos})
     ) else (
+    let hasTilde = match p.token with
+    | Tilde -> true
+    | _ -> false
+    in
     let (attrs, lbl, pat) = match p.Parser.token with
     | Tilde ->
       Parser.next p;
@@ -1641,11 +1650,18 @@ and parseParameter p =
     in
     match p.Parser.token with
     | Equal ->
+      if not hasTilde then (
+        let msg = match pat.ppat_desc with
+        | Ppat_var var -> ErrorMessages.missingTildeLabeledParameter var.txt
+        | _ -> ErrorMessages.missingTildeLabeledParameter ""
+        in
+        Parser.err ~startPos  p (Diagnostics.message msg)
+      );
       Parser.next p;
       let lbl = match lbl with
-      | Asttypes.Labelled lblName -> Asttypes.Optional lblName
-      | Asttypes.Optional _ as lbl -> lbl
-      | Asttypes.Nolabel -> Asttypes.Nolabel
+      | Asttypes.Labelled lblName ->
+        Asttypes.Optional lblName
+      | lbl -> lbl
       in
       begin match p.Parser.token with
       | Question ->
@@ -3848,9 +3864,9 @@ and parseTypeParameter p =
       begin match p.token with
       | Colon ->
         let () =
-          let error = Diagnostics.message
-            ("Parameter names start with a `~`, like: ~" ^ name)
-          in
+          let error = Diagnostics.message (
+            ErrorMessages.missingTildeLabeledParameter name
+          ) in
           Parser.err ~startPos:loc.loc_start ~endPos:loc.loc_end p error
         in
         Parser.next p;

tests/parsing/errors/other/__snapshots__/parse.spec.js.snap

@@ -32,6 +32,34 @@ module M = struct ;;match l with | None -> [] | Some l -> l ## prop end
   I'm not sure what to parse here when looking at \\"}\\".
 
 
+========================================================"
+`;
+
+exports[`labelledParameters.res 1`] = `
+"=====Parsetree==========================================
+let f x y z = (x + y) + z
+type nonrec f = x:int -> y:int -> int
+=====Errors=============================================
+
+  Syntax error!
+  parsing/errors/other/labelledParameters.res:1:13-14  
+  1 │ let f = (x, y=2, z) => x + y + z
+  2 │ 
+  3 │ type f = (x: int, y: int) => int
+  
+  A labeled parameter starts with a \`~\`. Did you mean: \`~y\`?
+
+
+  Syntax error!
+  parsing/errors/other/labelledParameters.res:3:11  
+  1 │ let f = (x, y=2, z) => x + y + z
+  2 │ 
+  3 │ type f = (x: int, y: int) => int
+  4 │ 
+  
+  A labeled parameter starts with a \`~\`. Did you mean: \`~x\`?
+
+
 ========================================================"
 `;
 

tests/parsing/errors/other/labelledParameters.res

@@ -0,0 +1,3 @@
+let f = (x, y=2, z) => x + y + z
+
+type f = (x: int, y: int) => int

tests/parsing/errors/typeDef/__snapshots__/parse.spec.js.snap

@@ -100,7 +100,7 @@ type nonrec draw = stroke:pencil -> unit
   1 │ type  draw = (stroke: pencil) => unit
   2 │ 
   
-  Parameter names start with a \`~\`, like: ~stroke
+  A labeled parameter starts with a \`~\`. Did you mean: \`~stroke\`?
 
 
 ========================================================"