More robust parse recovery after ".".

See rescript-lang/rescript-vscode#416 for examples where parser recovery goes wrong in template expressions.
Basically, if the token after "." is consumed in recovery, the entire template expression is eaten up and never recovers.

This PR changes error recovery to not consume the token after Dot so the parser can recover properly inside a template.

Author: cristianoc

src/res_core.ml

@@ -728,6 +728,16 @@ let parseValuePath p =
   Parser.next p;
   Location.mkloc ident (mkLoc startPos p.prevEndPos)
 
+let parseValuePathAfterDot p =
+  let startPos = p.Parser.startPos in
+  match p.Parser.token with
+  | Lident _
+  | Uident _ ->
+    parseValuePath p
+  | token ->
+    Parser.err p (Diagnostics.unexpected token p.breadcrumbs);
+    Location.mkloc (Longident.Lident "_") (mkLoc startPos p.prevEndPos)
+
 let parseValuePathTail p startPos ident =
   let rec loop p path =
     match p.Parser.token with
@@ -2052,7 +2062,7 @@ and parsePrimaryExpr ~operand ?(noCall=false) p =
     match p.Parser.token with
     | Dot ->
       Parser.next p;
-      let lident = parseValuePath p in
+      let lident = parseValuePathAfterDot p in
       begin match p.Parser.token with
       | Equal when noCall = false ->
         Parser.leaveBreadcrumb p Grammar.ExprSetField;
@@ -3591,11 +3601,16 @@ and parseValueOrConstructor p =
       let lident = buildLongident (ident::acc) in
       Ast_helper.Exp.ident ~loc (Location.mkloc lident loc)
     | token ->
-      Parser.next p;
-      let loc = mkLoc startPos p.prevEndPos in
-      Parser.err p (Diagnostics.unexpected token p.breadcrumbs);
-      let lident = buildLongident ("_"::acc) in
-      Ast_helper.Exp.ident ~loc (Location.mkloc lident loc)
+      if acc = [] then (
+        Parser.next p;
+        Parser.err p (Diagnostics.unexpected token p.breadcrumbs);
+        Recover.defaultExpr()
+      ) else (
+        let loc = mkLoc startPos p.prevEndPos in
+        Parser.err p (Diagnostics.unexpected token p.breadcrumbs);
+        let lident = buildLongident ("_"::acc) in
+        Ast_helper.Exp.ident ~loc (Location.mkloc lident loc)
+      )
   in
   aux p []