Improve error message when capturing this in lambdas (ponylang#1201)

Author: Praetonus

pony.g

@@ -209,7 +209,7 @@ tuple
   ;
 
 lambdacaptures
-  : ('(' | LPAREN_NEW) lambdacapture (',' lambdacapture)* ')'
+  : ('(' | LPAREN_NEW) (lambdacapture | 'this') (',' (lambdacapture | 'this'))* ')'
   ;
 
 lambdacapture

src/libponyc/ast/parser.c

@@ -26,6 +26,7 @@ DECL(parampattern);
 DECL(pattern);
 DECL(idseq_in_seq);
 DECL(members);
+DECL(thisliteral);
 
 
 /* Precedence
@@ -309,12 +310,13 @@ DEF(lambdacapture);
   IF(TK_ASSIGN, RULE("capture value", infix));
   DONE();
 
-// (LPAREN | LPAREN_NEW) lambdacapture {COMMA lambdacapture} RPAREN
+// (LPAREN | LPAREN_NEW) (lambdacapture | thisliteral)
+// {COMMA (lambdacapture | thisliteral)} RPAREN
 DEF(lambdacaptures);
   AST_NODE(TK_LAMBDACAPTURES);
   SKIP(NULL, TK_LPAREN, TK_LPAREN_NEW);
-  RULE("capture", lambdacapture);
-  WHILE(TK_COMMA, RULE("capture", lambdacapture));
+  RULE("capture", lambdacapture, thisliteral);
+  WHILE(TK_COMMA, RULE("capture", lambdacapture, thisliteral));
   SKIP(NULL, TK_RPAREN);
   DONE();
 

src/libponyc/pass/syntax.c

@@ -955,6 +955,18 @@ static ast_result_t syntax_lambda(pass_opt_t* opt, ast_t* ast)
     default: {}
   }
 
+  ast_t* capture = ast_child(captures);
+  while(capture != NULL)
+  {
+    if(ast_id(capture) == TK_THIS)
+    {
+      ast_error(opt->check.errors, capture,
+        "use a named capture to capture 'this'");
+      return AST_ERROR;
+    }
+    capture = ast_sibling(capture);
+  }
+
   return AST_OK;
 }