Continue parsing function after finding ... arg

When encountering a variadic argument in a function definition that
doesn't accept it, if immediately after there's a closing paren,
continue parsing as normal. Otherwise keep current behavior of emitting
error and stopping.

Author: estebank

src/libsyntax/parse/parser.rs

@@ -4859,19 +4859,30 @@ impl<'a> Parser<'a> {
                 |p| {
                     if p.token == token::DotDotDot {
                         p.bump();
+                        variadic = true;
                         if allow_variadic {
                             if p.token != token::CloseDelim(token::Paren) {
                                 let span = p.span;
                                 p.span_err(span,
                                     "`...` must be last in argument list for variadic function");
                             }
+                            Ok(None)
                         } else {
-                            let span = p.span;
-                            p.span_err(span,
-                                       "only foreign functions are allowed to be variadic");
+                            let span = p.prev_span;
+                            if p.token == token::CloseDelim(token::Paren) {
+                                // continue parsing to present any further errors
+                                p.struct_span_err(
+                                    span,
+                                    "only foreign functions are allowed to be variadic"
+                                ).emit();
+                                Ok(Some(dummy_arg(span)))
+                           } else {
+                               // this function definition looks beyond recovery, stop parsing
+                                p.span_err(span,
+                                           "only foreign functions are allowed to be variadic");
+                                Ok(None)
+                            }
                         }
-                        variadic = true;
-                        Ok(None)
                     } else {
                         match p.parse_arg_general(named_args) {
                             Ok(arg) => Ok(Some(arg)),

src/test/ui/invalid-variadic-function.rs

@@ -0,0 +1,13 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+extern "C" fn foo(x: u8, ...);
+//~^ ERROR only foreign functions are allowed to be variadic
+//~| ERROR expected one of `->`, `where`, or `{`, found `;`

src/test/ui/invalid-variadic-function.stderr

@@ -0,0 +1,14 @@
+error: only foreign functions are allowed to be variadic
+  --> $DIR/invalid-variadic-function.rs:11:26
+   |
+11 | extern "C" fn foo(x: u8, ...);
+   |                          ^^^
+
+error: expected one of `->`, `where`, or `{`, found `;`
+  --> $DIR/invalid-variadic-function.rs:11:30
+   |
+11 | extern "C" fn foo(x: u8, ...);
+   |                              ^ expected one of `->`, `where`, or `{` here
+
+error: aborting due to 2 previous errors
+