Add a note for unclosed delimiters

Currently, the parser doesn't give any context when it finds an unclosed
delimiter and it's not EOF. Report the most recent unclosed delimiter, to help
the user along.

Closes #10636

Author: emberian

src/libsyntax/parse/parser.rs

@@ -2086,15 +2086,13 @@ impl Parser {
         fn parse_non_delim_tt_tok(p: &Parser) -> token_tree {
             maybe_whole!(deref p, nt_tt);
             match *p.token {
-              token::RPAREN | token::RBRACE | token::RBRACKET
-              => {
-                p.fatal(
-                    format!(
-                        "incorrect close delimiter: `{}`",
-                        p.this_token_to_str()
-                    )
-                );
-              }
+              token::RPAREN | token::RBRACE | token::RBRACKET => {
+                  // This is a conservative error: only report the last unclosed delimiter. The
+                  // previous unclosed delimiters could actually be closed! The parser just hasn't
+                  // gotten to them yet.
+                  p.open_braces.last_opt().map(|sp| p.span_note(*sp, "unclosed delimiter"));
+                  p.fatal(format!("incorrect close delimiter: `{}`", p.this_token_to_str()));
+              },
               /* we ought to allow different depths of unquotation */
               token::DOLLAR if *p.quote_depth > 0u => {
                 p.bump();

src/test/compile-fail/issue-10636-1.rs

@@ -0,0 +1,13 @@
+// Copyright 2013 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.
+
+struct Obj { //~ NOTE: unclosed delimiter
+    member: uint
+) //~ ERROR: incorrect close delimiter

src/test/compile-fail/issue-10636-2.rs

@@ -0,0 +1,13 @@
+// Copyright 2013 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.
+
+pub fn trace_option(option: Option<int>) {
+    option.map(|some| 42; //~ NOTE: unclosed delimiter
+} //~ ERROR: incorrect close delimiter