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

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();