syntax: Allow 1-tuple expressions

This is for greater uniformity (for example, macros that generate
tuples). rustc already supported 1-tuple patterns, but there was no
way to construct a 1-tuple term.

Author: catamorphism

src/libsyntax/parse/parser.rs

@@ -1061,6 +1061,9 @@ pub impl Parser {
 
         if self.token == token::LPAREN {
             self.bump();
+            // (e) is parenthesized e
+            // (e,) is a tuple with only one field, e
+            let mut one_tuple = false;
             if self.token == token::RPAREN {
                 hi = self.span.hi;
                 self.bump();
@@ -1069,12 +1072,18 @@ pub impl Parser {
             }
             let mut es = ~[self.parse_expr()];
             while self.token == token::COMMA {
-                self.bump(); es.push(self.parse_expr());
+                self.bump();
+                if self.token != token::RPAREN {
+                    es.push(self.parse_expr());
+                }
+                else {
+                    one_tuple = true;
+                }
             }
             hi = self.span.hi;
             self.expect(token::RPAREN);
 
-            return if es.len() == 1 {
+            return if es.len() == 1 && !one_tuple {
                 self.mk_expr(lo, self.span.hi, expr_paren(es[0]))
             }
             else {
@@ -2158,11 +2167,13 @@ pub impl Parser {
                 pat = pat_lit(expr);
             } else {
                 let mut fields = ~[self.parse_pat(refutable)];
-                while self.token == token::COMMA {
-                    self.bump();
-                    fields.push(self.parse_pat(refutable));
+                if self.look_ahead(1) != token::RPAREN {
+                    while self.token == token::COMMA {
+                        self.bump();
+                        fields.push(self.parse_pat(refutable));
+                    }
                 }
-                if vec::len(fields) == 1u { self.expect(token::COMMA); }
+                if fields.len() == 1 { self.expect(token::COMMA); }
                 hi = self.span.hi;
                 self.expect(token::RPAREN);
                 pat = pat_tup(fields);

src/libsyntax/print/pprust.rs

@@ -1199,6 +1199,9 @@ pub fn print_expr(s: @ps, &&expr: @ast::expr) {
       ast::expr_tup(exprs) => {
         popen(s);
         commasep_exprs(s, inconsistent, exprs);
+        if exprs.len() == 1 {
+            word(s.s, ~",");
+        }
         pclose(s);
       }
       ast::expr_call(func, args, sugar) => {
@@ -1634,6 +1637,9 @@ pub fn print_pat(s: @ps, &&pat: @ast::pat, refutable: bool) {
       ast::pat_tup(elts) => {
         popen(s);
         commasep(s, inconsistent, elts, |s, p| print_pat(s, p, refutable));
+        if elts.len() == 1 {
+            word(s.s, ~",");
+        }
         pclose(s);
       }
       ast::pat_box(inner) => {

src/test/run-pass/one-tuple.rs

@@ -0,0 +1,20 @@
+// 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.
+
+// Why one-tuples? Because macros.
+
+fn main() {
+    match ('c',) {
+        (x,) => {
+            assert x == 'c';
+        }
+    }
+}
+