syntax: allow stmt/expr macro invocations to be delimited by [].

this is useful for macros like vec! which construct containers

src/libsyntax/parse/token.rs

@@ -297,21 +297,17 @@ pub fn can_begin_expr(t: &Token) -> bool {
     }
 }
 
-/// what's the opposite delimiter?
-pub fn flip_delimiter(t: &token::Token) -> token::Token {
+/// Returns the matching close delimiter if this is an open delimiter,
+/// otherwise `None`.
+pub fn close_delimiter_for(t: &Token) -> Option<Token> {
     match *t {
-      LPAREN => RPAREN,
-      LBRACE => RBRACE,
-      LBRACKET => RBRACKET,
-      RPAREN => LPAREN,
-      RBRACE => LBRACE,
-      RBRACKET => LBRACKET,
-      _ => fail!()
+        LPAREN   => Some(RPAREN),
+        LBRACE   => Some(RBRACE),
+        LBRACKET => Some(RBRACKET),
+        _        => None
     }
 }
 
-
-
 pub fn is_lit(t: &Token) -> bool {
     match *t {
       LIT_CHAR(_) => true,