Rephrase error message on invalid fragment specifiers in macros.

Also, print help on valid fragment specifiers.

src/libsyntax/ext/tt/macro_parser.rs

@@ -83,7 +83,7 @@ use self::TokenTreeOrTokenTreeVec::*;
 use ast;
 use ast::{TokenTree, Ident};
 use ast::{TtDelimited, TtSequence, TtToken};
-use codemap::{BytePos, mk_sp};
+use codemap::{BytePos, mk_sp, Span};
 use codemap;
 use parse::lexer::*; //resolve bug?
 use parse::ParseSess;
@@ -483,11 +483,11 @@ pub fn parse(sess: &ParseSess,
 
                 let mut ei = bb_eis.pop().unwrap();
                 match ei.top_elts.get_tt(ei.idx) {
-                  TtToken(_, MatchNt(_, name, _, _)) => {
+                  TtToken(span, MatchNt(_, name, _, _)) => {
                     let name_string = token::get_ident(name);
                     let match_cur = ei.match_cur;
                     (&mut ei.matches[match_cur]).push(Rc::new(MatchedNonterminal(
-                        parse_nt(&mut rust_parser, name_string.get()))));
+                        parse_nt(&mut rust_parser, span, name_string.get()))));
                     ei.idx += 1us;
                     ei.match_cur += 1;
                   }
@@ -505,7 +505,7 @@ pub fn parse(sess: &ParseSess,
     }
 }
 
-pub fn parse_nt(p: &mut Parser, name: &str) -> Nonterminal {
+pub fn parse_nt(p: &mut Parser, sp: Span, name: &str) -> Nonterminal {
     match name {
         "tt" => {
             p.quote_depth += 1us; //but in theory, non-quoted tts might be useful
@@ -541,7 +541,11 @@ pub fn parse_nt(p: &mut Parser, name: &str) -> Nonterminal {
       }
       "meta" => token::NtMeta(p.parse_meta_item()),
       _ => {
-          p.fatal(&format!("unsupported builtin nonterminal parser: {}", name)[])
+          p.span_fatal_help(sp,
+                            &format!("invalid fragment specifier `{}`", name)[],
+                            "valid fragment specifiers are `ident`, `block`, \
+                             `stmt`, `expr`, `pat`, `ty`, `path`, `meta`, `tt` \
+                             and `item`")
       }
     }
 }

src/libsyntax/ext/tt/macro_rules.rs

@@ -457,7 +457,7 @@ fn is_in_follow(_: &ExtCtxt, tok: &Token, frag: &str) -> Result<bool, String> {
                 // harmless
                 Ok(true)
             },
-            _ => Err(format!("unrecognized builtin nonterminal `{}`", frag))
+            _ => Err(format!("invalid fragment specifier `{}`", frag))
         }
     }
 }

src/test/compile-fail/issue-21356.rs

@@ -9,6 +9,6 @@
 // except according to those terms.
 
 macro_rules! test { ($wrong:t_ty ..) => () }
-                  //~^ ERROR: unrecognized builtin nonterminal `t_ty`
+                  //~^ ERROR: invalid fragment specifier `t_ty`
 
 fn main() {}