Change:
In this case, it should be looking for an ident or a , in which there is no ambiguity. All channels are affected. This is affecting my ability to write macros accepting generics using angle brackets, e.g. , with a supporting pattern to turn into . May be related to .
In theory there is no ambiguity because an ident can't start with . As far as I can tell the macro evaluator doesn't reason about this at all. It sure is annoying and I hope it can be fixed. I suppose it introduces a forwards compatibility consideration, just like follow sets: if we let macros distinguish between and , then can never be to the ident grammar.
the error is spurious; this is definitely a bug. The issue is purely when the repeating group is empty. Hence working.

Discussion:
return Error(span, "missing fragment specifier".to_string()); } } <del> TokenTree::MetaVarDecl(..) => { </del> <ins> TokenTree::MetaVarDecl(_, _, id) => { </ins> // Built-in nonterminals never start with these tokens, // so we can eliminate them from consideration. <del> match *token { token::CloseDelim(_) => {}, _ => bb_eis.push(ei), </del> <ins> if may_begin_with(&*id.name.as_str(), token) { bb_eis.push(ei); </ins> } } seq @ TokenTree::Delimited(..) | seq @ TokenTree::Token(_, DocComment(..)) => {

} } <ins> /// Checks whether a non-terminal may begin with a particular token. /// /// Returning `false` is a *stability guarantee* that such a matcher will *never* begin with that /// token. Be conservative (return true) if not sure. fn may_begin_with(name: &str, token: &Token) -> bool { /// Checks whether the non-terminal may contain a single (non-keyword) identifier. fn may_be_ident(nt: &token::Nonterminal) -> bool { match *nt { token::NtItem(_) | token::NtBlock(_) | token::NtVis(_) => false, _ => true, } } match name { "expr" => token.can_begin_expr(), "ty" => token.can_begin_type(), "ident" => token.is_ident(), "vis" => match *token { // The follow-set of :vis + "priv" keyword + interpolated Token::Comma | Token::Ident(_) | Token::Interpolated(_) => true, _ => token.can_begin_type(), }, "block" => match *token { Token::OpenDelim(token::Brace) => true, Token::Interpolated(ref nt) => match nt.0 { token::NtItem(_) | token::NtPat(_) | token::NtTy(_) | token::NtIdent(_) | token::NtMeta(_) | token::NtPath(_) | token::NtVis(_) => false, // none of these may start with '{'. _ => true, }, _ => false, }, "path" | "meta" => match *token { Token::ModSep | Token::Ident(_) => true, Token::Interpolated(ref nt) => match nt.0 { token::NtPath(_) | token::NtMeta(_) => true, _ => may_be_ident(&nt.0), }, _ => false, }, "pat" => match *token { Token::Ident(_) |               // box, ref, mut, and other identifiers (can stricten) Token::OpenDelim(token::Paren) |    // tuple pattern Token::OpenDelim(token::Bracket) |  // slice pattern Token::BinOp(token::And) |          // reference Token::BinOp(token::Minus) |        // negative literal Token::AndAnd |                     // double reference Token::Literal(..) |                // literal Token::DotDot |                     // range pattern (future compat) Token::DotDotDot |                  // range pattern (future compat) Token::ModSep |                     // path Token::Lt |                         // path (UFCS constant) Token::BinOp(token::Shl) |          // path (double UFCS) Token::Underscore => true,          // placeholder Token::Interpolated(ref nt) => may_be_ident(&nt.0), _ => false, }, _ => match *token { token::CloseDelim(_) => false, _ => true, }, } } </ins> fn parse_nt<'a>(p: &mut Parser<'a>, sp: Span, name: &str) -> Nonterminal { if name == "tt" { return token::NtTT(p.parse_token_tree());

// except according to those terms. fn main() { <del> panic!(@); //~ ERROR expected expression, found `@` </del> <ins> panic!(@); //~ ERROR no rules expected the token `@` </ins> }

// except according to those terms. pub fn main() { <del> vec![,]; //~ ERROR expected expression, found `,` </del> <ins> vec![,]; //~ ERROR no rules expected the token `,` </ins> }

<ins> // Copyright 2017 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. #![feature(macro_vis_matcher)] //{{{ issue 40569 ============================================================== macro_rules! my_struct { ($(#[$meta:meta])* $ident:ident) => { $(#[$meta])* struct $ident; } } my_struct!(#[derive(Debug, PartialEq)] Foo40569); fn test_40569() { assert_eq!(Foo40569, Foo40569); } //}}} //{{{ issue 26444 ============================================================== macro_rules! foo_26444 { ($($beginning:ident),*; $middle:ident; $($end:ident),*) => { stringify!($($beginning,)* $middle $(,$end)*) } } fn test_26444() { assert_eq!("a , b , c , d , e", foo_26444!(a, b; c; d, e)); assert_eq!("f", foo_26444!(; f ;)); } macro_rules! pat_26444 { ($fname:ident $($arg:pat)* =) => {} } pat_26444!(foo 1 2 5...7 =); pat_26444!(bar Some(ref x) Ok(ref mut y) &(w, z) =); //}}} //{{{ issue 40984 ============================================================== macro_rules! thread_local_40984 { () => {}; ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = $init:expr; $($rest:tt)*) => { thread_local_40984!($($rest)*); }; ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = $init:expr) => {}; } thread_local_40984! { // no docs #[allow(unused)] static FOO: i32 = 42; /// docs pub static BAR: String = String::from("bar"); // look at these restrictions!! pub(crate) static BAZ: usize = 0; pub(in foo) static QUUX: usize = 0; } //}}} //{{{ issue 35650 ============================================================== macro_rules! size { ($ty:ty) => { std::mem::size_of::<$ty>() }; ($size:tt) => { $size }; } fn test_35650() { assert_eq!(size!(u64), 8); assert_eq!(size!(5), 5); } //}}} //{{{ issue 27832 ============================================================== macro_rules! m { ( $i:ident ) => (); ( $t:tt $j:tt ) => (); } m!(c); m!(t 9); m!(0 9); m!(struct); m!(struct Foo); macro_rules! m2 { ( $b:expr ) => (); ( $t:tt $u:tt ) => (); } m2!(3); m2!(1 2); m2!(_ 1); m2!(enum Foo); //}}} //{{{ issue 39964 ============================================================== macro_rules! foo_39964 { ($a:ident) => {}; (_) => {}; } foo_39964!(_); //}}} //{{{ issue 34030 ============================================================== macro_rules! foo_34030 { ($($t:ident),* /) => {}; } foo_34030!(a, b/); foo_34030!(a/); foo_34030!(/); //}}} //{{{ issue 24189 ============================================================== macro_rules! foo_24189 { ( pub enum $name:ident { $( #[$attr:meta] )* $var:ident } ) => { pub enum $name { $( #[$attr] )* $var } }; } foo_24189! { pub enum Foo24189 { #[doc = "Bar"] Baz } } macro_rules! serializable { ( $(#[$struct_meta:meta])* pub struct $name:ident { $( $(#[$field_meta:meta])* $field:ident: $type_:ty ),* , } ) => { $(#[$struct_meta])* pub struct $name { $( $(#[$field_meta])* $field: $type_ ),* , } } } serializable! { #[allow(dead_code)] /// This is a test pub struct Tester { #[allow(dead_code)] name: String, } } macro_rules! foo_24189_c { ( $( > )* $x:ident ) => { }; } foo_24189_c!( > a ); fn test_24189() { let _ = Foo24189::Baz; let _ = Tester { name: "".to_owned() }; } //}}} //{{{ some more tests ========================================================== macro_rules! test_block { (< $($b:block)* >) => {} } test_block!(<>); test_block!(<{}>); test_block!(<{1}{2}>); macro_rules! test_ty { ($($t:ty),* $(,)*) => {} } test_ty!(); test_ty!(,); test_ty!(u8); test_ty!(u8,); macro_rules! test_path { ($($t:path),* $(,)*) => {} } test_path!(); test_path!(,); test_path!(::std); test_path!(std::u8,); test_path!(any, super, super::super::self::path, X<Y>::Z<'a, T=U>); macro_rules! test_meta_block { ($($m:meta)* $b:block) => {}; } test_meta_block!(windows {}); //}}} fn main() { test_26444(); test_40569(); test_35650(); test_24189(); } </ins>