Code change:
Here is a minimum example of the problem. Compilation fails with following error. Which means that the fragment qualifier is not taken into account to choose which variant of the macro is to be used.
Yeah, macros don't do backtracking. You'll need to adjust the matchers to provide some other way to disambiguate.
Yes, I obviously know that I could just put brackets or whatever around one of the patterns to make it distinct. But the whole point of having fragment qualifiers is precisely to distinguish between two otherwise identical patterns. If macros cannot do it although they have the perfect tool for it, then they are stupid and ought to be improved.
It wasn't obvious to me that you knew the workaround -- sorry for assuming! Also see .
Thanks for pointing the PR. It is blocked for now, but I suppose there is still hope. Sorry for being harsh. I have been raging for months over how annoyingly limited and stupid Rust macros are, and I finally found a case general and obvious enough to justify my complaining…
triage: Macros still don't backtrack. I can't find an issue for that, though I do suspect that one exists in some form. Labeling as A-macros, though.
here is (closed).
Closing in favor of -- macro rules do not backtrack.

Related discussion:
text: } } <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());