Standalone `$` is accepted as a token in declarative macros.

[cafce25]

I tried this code:

macro_rules! foo {
    ($) => {}
}

I expected to see this happen: an error because a standalone $ doesn't match any of the grammar rules from the reference

Instead, this happened: the code compiles, you can successfully invoke the macro like this foo!($);

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Latest stable on the playground (1.93.1)
Also on the nightly verson:

Nightly version: 1.95.0-nightly
(2026-02-24 859951e3c7c9d0322c39)

[cafce25]

@rustbot label A-macros

[PatchMixolydic]

This seems like a reference bug. Until $$ is stabilized, macro matchers must support matching against $ to allow macros to reliably expand to macros. (This requires matching with $tok:tt, but allowing $ as tt but not as a literal token is likely to result in confusion.)

Edit: Actually, it seems like adding any token after $ (eg. $a, $loop, $!loop; excluding $a:specifier) leads to a compiler error:

error: expected identifier, found `!`
 --> src/main.rs:2:7
  |
2 |     ($!loop) => {}
  |       ^

error: missing fragment specifier
 --> src/main.rs:2:7
  |
2 |     ($!loop) => {}
  |       ^
  |
  = note: fragment specifiers must be provided
  = help: valid fragment specifiers are `ident`, `block`, `stmt`, `expr`, `pat`, `ty`, `lifetime`, `literal`, `path`, `meta`, `tt`, `item` and `vis`, along with `expr_2021` and `pat_param` for edition compatibility
help: try adding a specifier here
  |
2 |     ($!:specloop) => {}
  |        +++++

Perhaps accepting literal $ is unintentional after all.