It turns out _ is one of very few things still hitting the parser-based slow path after the recent optimizations for literals and lifetimes. Although _ is considered an Ident by the proc macro API, it does not match macro_rules's $:ident matcher and must be treated separately.
It turns out
_is one of very few things still hitting the parser-based slow path after the recent optimizations for literals and lifetimes. Although_is considered an Ident by the proc macro API, it does not match macro_rules's $:ident matcher and must be treated separately.